working near energized parts exposes employees to electric-arc hazards
whenever the employee or another conductive object can contact those
energized parts and other parts at a different potential (id. \288\).
(See the definition of ``exposed'' and the summary and explanation for
final Sec.  1926.960(b)(3), earlier in this section of the preamble.)
---------------------------------------------------------------------------

    \286\ See, for example, the three accidents described at http://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=14328736&id=200962322&id=170197156.
    \287\ See, for example, the two accidents described at http://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=170762769&id=170204622.
    \288\ See, for example, the three accidents described at http://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=170054258&id=170614002&id=170611057.
---------------------------------------------------------------------------

    With respect to the American Forest & Paper Association's comment
about opening and closing disconnects in an enclosed electrical box,
evidence in the record indicates that equipment enclosures do not
always provide adequate protection against electrical faults (Ex.
0373). A paper by Jones et al. \289\ described the results of one
arcing-fault test as follows: ``the fault blew the door open and
progressed up the vertical bus, completely destroying the vertical
section of the [motor control center]'' (id.). A paper by Land \290\
described problems the Navy had in 1979 with arcing faults in
switchboards: ``These arcs could completely destroy a switchboard
within a matter of seconds'' (id.). Although these events may be
uncommon, OSHA believes that it is appropriate for the standard to
require the employer to assess the hazards posed by different
operations and distinguish conditions that expose employees to
electric-arc hazards from conditions that do not. For example,
employers may consider a properly maintained switch as posing no
electric-arc hazards when an employee is opening it under normal
conditions. On the other hand, if there is evidence that the switch may
be faulty or if the employee is opening the switch to troubleshoot the
circuit, OSHA would expect the employer to assume that the switch does
pose electric-arc hazards. Evidence that a switch may be faulty can
include the presence of arcing or unusual noise from the switch,
abnormally high temperatures around the switch, and safety bulletins
from the switch manufacturer indicating that the device might fail
under certain operating conditions. Thus, OSHA concludes that it is not
always safe to operate an enclosed switch and, therefore, is not
generally exempting such activities from the hazard-assessment
requirement in final paragraph (g)(1) or any of the other provisions in
final paragraph (g).
---------------------------------------------------------------------------

    \289\ Jones, R. A., Liggett, D. P., Capelli-Schellpfeffer, M.,
Macalady, T., Saunders, L. F., Downey, R. E., McClung, L. B., Smith,
A., Jamil, S., Saporita, V. J., ``Staged Tests Increase Awareness of
Arc-Flash Hazards in Electrical Equipment,'' IEEE Transactions on
Industry Applications Society, 36(2): 659-667, March-April 2000.
    \290\ Land III, H. B., ``The Behavior of Arcing Faults in Low
Voltage Switchboards,'' 2005 IEEE ESTS, Philadelphia, pp. 133-140,
2005.
---------------------------------------------------------------------------

    OSHA does not believe that applying paragraph (g)(1) of the final
rule in this manner will impose substantial extra costs on employers.
The Agency anticipates that, in the vast majority of cases, the
employer will determine that employees operating enclosed switches will
have no exposure to hazards from electric arcs. On the basis of the
foregoing discussion, it should be clear that the only occasions that
an employee performing a switching operation would have exposure to
electric-arc hazards under paragraph (g)(1), and, thus, be required to
use arc-rated protection, would be if: a switch or other disconnect may
be faulty (which should be rare); an employee operates a switch outside
its rating \291\ (which also should be rare), or an employee is
performing troubleshooting or repair on the switch or a circuit
controlled by the switch. In the latter case, the employee will be
exposed to those same hazards during the troubleshooting or repair
activities, when appropriate arc-flash protection would be required
anyway. For the rare cases in which the employer has reason to believe
that the switch might fail and expose an employee to an electric-arc
hazard, the protection afforded by arc-flash protection would be
necessary.

However, the need to outfit the employee in arc-flash protection in
such cases will serve as an incentive to effect repair of the switch
and remove the hazard.
---------------------------------------------------------------------------

    \291\ Operating a switch or other disconnect outside its rating
is prohibited by Sec.  1926.960(k) of the final rule.
---------------------------------------------------------------------------

    Some commenters argued that some utilities perform work with live-
line tools, which limits employee exposure to hazards posed by electric
arcs and makes FR clothing unnecessary. (See, for example, Exs. 0125,
0171, 0179, 0188, 0226.) NECA also argued that 40-cal/cm\2\ arc-flash
suits with hoods would reduce manual dexterity to the point that they
would interfere with the employee's ability to use live-line tools (Ex.
0171).
    OSHA agrees that work with live-line tools exposes employees to a
lower incident-energy level than work directly on energized parts with
rubber insulating gloves because employees working with live-line tools
are normally farther from an electric arc than employees using gloves.
(The tables in Appendix E use a method of estimating heat energy that
assumes that employees using live-line tools will be substantially
further away from the arc than employees using rubber insulating
gloves.) All of the incident-energy calculation methods (described
later in this section of the preamble) result in energy estimates that
are approximately inversely proportional to the square of the distance.
This proportion means that, when the employee is twice as far from the
electric arc, he or she has exposure to no more than a quarter of the
energy. OSHA does not believe that there are many, if any, working
conditions that would expose an employee using a live-line tool to an
incident energy of 40-cal/cm\2\. NECA's example using clothing
appropriate for such high exposure contradicts its claim that employees
using live-line tools face reduced exposures.
    As discussed later in this section of the preamble, final paragraph
(g)(4)(iv) requires FR clothing when the estimated incident-energy
levels are more than 2.0 cal/cm\2\. If live-line tool work practices
limit incident-energy levels to that value or less, then paragraph
(g)(4) may not require flame-resistant clothing. However, clothing can
ignite even at low incident-energy levels. For example, an arc can
ignite insulating fluid in transformers and other equipment, which
could ultimately ignite clothing (Ex. 0004 \292\). Current passing
through grounding conductors can melt those conductors and ignite
clothing (id. \293\). Hot debris from faulted equipment can spew out
and ignite clothing (Exs. 0342, 0373). Final paragraph (g)(4), as
described more fully later in this section of the preamble, requires
flame-resistant clothing in those scenarios. OSHA is not exempting
live-line tool work from the hazard assessment or other requirements in
paragraph (g) of the final rule. Employers must account for the
possibility of clothing ignition from sources other than incident heat
energy in the hazard assessment required by paragraph (g)(1) of the
final rule.
---------------------------------------------------------------------------

    \292\ See the seven accidents described at http://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=200671253&id=201340395&id=170762769&id=170632699&id=1450477
3&id=14343594&id=837815.
    \293\ See the accident described at http://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=596304.
---------------------------------------------------------------------------

    The American Forest & Paper Association commented that the proposed
definition of ``exposed'' in Sec.  1926.968 does not seem applicable to
the use of the word ``exposed'' in proposed Sec.  1926.960(g) because
the definition refers to a conductor or part rather than a person (Ex.
0237).
    OSHA agrees that the definition in final Sec.  1926.968 relates
only to parts of electric circuits; it does not address employee
exposure to hazards other than exposure to live parts.\294\ To clarify
the application of the definition of ``exposed'' in Sec.  1926.968 of
the final rule, OSHA is adding the parenthetical phrase ``(as applied
to energized parts)'' to the defined term ``exposed.''
---------------------------------------------------------------------------

    \294\ Several provisions in subpart V in addition to final Sec.
1926.960(g) refer to employee exposure.
---------------------------------------------------------------------------

    Estimating incident heat energy.\295\ Once an employer determines
the employees exposed to hazards from flames or electric arcs, the next
step in protecting these employees is to determine the extent of the
hazard. Paragraph (g)(2) of the final rule, which OSHA revised from the
proposal as described later in this section of the preamble, requires
the employer to make a reasonable estimate of the incident heat energy
to which each employee exposed to electric-arc hazards would be
exposed. Under final paragraph (g)(5), employers must use this estimate
to select appropriate PPE.
---------------------------------------------------------------------------

    \295\ This preamble uses the term ``incident energy'' as a
synonym for ``incident heat energy.''
---------------------------------------------------------------------------

    As noted in the preamble to the proposal, OSHA is aware of various
methods of calculating values of available heat energy from an electric
circuit (70 FR 34866-34867). Table 10, later in this section of the
preamble, lists methods that were available when OSHA proposed
paragraph (g)(2). Each method requires the input of various parameters,
such as fault current, the expected length of the electric arc, the
distance from the arc to the employee, and the clearing time for the
fault (that is, the time the circuit protective devices take to open
the circuit and clear the fault). Some of these parameters, such as the
fault current and the clearing time, are known quantities for a given
system. Other parameters, such as the length of the arc and the
distance between the arc and the employee, vary depending on what
happens to initiate the electric arc and are estimated parameters. It
should be noted that NFPA 70E-2004 Annex D contains three different
methods of estimating incident heat energy: (1) a method based on a
paper by Lee entitled ``The Other Electrical Hazard: Electric Arc Blast
Burns,'' \296\ also known as the ``Lee equation''; (2) a method based
on the Doughty, Neal, and Floyd paper, which Table 10 lists separately;
and (3) the IEEE 1584 method, which Table 10 also lists
separately.\297\ The following discussion refers to the method based on
the Lee equation as the NFPA 70E Annex D method.\298\
---------------------------------------------------------------------------

    \296\ Lee, R. H., ``The Other Electrical Hazard: Electric Arc
Blast Burns, '' IEEE Transactions on Industry Applications, 1A-
18(3):246--251, May/June 1982 (Ex. 0433).
    \297\ NFPA 70E-2012, Annex D, contains the same three methods
plus an additional method for calculating incident heat energy for
dc systems. Although OSHA has not evaluated this new method,
employers may use it to calculate incident heat energy if it
reasonably predicts the incident energy for the system involved.
    \298\ NFPA 70E-2012, Annex D, also contains the Lee equation.
Consequently, OSHA's conclusions regarding the NFPA 70E-2004 Annex D
method also apply to NFPA 70E-2012, and Appendix E to final Subpart
V references NFPA 70E-2012. Unless otherwise noted, the preamble
references to the content of NFPA 70E-2004, Annex D, apply equally
to NFPA 70E-2012.

 Table 10--Methods of Calculating Incident Heat Energy From an Electric
                                   Arc
------------------------------------------------------------------------

-----------------------------------------------------------------------------
1. Standard for Electrical Safety Requirements for Employee Workplaces,
 NFPA 70E-2004, Annex D, ``Sample Calculation of Flash Protection
 Boundary.''
2. Doughty, T. E., Neal, T. E., and Floyd II, H. L., ``Predicting
 Incident Energy to Better Manage the Electric Arc Hazard on 600 V Power
 Distribution Systems,'' Record of Conference Papers IEEE IAS 45th
 Annual Petroleum and Chemical Industry Conference, September 28-30,
 1998.
3. Guide for Performing Arc Flash Hazard Calculations, IEEE Std 1584-
 2002.
4. Heat Flux Calculator, a free software program created by Alan
 Privette (widely available on the Internet).
5. ARCPRO, a commercially available software program developed by
 Kinectrics, Toronto, ON, CA.
------------------------------------------------------------------------

    Employee arc exposures. One of the following three separate types
of electric arcs typically serves as the basis for the methods used to
estimate incident energy: single-phase arc in open air, three-phase arc
in open air, and three-phase arc in an enclosure (arc in a box) (Exs.
0425, 0430, 0433, 0463, 0468, 0469). A single-phase arc occurs when
electric current arcs from a circuit part for one phase to ground or to
a circuit part for another phase. A three-phase arc involves arcing
between all three phases of a three-phase circuit. A single-phase arc
can escalate into a three-phase arc as the air around the arc ionizes
and becomes more conductive (Ex. 0425). Both kinds of arcs can occur in
open air or inside an enclosure. The incident-energy levels vary
between the types of arcs, with energy levels progressively increasing
from single-phase arcs in open air, to three-phase arcs in open air, to
three-phase arcs in a box (Exs. 0425, 0430, 0468). OSHA finds that, for
an estimate of heat energy to be reasonable, it must account for the
type of exposure the employee likely will encounter.
    Varying results using different calculation methods. Many
rulemaking participants objected to the proposed requirement that
employers make a reasonable estimate of the incident heat energy
associated with an employee's exposure to an electric-arc hazard. (See,
for example, Exs. 0152, 0173, 0178, 0201, 0209, 0227, 0233, 0501; Tr.
374-376, 547-548, 1094-1098, 1100-1102.) Some of these rulemaking
participants focused on purported problems with methods of calculating
incident heat energy. (See, for example, Exs. 0152, 0173, 0201, 0209,
0227, 0233, 0501; Tr. 547, 1094-1098, 1100-1102.) These commenters
maintained that the results of calculations from the different methods
varied widely or are subject to manipulation that would make the
calculation methods unreliable or unscientific (id.). For example, Ms.
Kathy Wilmer, testifying on behalf of EEI, spoke to the wide variations
she found in calculating incident heat energy using the methods listed
in the proposed rule:

    OSHA does not endorse any of the methods listed in the table.
OSHA further acknowledges that the method of calculation can affect
the results inasmuch as each method yields somewhat different values
using the same input parameters.
* * * * *
    [F]our methods, including two tables and two formulas, were
compared for the conditions of 15,000 volts, 5,000 amps, and 34.5
cycles. The heat energies determined were, No. 1, from Appendix F,
Table 8,\[\\299\\]\ of the proposal, 5 calories per square
centimeter; No. 2, from the HeatFlux Calculator, 2.9 calories per
square centimeter; No. 3, from NFPA 70E, Table
130.7(c)(9)(a),\[\\300\\]\ 40 calories per square centimeter, as it
is listed as risk category 4 \[\\301\\]\ for work on energized parts
in the other equipment over 1,000-fold category; No. 4, from NFPA
70E, Annex D, D7, formula, 153 calories per square centimeter.
---------------------------------------------------------------------------

    \299\ Table 8 in proposed Appendix F listed estimates of
incident energy for different parts of an electrical system
operating at 4 to 46 kilovolts. OSHA based these estimates on the
ARCPRO method.
    \300\ NFPA 70E-2004 Table 130.7(C)(9)(a) is a method for
selecting PPE based on hazard/risk categories. Proposed Appendix F
did not list NFPA 70E-2004, Table 130.7(C)(9)(a), as an acceptable
method of estimating incident-energy level.
    \301\ NFPA 70E-2004, Table 130.7(C)(11) lists the following
hazard-risk categories (HRC) with the corresponding minimum required
arc ratings: 0-none, 1-4 cal/cm\2\, 2-8 cal/cm\2\, 3-25 cal/cm\2\,
4-40 cal/cm\2\.
---------------------------------------------------------------------------

    In summary, the results were 2.9, 5, 40, and 153 calories per
square centimeter for the same conditions: 15,000 volts, 5,000 amps,
34.5 cycles. Again, this example illustrates serious concerns about
the reliability of methods offered to determine heat energy on
transmission and distribution systems. [Tr. 1096, 1101-1102]

    OSHA applied the same methods Ms. Wilmer described in this comment
and arrived at values similar to the values provided in her testimony,
as shown in Table 11.

                      Table 11--Sample Incident-Energy Calculations Using Different Methods
----------------------------------------------------------------------------------------------------------------
                  Method                                         Incident energy (cal/cm\2\)
----------------------------------------------------------------------------------------------------------------
Heat flux calculator......................  3.0 (results must be rounded up to ensure that the protective
                                             equipment rating equals or exceeds this value).
Table 8 from proposed Appendix F..........  5.0.
NFPA 70E-2004, Annex D, section D.7.......  152.
NFPA 70E-2004, Table 130.7(C)(9)(a).......  Not applicable. Table 130.7(C)(9)(a) lists a Hazard-Risk Category of
                                             2 (8 cal/cm\2\) for insulated cable examination in open areas,
                                             which is an exposure comparable to that of a single-phase arc in
                                             open air represented by the Heat Flux calculator and Table 8 from
                                             proposed Appendix F. Table 130.7(C)(9)(a) lists a Hazard-Risk
                                             Category of 4 (40 cal/cm\2\) for work on energized parts, which is
                                             an exposure comparable to the three-phase arc in an enclosure
                                             represented by the method in NFPA 70E-2004, Annex D, section D.7.
                                             However, as explained later in this section of the preamble, Table
                                             130.7(C)(9)(a) combines a risk assessment with incident-energy
                                             calculation and does not represent incident energy alone.
----------------------------------------------------------------------------------------------------------------

    A closer look at these results shows that the two software
programs, heat flux calculator and ARCPRO (upon which OSHA based Table
8 of proposed Appendix F), produce similar results: 3.0 cal/cm\2\ for
the heat flux calculator and 5.0 cal/cm\2\ for ARCPRO. Because the arc
rating for the lightest weight arc-rated clothing ranges from 4.0 to
5.0 cal/

cm\2\, both programs would lead generally to the use of the same
minimum level of protection for the system parameters at issue.\302\
---------------------------------------------------------------------------

    \302\ As explained later in this section of the preamble, Table
6 and Table 7 in Appendix E in the final rule set a minimum level of
4.0 cal/cm\2\, which is the minimum level of arc-rated clothing
currently available.
---------------------------------------------------------------------------

    The heat flux calculator and ARCPRO both calculate incident energy
produced by single-phase arcs in air, which is clear in the ARCPRO
documentation (Ex. 0468). Also, the preamble to the proposal clearly
stated that the results from the heat flux calculator require
adjustment for application to exposures involving three-phase arcs or
arcs in enclosures (70 FR 34867), and other evidence in the record
indicates that the calculator is designed for application to single-
phase arc exposures (Exs. 0430, 0463).
    The incident-energy estimate resulting from application of the
formula in NFPA 70E-2004, Annex D, is significantly higher than the
results obtained using either of the software programs. There are two
reasons for this difference. First, the formula that appears in section
D.7 of NFPA 70E, Annex D, is designed to calculate the incident energy
produced by a three-phase arc in open air. The corresponding single-
phase exposure, based on an ARCPRO conversion factor (multiplying
single-phase values by 2.2 to convert them to three-phase values or,
conversely, dividing three-phase values by 2.2 to convert them to
single-phase values), would be 70 cal/cm\2\ (Ex. 0468). Second,
although NFPA 70E states that the formula in section D.7 of Annex D can
be used to predict the incident energy produced by arcs on systems
operating at more than 600 volts, it also explicitly warns about doing
so, noting:

    The following example is conservative at voltage levels above
600 volts. Experience suggests that the example is conservative at
voltage levels above 600 volts and becomes more conservative as the
voltage increases. [Ex. 0134; annex section D.1\303\]

    \303\ NFPA 70E-2012, Annex D, contains the same equation in
Section D.6. Similar language warning about conservative results
from using the Lee paper for voltages over 600 volts appears in
Table D.1, Limitation of Calculation Methods.

    Consequently, it is not surprising that the incident-energy
estimate calculated using Annex D of NFPA 70E-2004 for a scenario
involving a single-phase arc on a 15-kilovolt system \304\ is
substantially higher than the values derived using the two software
programs.
---------------------------------------------------------------------------

    \304\ Although Ms. Wilmer did not state that her scenario
involved a single-phase exposure, her use of Table 8 in proposed
Appendix F, the use of which is limited to such exposures, implies
that the scenario is for a single-phase arc.
---------------------------------------------------------------------------

    Ms. Wilmer also mentioned Table 130.7(C)(9)(a) of NFPA 70E-2004.
The closest hazard-risk category from Table 130.7(C)(9)(a) is 2
(requiring clothing rated at 8 cal/cm\2\), which is for the task of
``[i]nsulated cable examination in open air'' (Ex. 0134). The other
tasks in the category entitled ``Other Equipment 1 kV and Above''
appear to represent exposures from arcs in enclosures, and all of those
tasks, including the one for cable examination, represent three-phase
exposures. Moreover, OSHA examined this table more closely and found
that it does not represent incident-energy calculations alone. The
hazard-risk categories listed in NFPA 70E-2004, Table
130.7(C)(9)(a),\305\ include a risk component, as well as an incident-
energy component, as can be seen from the entries for the various tasks
on 600-volt class motor control centers. The hazard-risk categories for
this equipment vary from 1 to 3 (which require clothing rated from 4 to
25 cal/cm\2\) depending on the task, even though, according to the
notes to the table, the system parameters are the same for all the
tasks; thus, the calculated incident energy for all the tasks for this
equipment should be the same. While not clear from NFPA 70E-2004, it
appears that the NFPA 70E Committee chose to reduce the amount of
protection for a task based on the likelihood that an electric arc
would occur.\306\ The level of protection needed for a particular
incident heat energy is the same regardless of the probability that an
electric arc will occur. In other words, whether there is a 5-percent
risk or a 10-percent risk is not relevant to whether the employee's PPE
is adequate. As will be explained later in this section of the
preamble, OSHA based the determination of the level of PPE required
under the final rule solely on incident heat energy. OSHA's final rule
separates the determination of risk (that is, whether an employee is
exposed to hazards posed by electric arcs), as required by final
paragraph (g)(1), from the calculation of incident energy, as required
by final paragraph (g)(2). Therefore, the Agency concludes that NFPA
70E-2004, Table 130.7(C)(9)(a), is not a reasonable method of
estimating incident energy under final paragraph (g)(2) and, therefore,
is not referencing that table in Appendix E in the final rule.
---------------------------------------------------------------------------

    \305\ NFPA 70E-2012 contains an equivalent table in Table
130.7(C)(15)(a). As noted earlier, NFPA 70E-2004, Table 130.7(C)(11)
lists the minimum arc rating for each hazard-risk category. NFPA
70E-2012 lists minimum arc ratings for each hazard-risk category in
Table 130.7(C)(16). OSHA's conclusions regarding NFPA 70E-2004 Table
130.7(C)(9)(a) apply equally to NFPA 70E-2012 Table 130.7(C)(15)(a).
    \306\ Earlier editions of NFPA 70E, such as the 2000 edition,
and NFPA documentation on the adoption of the task table show that
the hazard/risk category is reduced by 1 if the probability of an
arc is low and reduced by 2 if the probability is very low.
---------------------------------------------------------------------------

    In the following discussion, the Agency evaluates the various
methods listed in Table 10 across three distinct voltage categories
(600 volts and less, 601 to 1,000 volts, and more than 1,000 volts),
and for each type of electric arc (single-phase arc in open air, three-
phase arc in open air, and three-phase arc in an enclosure).
    Voltages of 600 volts and less. As can be seen from the tasks
listed in Table 130.7(C)(9)(a), much of the work addressed by NFPA 70E-
2004 involves voltages of 600 volts or less (Ex. 0134). This category
represents the dominant voltage class for utilization equipment
installed in buildings, including electric power generation stations.
It also includes service-class equipment, such as meters, installed on
distribution circuits. There is wide experience using the incident-
energy calculation methods included in Annex D of NFPA 70E-2004 and in
IEEE Std 1584a-2004,\307\ and there is evidence that some electric
utilities use these methods successfully (Exs. 0216 (showing TVA's use
of IEEE Std 1584 to calculate incident-energy levels), 0444 (``INPO
(Institute for Nuclear Power Operations) was and is a huge factor in
driving the use of NFPA 70E as a recognized `best practice' for
electrical safety programs in the nuclear power industry'')). A
national consensus standard recognizes these methods

(NFPA 70E),\308\ and there is considerable test data validating them
(Exs. 0425 (``[the IEEE 1584 committee] has overseen a significant
amount of testing and has developed new models of incident energy'' and
``[IEEE Std 1584a-2004 provides calculations based on] new, empirically
derived models based on statistical analysis and curve fitting of the
overall test data available''), 0430 (this paper, which the IEEE 1584
committee referenced, reported on the results of 25 tests that
supplemented ``previously completed extensive arc testing'').)
---------------------------------------------------------------------------

    \307\ IEEE adopted two amendments after it published IEEE Std
1584-2002: IEEE Std 1584a-2004 (Amendment 1 to IEEE Std 1584-2002),
and IEEE Std 1584b-2011 (Amendment 2: Changes to Clause 4 of IEEE
Std 1584-2002). (Ex. 0425 contains both the IEEE Std 1584-2002
standard and the 1584a-2004 amendment.) This preamble refers to
specific versions of IEEE Std 1584 as follows:
    IEEE Std 1584-2002: the base IEEE Std 1584 standard
    IEEE Std 1584a-2004: IEEE Std 1584-2002 as amended by IEEE Std
1584a-2004
    IEEE Std 1584b-2011: IEEE Std 1584-2002 as amended by IEEE Std
1584a-2004 and IEEE Std 1584b-2011.
    IEEE Std 1584a-2004 and IEEE Std 1584b-2011 use the same basic
methodology to calculate incident-energy levels as IEEE Std 1584-
2002. In this section of the preamble, OSHA analyzed IEEE Std 1584a-
2004 (Ex. 0425) to determine whether employers can use that standard
to make reasonable estimates of incident energy. The Agency also
examined the latest version of IEEE Std 1584 and found that, because
the calculation method did not change from IEEE Std 1584a-2004 to
IEEE Std 1584b-2011, OSHA's conclusions regarding IEEE Std 1584a-
2004 also apply to IEEE Std 1584b-2011, and Appendix E to final
Subpart V references IEEE Std 1584b-2011. Unless otherwise noted,
the preamble references to the content of IEEE Std 1584a-2004 apply
equally to IEEE Std 1584b-2011.
    \308\ As previously mentioned, NFPA 70E-2004, Annex D,
recognizes IEEE Std 1584-2002 as a valid method of calculating
incident heat energy (Ex. 0134).
---------------------------------------------------------------------------

    OSHA concludes that the methods of calculating incident heat energy
in NFPA 70E-2004, Annex D, and IEEE Std 1584a-2004 are reasonable at
voltages of 600 volts and less for the exposures these methods address,
as explained more fully later in this section of the preamble. No
evidence in the record persuades OSHA otherwise. A paper by Stokes and
Sweeting entitled ``Electric Arcing Burn Hazards'' criticized both the
NFPA 70E Annex D and IEEE 1584 methods (Ex. 0452).\309\ That paper
notes that the NFPA and IEEE methods use a predominantly radiant model
of incident heat energy from an electric arc, in which 90 percent of
the heat is radiant heat and in which the entire exposure will be
outside the electric arc plasma. The Stokes and Sweeting paper
disagrees that radiant heat is the predominant hazard and shows that
orienting the test electrodes in a horizontal configuration can result
in the transference of a greater degree of convective heat and that the
amount of heat within the electric arc plasma \310\ is more than three
times higher than predicted by the NFPA and IEEE models. The Stokes and
Sweeting paper also noted that the Lee paper, which is the basis of the
NFPA method, predicts a smaller plasma diameter than the plasma
diameter found during testing. The Stokes and Sweeting paper explained:
---------------------------------------------------------------------------

    \309\ Stokes, A. D., Sweeting, D. K., ``Electric Arcing Burn
Hazards,'' IEEE Transactions on Industry Applications. Vol. 42. No.
1, January/February 2006, pp. 134-141.
    \310\ Plasma is the high-temperature ionized gas cloud that
results from the electric arc.

    As an example, for a three-phase arcing exposure of 5000 V and
20000 A, the Lee prediction forecasts a plasma diameter of 170 mm [7
inches]. . . . The authors' test results for this condition, for an
arc duration of 0.5 s, show a brilliant plasma cloud some 3000 mm
[118 inches] long and around 1500 mm [59 inches] tall in the plane
---------------------------------------------------------------------------
of the camera. [Id.]

    OSHA recognizes that exposures within the plasma field of an
electric arc will produce heat that is several times the incident
energy predicted by any of the methods used to calculate heat energy
recognized by the final rule. However, the Agency believes that the
predominant exposure for employees covered by this final rule will be
outside the plasma field. Although, in the Stokes and Sweeting paper,
the plasma field extended beyond the distance provided for in the NFPA
and IEEE methods, the paper did not indicate how to estimate the
field's reach. Furthermore, all of the calculation methods require an
estimate of the distance from the electric arc to the employee. The
IEEE 1584 method uses 455 to 610 millimeters (18 to 24 inches) for low-
voltage (600 volts and less) equipment such as switchboards,
panelboards, and motor control centers. As explained later in this
section of the preamble, those distances are reasonable estimates of
the distance from the employee to the arc. In addition, the testing
supporting the IEEE 1584 method, which is representative of typical
exposures, confirms the incident-energy results derived using that
method (Ex. 0425). There is no evidence in the record that indicates
that employees will typically be closer than these distances for this
type of work or will be in the plasma field at these working distances.
Therefore, OSHA concludes that, in general, the incident-energy
calculation methods in NFPA 70E-2004, Annex D, and IEEE Std 1584a-2004
reasonably represent employee exposure for voltages of 600 volts and
less.
    The IEEE 1584 method accounts for differences between single-phase
and three-phase arcs and between arcs in open air and arcs in an
enclosure (id. (``The arc-flash hazard calculations included in this
guide will enable quick and comprehensive solutions for arcs in single-
or three-phase electrical systems either of which may be in open air or
in a box, regardless of the low or medium voltage available'')). In
addition, as noted earlier, this method is based on extensive testing,
and a consensus standard recognizes this method. Therefore, OSHA
concludes that this method reasonably represents employee exposures for
single-phase and multiphase arcs in enclosures and open air.
    Proposed Appendix F also listed a paper by Doughty, Neal, and Floyd
as a method of estimating incident energy from an electric arc. (See
Table 10 earlier in this section of the preamble.) This paper describes
the results of tests performed on a 600-volt power system with a 36.25-
kiloampere prospective fault current and contains algorithms to
estimate incident energy at a specified distance from an arc as a
function of the available bolted-fault current on a 600-volt system
(Ex. 0430). The tests included three-phase arcs in enclosures and in
open air (id.). Because this paper was peer reviewed and the methods it
uses are based on testing electric arcs, OSHA finds that the method in
this paper reliably estimates incident energy for the 600-volt systems
it represents.\311\ The Agency also finds that it reasonably represents
incident energy for systems of lower voltages and for single-phase
systems because the power produced by these systems should be
comparable to, and not exceed, the power from a three-phase 600-volt
system with an equivalent supply. The Doughty, Neal, and Floyd method
will produce conservative results for lower-voltage and single-phase
systems. On the other hand, this method does not estimate incident
energy for systems of higher voltages. Therefore, OSHA finds that it is
not reasonable to use this method to estimate incident energy for
systems of voltages of more than 600 volts.
---------------------------------------------------------------------------

    \311\ The equations given in this paper are for an arc lasting 6
cycles. An employer using the Doughty, Neal, and Floyd method will
need to adjust the results to account for any clearing times
different from 6 cycles by multiplying the incident energy
calculated using these equations by the ratio of the actual clearing
time to 6 cycles.
---------------------------------------------------------------------------

    The Doughty, Neal, Floyd paper compared the results of its authors'
testing with other methods of estimating incident-energy levels,
including the NFPA Annex D method, the heat flux calculator, and a
commercial software program (apparently ARCPRO), which OSHA listed in
the proposal (id.). The paper compared the incident energy it found for
three-phase electric arcs with the incident energy calculated by the
Lee equation used in NFPA 70E, Annex D, by examining the distance
required to achieve an incident-energy level of 1.2 cal/cm\2\. This
distance is the ``curable burn distance,'' which is the distance at
which an employee will begin to sustain a second-degree, or curable,
burn. The paper explained the results of this comparison as follows:

    The Lee ``curable burn'' distances coincide almost exactly with
the second-degree burn distances for the open three-phase arc. The
second-degree burn distances for the arc in the cubic box, however,
are significantly higher. The difference is more pronounced at
higher bolted fault levels. [id.]

Figure 8 depicts these functions.

[GRAPHIC] [TIFF OMITTED] TR11AP14.012

    Based on this analysis, the Agency finds that the Lee equation from
NFPA 70E-2004, Annex D, is a reasonable method of estimating the
incident energy of a three-phase electric arc in open air for systems
of 600 volts or less. However, because the Lee equation significantly
underestimates incident energy from three-phase arcs in an enclosure,
OSHA finds that this is not a reasonable method to estimate incident
energy from such exposures. The Agency also finds that the NFPA 70E-
2004, Annex D, method reasonably represents incident energy for single-
phase systems because the power produced by these systems should be
comparable to, and not exceed, the power from a three-phase system with
an equivalent supply. Thus, this method will produce conservative
results for single-phase systems.
    The Doughty, Neal, and Floyd paper also compared the results of its
authors' testing with the heat flux calculator and ``a commercially
available computer program'' (id.).\312\ The paper found that:
---------------------------------------------------------------------------

    \312\ Although the paper did not identify the ``commercially
available computer program'' by name, OSHA closely examined the
results from ARCPRO and compared them with the commercial software
program incident-energy estimates reported by the paper and found
them to be equivalent.
---------------------------------------------------------------------------

     The three-phase test values of maximum incident energy for
open arcs were 2.5 to 3.0 times the amounts calculated for single-phase
arcs in air by the two programs; and
     The three-phase test values of maximum incident energy for
arcs in a box were 5.2 to 12.2 times the amounts calculated for single-
phase arcs in air by the two programs (id.).
    This comparison clearly shows that neither program reasonably
estimates incident heat energy from three-phase electric arcs or
electric arcs in an enclosure. Although there are conversion factors
recommended for these programs, these conversion factors do not account
for the wide variation between the incident energies the programs
calculate and the actual incident energy found during testing. Thus,
OSHA finds that the heat flux calculator and ARCPRO do not reasonably
estimate incident heat energy for three-phase arcs or arcs in a box for
systems of 600 volts or less.
    On systems of 600 volts or less, the phase conductors are typically
relatively close together, approximately 30 millimeters (1.25 inches),
as noted in the Doughty, Neal, and Floyd paper (id.). When an arc
occurs between one phase and ground, or between two phases, the
surrounding air becomes ionized (and, thus, conductive), and it can
relatively easily escalate to a three-phase arc (Ex. 0425). In
addition, as seen from NFPA 70E-2004, Table 130.7(C)(9)(a), most of the
exposures at this voltage level, with the exception of work on service
drops, involve equipment in enclosures (Ex. 0134).\313\ Consequently,
OSHA concludes that it normally would be unreasonable to estimate
incident-energy levels for systems of 600 volts using methods based on
single-phase open air arcs. However, the employer may use such methods
when it can demonstrate that there is only one phase present or that
the spacing of the phases is sufficient to prevent the formation of a
three-phase arc. The incident energy results from the electric-arc
model used by ARCPRO ``have shown good agreement with measured values
from a series of tests covering the following ranges of parameters:
Currents from 3.5 kA to 21.5 kA, arc durations from 4 cycles to 30
cycles, arc lengths from 1 inches to 12 inches, and distances of 8
inches to 24 inches from the arc'' (Ex. 0469). The ARCPRO documentation
does not indicate the voltage range verified by the test results;
however, the model used by this program uses voltage only to ensure
that an arc can be sustained over the distance between electrodes.
Consequently, OSHA finds that this program can reasonably estimate
incident energy from a single-phase arc in open air for systems of 600
volts or less, and the employer may use the program as long as the
employer can demonstrate that there is only one phase present or that
the spacing of the phases is sufficient to prevent the formation of a
three-phase arc.
---------------------------------------------------------------------------

    \313\ OSHA acknowledges that NFPA 70E exempts work on electric
power generation, transmission, and distribution installations.
However, the electric equipment installed in generating plants is of
the same type as that covered by NFPA 70E (Ex. 0077), and OSHA
concludes that the tasks performed on this equipment would be of a
similar nature.
---------------------------------------------------------------------------

    For reasons explained later in this section of the preamble, OSHA
finds that the heat flux calculator is not a reasonable method for
estimating incident energy for any type of exposures, irrespective of
voltage.

    Table 12 summarizes OSHA's findings regarding the reasonableness of
using the various methods of estimating incident heat energy for
exposures involving single-phase and three-phase arcs in open air and
in an enclosure for voltages of 600 volts and less.
    Voltages of 601 volts to 15 kilovolts. Work at voltages from 601
volts to 15 kilovolts is common to both electric power distribution
work and to work in industrial and electric utility substations and
plants. Industrial installations use equipment similar to that used by
electric utilities (see, for example, 59 FR 4333-4334). Therefore, any
method that is appropriate for use with industrial systems operating at
these voltages should be appropriate for use with electric power
generation and distribution installations.
    Again, there is wide experience using the incident-energy methods
included in Annex D of NFPA 70E-2004 and in IEEE Std 1584, and there is
evidence that some electric utilities use these methods successfully
(Exs. 0216, 0444). A national consensus standard (NFPA 70E) recognizes
these methods, and there is considerable test data validating them
(Exs. 0425, 0430). OSHA, therefore, finds that the IEEE 1584 method
reasonably estimates incident-energy levels for systems operating at
voltages of 601 volts to 15 kilovolts for exposures involving single-
phase and three-phase arcs in open air or in enclosures. As explained
previously in the discussion of Ms. Wilmer's comments, the method in
NFPA 70E, Annex D (the Lee method), is conservative at more than 600
volts. In addition, this method estimates incident-energy levels for
three-phase arcs and, thus, is even more conservative for exposures
involving single-phase arcs. Because the NFPA 70E Annex D method is
conservative, OSHA finds that it reasonably estimates incident-energy
levels for systems operating at voltages of 601 volts to 15 kilovolts,
that is, it will provide employees with adequate protection.\314\
However, clothing appropriate for the levels of incident energy
calculated by the NFPA 70E Annex D method will be heavier and bulkier,
as well as more expensive, than clothing appropriate for incident
energy calculated using other acceptable methods. (See, for example,
Ex. 0213, ``[The NFPA 70E Annex D method] could be used to calculate
incident energies for transmission system voltages, but [it] will
produce very conservative (high heat energy) results. This will result
in employees wearing unnecessarily heavy arc flash protection when
working on lines.'') Consequently, the Agency anticipates that
employers will only use this method to estimate incident-energy levels
at voltages of 601 volts to 15 kilovolts when it would result in the
use of clothing with a relatively low arc rating.
---------------------------------------------------------------------------

    \314\ For reasons already explained, the NFPA 70E Annex D method
is not reasonable for estimating incident energy exposures from
three-phase arcs in an enclosure.
---------------------------------------------------------------------------

    The method in the Doughty, Neal, and Floyd paper described earlier
in this section of the preamble is based on testing performed
exclusively with an electrode spacing of 32 millimeters (1.25 inches)
at 600 volts (Ex. 0430). There is no evidence in the record that
suggests that this method is suitable at higher voltages, at which
electrode gaps likely are significantly longer. Therefore, OSHA finds
that this method does not reasonably estimate incident-energy levels
for systems operating at voltages above 600 volts.
    The Agency closely examined the two software calculation methods,
ARCPRO and the heat flux calculator, over the voltage range 601 volts
to 15 kilovolts. OSHA performed this examination in part by looking at
the estimates of heat flux for different system parameters. Heat flux
is a measure of the flow of heat energy per unit area per second. The
incident energy from an electric arc can be computed by multiplying the
heat flux, which has the units cal/cm\2\-sec, by the number of seconds
the arc lasts (that is, the clearing time or the amount of time the
devices protecting a circuit take to open the circuit). The clearing
time for circuit protective devices typically is given in cycles, which
then is converted to seconds by dividing the number of cycles by the
number of cycles per second, usually 60. The two software programs,
ARCPRO and the heat flux calculator, can be used to calculate the heat
flux at a given distance from an electric arc with varying parameters
(for example, arc length, system voltage, and current). Figure 9
compares the heat flux calculated by these two programs at 380
millimeters (15 inches) from an arc with an electrode spacing of 51
millimeters (2 inches).\315\ Note that, although 15 kilovolts is the
voltage input to these programs, the incident energy calculated by both
programs would be the same at 601 volts. The two programs only use the
voltage to verify that an arc can be sustained across the given
electrode gap. Figure 9 shows that the heat flux calculator produces
results that can be more than 50 percent less than the results produced
by ARCPRO.
---------------------------------------------------------------------------

    \315\ In preparing Figure 9, OSHA used the values from Table 6
in Appendix E for the distance to the arc and the electrode spacing
corresponding to 15 kilovolts.
---------------------------------------------------------------------------

    After calculating the incident heat energy using ARCPRO or the heat
flux calculator, an employer can select arc-rated protective equipment.
NFPA 70E-2004 contains a widely used, five-level system for selecting
protective clothing based on different incident-energy levels (Ex.
0134). Figure 10 shows the protective-clothing arc rating, based on the
NFPA 70E levels, that employers would select based on the heat-flux
results shown in Figure 9 for each software program using clearing
times of 6, 12, and 36 cycles. The figures clearly show that incident-
energy calculations from the heat flux calculator can be more than 50
percent lower than the calculations from ARCPRO. This difference
generally increases with increasing fault current.
    The documentation for ARCPRO describes the formulas for calculating
energy and heat estimates and the basis for that program's formulas, as
follows:

    The ARCPRO computer program is based on a state-of-the-art
electrical arc model . . . Temperature-dependent gas properties, the
electrode materials and configuration are taken into account in the
model . . .
    Energy and heat values computed by ARCPRO have been verified by
comparison with measured results from high current laboratory tests
involving controlled vertical arcs in air. ARCPRO results have shown
good agreement with measured values from a series of tests covering
the following ranges of parameters: Currents from 3.5 kA to 21.5 kA,
arc durations from 4 cycles to 30 cycles, arc lengths from 1 inches
to 12 inches, and distances of 8 inches to 24 inches from the arc.
[Ex. 0469]

    Ontario Hydro Technologies (now known as Kinectrics), the same
company that performs high-voltage and high-current electrical testing,
including arc testing, developed this program for numerous purposes.
(See, for example, Exs. 0469, 0501; Tr. 283.\316\) Consequently, OSHA
concludes that the incident-energy values calculated by this program
relate reasonably to the heat energy faced by employees facing
exposures involving single-phase electric arcs in open air. (As
explained previously, ARCPRO's conversion factors for exposures
involving three-phase arcs and arcs in enclosures do not reasonably
estimate employee exposures and would result in significant
underprotection for workers.) The Agency believes that this program is
highly accurate over the range of input parameters for which testing
validated the results, that is, single-phase arcs in

open air only. Therefore, OSHA finds that ARCPRO reasonably estimates
incident-energy levels for single-phase arcs in open air for systems
operating at 601 volts to 15 kilovolts.
BILLING CODE 4510-26-P
---------------------------------------------------------------------------

    \316\ See also http://www.kinectrics.com/en/serviceline/ElectricalTesting.html.
[GRAPHIC] [TIFF OMITTED] TR11AP14.013

[GRAPHIC] [TIFF OMITTED] TR11AP14.014

BILLING CODE 4510-26-C

    On the other hand, there is little documentation supporting use of
the heat flux calculator beyond the documentation provided by the NASCO
Electric Arc Hazard Support Page, which describes the program (Ex.
0467).\317\ OSHA is aware that some employers, electric utilities and
others, use this program to estimate incident-energy levels and select
appropriate PPE (Ex. 0430). However, there is little information in the
record on which to judge the heat flux calculator on its own merits or
the results it produces. In fact, TVA commented that it is ``not aware
of any test verification of the results derived from the Heat Flux
Calculator'' (Ex. 0213). Because the heat flux calculator provides
incident-energy levels that are substantially below the levels
resulting from the testing that supports ARCPRO and because there is no
other means of validating the incident energy results from this
program, OSHA cannot find that the heat flux calculator reasonably
estimates incident heat energy levels for any exposures covered by this
final rule.
---------------------------------------------------------------------------

    \317\ The updated online version of this page contains a link to
download the free program (http://www.nascoinc.com/quick_links/heatflux.htm). The program is also available on other Internet Web
sites.
---------------------------------------------------------------------------

    Table 12 summarizes OSHA's findings regarding the reasonableness of
using the various methods of estimating incident heat energy for
exposures involving single-phase and three-phase arcs in open air and
in an enclosure for voltages of 601 volts to 15 kilovolts.
    OSHA expects employers to determine the type of exposure employees
will face. If the energized parts are not in an enclosure, the employer
may use a method appropriate for single-phase arcs in open air as long
as the employer can demonstrate that there is only one phase present or
if the spacings of the phases is sufficient to prevent the formation of
a three-phase arc. Otherwise, employers must use a method suitable for
three-phase arcs in open air or in an enclosure, as appropriate.
    Voltages of more than 15 kilovolts. Systems that operate at more
than 15 kilovolts generally are electric power distribution or
transmission systems covered by existing Sec.  1910.269 and subpart V.
Although some industrial plants operate systems at these voltages,
these existing OSHA standards typically cover systems operating at more
than 15 kilovolts regardless of whether an electric utility or an
industrial operation operates the system. (See, for example, the
preamble to the 1994 final rule adopting existing Sec.  1910.269 (59 FR
4333-4335).)
    IEEE Std 1584a-2004 describes the limits of its application as
follows:
    This model is designed for systems having:

--Voltages in the range of 208 V-15 000 V, three-phase.
* * * * *
    Use of this model is recommended for applications within the
parameters stated in this subclause. [Ex. 0425]

    Systems operating at voltages above 15 kilovolts are, thus,
outside the recommended range of applications for the IEEE standard.
Consequently, OSHA finds that the IEEE 1584 method does not
reasonably estimate incident-energy levels for systems operating at
voltages of more than 15 kilovolts.
    As noted earlier, the NFPA 70E Annex D method gives conservative
results for voltages over 600 volts. For example, as explained in
the discussion of Ms. Wilmer's comment earlier in this section of
the preamble, that method produces an incident heat energy level of
152 cal/cm\2\ for an exposure involving a three-phase arc in open
air for a system of 15 kilovolts with a fault current of 5,000
amperes, a clearing time of 34.5 cycles, and a distance from the
employee to the arc of 381 millimeters (15 inches). In addition, the
NFPA 70E Annex D method produces an incident-energy level of 1254
cal/cm\2\ for an exposure involving a three-phase arc in open air
for a system of 800 kilovolts with a fault current of 20,000
amperes, a clearing time of 54.5 cycles, and a distance from the
employee to the arc of 2,200 meters (86.6 inches).\318\ These values
are too high to be meaningful, particularly at the higher end of the
voltage range. Employers using the NFPA 70E Annex D method to select
arc-rated clothing would outfit employees in clothing that exposes
employees to severe heat-stress hazards even though the incident
energy is not high enough to warrant such protection. Thus, OSHA
finds that it is not reasonable to use this method to estimate
incident energy for systems of voltages of more than 15 kilovolts.
However, in some cases, employees may be far enough away from any
potential arc that even the NFPA 70E Annex D method does not result
in an estimated incident energy that is sufficient to ignite
flammable clothing (2.0 cal/cm\2\ or less, as explained later in
this section of the preamble). Because that method is conservative,
employers may use it to determine that employee exposure to
estimated incident-heat energy is not more than 2.0 cal/cm\2\ and,
thus, that employees need not wear FR clothing under final paragraph
(g)(4)(iv).
---------------------------------------------------------------------------

    \318\ Table 9 in proposed Appendix F listed incident heat
energies for various voltage ranges of more than 46 kilovolts and
fault currents. These are the values for the distance to the arc and
the electrode spacing used in that table for 765 to 800 kilovolts.
The corresponding table in the final rule (Table 7 of Appendix E)
has been revised, as explained later in this section of the
preamble, but those parameters are the same for that voltage range.
---------------------------------------------------------------------------

    For reasons explained previously, OSHA finds the Doughty, Neal,
and Floyd method does not reasonably estimate incident energy for
systems at voltages of more than 600 volts.
    OSHA compared incident-energy values evaluated by the heat flux
calculator to the values computed by ARCPRO at voltages higher than
15 kilovolts using parameters from Table 8 and Table 9 of proposed
Appendix F. The results of this comparison were similar to the
results of the comparison using voltages of 601 volts to 15
kilovolts described earlier. The incident energies computed by the
heat flux calculator were substantially lower than the results
computed by ARCPRO using the same parameters for systems of more
than 15 kilovolts. In addition, as noted earlier, there is no
information in the record validating the incident-energy results
obtained using the heat flux calculator. Therefore, OSHA concludes
that the heat flux calculator does not reasonably estimate incident
energy from systems of more than 15 kilovolts.
    As noted earlier, verification of the ARCPRO incident-energy
calculation model occurred by testing a wide range of input
parameters (Ex. 0469). This model is mostly independent of voltage
(in other words, the results do not vary with voltage); the program
only checks that the voltage will sustain an arc across the
electrode gap (id.). The program accepts parameters outside the
range verified by testing,\319\ and there is no evidence in the
record to indicate that results using parameters outside that range
would be invalid (id.). As noted earlier, this program calculates
incident energy from a single-phase arc in open air. OSHA concludes
that this program accurately calculates incident heat energy from
such arcs. Therefore, the Agency finds that ARCPRO reasonably
estimates incident energy from single-phase arcs in open air on
systems of more than 15 kilovolts.
---------------------------------------------------------------------------

    \319\ ``ARCPRO results have shown good agreement with measured
values from a series of tests covering the following ranges of
parameters: currents from 3.5 kA to 21.5 kA, arc durations from 4
cycles to 30 cycles, arc lengths from 1 [inch] to 12 inches, and
distances of 8 inches to 24 inches from the arc'' (Ex. 0469).
---------------------------------------------------------------------------

    As mentioned previously, the incident energy calculated by
ARCPRO was significantly less than the actual heat energy found when
testing 600-volt, three-phase arcs in open air and in an enclosure
(Ex. 0430). Regardless of voltage, three-phase arcs consume more
power and, therefore, produce more energy, and three-phase arcs in
an enclosure produce even more heat energy because the heat energy
radiating away from the worker reflects back towards the worker and
because all of the convective heat energy is directed toward the
worker (Exs. 0430, 0433).\320\ Therefore, OSHA concludes that using
unmodified ARCPRO results would significantly underestimate the
amount of incident heat energy from these exposures. ARCPRO provides
multiplication factors for adjusting the results to estimate
incident energy from three-phase arcs in open air and

in enclosures.\321\ However, the Agency found that those adjustments
were not reasonable for systems up to 15 kilovolts. In those cases,
there are alternative calculation methods, identified in Table 12,
that more accurately estimate incident energy for those exposures.
In contrast, there is no reasonable alternative for voltages of more
than 15 kilovolts. Therefore, because ARCPRO is the best available
technology for estimating incident energy for three-phase arcs in
open air and in an enclosure for systems operating at more than 15
kilovolts, OSHA will treat this program as reasonably estimating
incident energy for these exposures provided the employer adjusts
the results using the conversion factors in the instructions
included with the program.
---------------------------------------------------------------------------

    \320\ Convection occurs in fluids (liquids and gases) through
the mixing of hot and cold fluid regions driven by pressure,
gravity, or mechanical agitation. This is the type of heating that
occurs as a pot of water is heated to boiling on a stove. Thermal
radiation occurs when radiation (such as infrared radiation) is
emitted from an object and is absorbed by another object. This is
the type of heating provided by the sun.
    \321\ Here are the conversion factors listed in ARCPRO's help
system:
    Energy for: Multiply by:
     1-phase in a box.... 1.5
     3-phase.................. 1.2 to 2.2
     3-phase in a box.... 3.7 to 6.5
    (Ex. 0468).
---------------------------------------------------------------------------

    Mr. Tommy Lucas with TVA maintained that there are no nationally
recognized methods of reasonably estimating incident energy over 60
kilovolts (Ex. 0213).
    As noted previously, however, OSHA evaluated the ARCPRO computer-
software method and found that it provides a reasonable estimate of
incident energy for voltages above 15 kilovolts, including voltages of
more than 60 kilovolts.
    Table 12 summarizes OSHA's findings regarding the reasonableness of
using the various methods of estimating incident heat energy for
exposures involving single-phase and three-phase arcs in open air and
in an enclosure for voltages higher than 15 kilovolts.
    Underground exposures, internal transformer faults, and other
potentially high exposures. Consolidated Edison Company of New York
(Con Edison), commented that the methodologies included in the proposal
would not be useful for exposures faced by its employees, explaining:

    Con Edison has spent millions of dollars to recreate real life
fault situations on our system at a high power testing laboratory.
In these recreation scenarios we deliberately caused cable faults
both in open air and in manholes and had mannequins wired with heat
sensors to measure the incident energies our employees could
potentially be exposed to. Based on the experience gained through
thousands of these faults, both open air and in manholes, we
realized that none of the methodologies OSHA now proposes would be
useful in conducting an analysis to arrive at a protective scheme
for our employees. [Ex. 0157]

    Although Con Edison did not provide the results of its tests, Dr.
Mary Capelli-Schellpfeffer submitted a presentation that Con Edison
prepared describing the company's tests (Ex. 0371). This presentation
did not include any quantitative comparisons with OSHA's proposed
methods of estimating incident energy. However, it did indicate that
Con Edison was able to select appropriate protective garments that
``have proven to be effective in the protection of [its employees]''
(id.).
    The company's tests included tests of faulted transformers and
cable faults in manholes, and OSHA acknowledges that it is possible for
the incident energy for these exposures to exceed results obtained
using the IEEE 1584 method, which addresses exposures involving three-
phase arcs in both open air and enclosures.\322\ If a transformer
experiences an internal fault, the transformer oil can ignite, and the
burning oil will contribute additional heat energy not accounted for by
that method (Ex. 0004).\323\ For underground exposures in manholes and
vaults, it is possible not only for the wall of the enclosure close to
the arc to reflect the heat energy, but for the far walls to do so as
well. The IEEE 1584 method accounts for the former but not the latter
reflections (Ex. 0425). Because the IEEE 1584 method, if the voltage is
15 kilovolts or less, and ARCPRO, if the voltage exceeds 15 kilovolts,
are the best available methods for estimating incident energy for
three-phase arcs in open air or in enclosures, OSHA will treat those
two methods as reasonably estimating incident energy for the exposures
cited by Con Edison. However, these estimates may not fully protect
employees from electric-arc exposures resulting from internal faults in
transformers or similar equipment or from arcs in underground manholes
or vaults. Despite this shortcoming, the Agency believes that using
these methods to estimate incident energy and to select appropriate
protective equipment in accordance with the other provisions of final
paragraph (g) will better protect employees than if employers permitted
employees to work without arc-rated protective equipment. (See, also,
the summary and explanation of paragraph (g)(5), later in this section
of the preamble.)
---------------------------------------------------------------------------

    \322\ Because Con Edison did not provide the parameters involved
in its tests, OSHA cannot determine for certain what the exposure
was. However, the Agency assumes that the manhole and cable testing
was performed with three-phase voltages between 601 volts and 15
kilovolts. From Table 12, the IEEE 1584 method is the only method
that provides a reasonable estimate for three-phase arcs in an
enclosure, which is the exposure most common in manholes; and the
IEEE 1584 and NFPA 70E Annex D methods are the only methods that
provide a reasonable estimate for three-phase arcs in open air,
which is the exposure associated with three-phase cables.
    \323\ See, for example, the two accidents described at http://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=170632699&id=14343594.
---------------------------------------------------------------------------

    Manipulation of results. Some rulemaking participants maintained
that employers could manipulate the estimate of incident energy by
selecting an inappropriate calculation method or by varying the
parameters, such as arc length or distance from the arc, to achieve
desired results. (See, for example, Exs. 0156, 0161, 0183.) Others
commented more generally that the results of incident-energy
calculations will vary depending on the parameters selected. (See, for
example, Exs. 0163, 0173, 0181.) For instance, Mr. Alan Blackmon with
Blue Ridge Electric Cooperative commented:

    Estimates of maximum amounts of heat energy to which an employee
would be exposed require making so many subjective assumptions as to
render the calculations useless. OSHA therefore should drop this
requirement. There is no value in an estimation that so easily can
be manipulated through choosing of, for example, duration of arc and
distance from arc to employee. [Ex. 0183]

    The parameters used by the calculation methods discussed earlier
include: the fault current (usually the maximum available fault
current), the system voltage, the arc length, the arc duration, and the
distance from the arc to the employee.\324\ The system fixes most of
these parameters. Each system has a fixed system voltage, fault
current, and fault clearing time.\325\ The system voltage is a known
``quantity.'' IEEE Std 1584a-2004, Section 4.4, explains the
calculation of the maximum fault current based on known characteristics
about the circuit involved (Ex. 0425). IEEE Std 1584a-2004 describes
how to determine the corresponding fault-clearing time by checking the
maximum fault current against the time characteristics provided by the
protective device manufacturer as follows:
---------------------------------------------------------------------------

    \324\ IEEE Std 1584a-2004 also expects the user to select the
overcurrent device protecting the circuit (Ex. 0425). However, that
method makes certain assumptions about some of the other parameters,
in particular, arc duration, that avoid the need to enter those
parameters. The consensus standard also provides a generic case in
which all of the typical parameters are input. IEEE Std 1584b-2011
provides additional guidance on selecting arc-duration times for
different types of overcurrent protective devices (that is, fuses,
integral-trip circuit breakers, and relay-operated circuit breakers)
for the generic case.
    \325\ The arc will last until the protective device opens the
circuit. Thus, the fault clearing time equals the duration of the
arc.
---------------------------------------------------------------------------

    An arc-flash hazard analysis should be performed in association
with or as a continuation of the short-circuit study and protective-
device coordination study. The process and methodology of
calculating

short-circuit currents and performing protective-device coordination
is covered in IEEE Std 141-1993 (IEEE Red Book TM) and
IEEE Std 242-2001 (IEEE Buff Book TM), respectively.
Results of the short-circuit study are used to determine the fault
current momentary duty, interrupting rating, and short-circuit
(withstand) rating of electrical equipment. Results of the
protective-device coordination study are used to determine the time
required for electrical circuit protective devices to isolate
overload or short-circuit conditions. Results of both short-circuit
and protective-device coordination studies provide information
needed to perform an arc-flash hazard analysis. [id. \326\]
---------------------------------------------------------------------------

    \326\ IEEE Std 1584b-2011 revises this paragraph and separates
it into five paragraphs. The revisions are editorial, except for
updated references to relevant IEEE standards, including the
substitution of IEEE Std 551 TM-2006 (IEEE Violet Book
TM) for IEEE Std 141-1993 (IEEE Red Book TM),
and additional language explaining that ``electrical system analysis
software may be used to simplify the calculations for complex
distribution systems . . .'' and explaining the limitations and
advantages of such software.

    Engineers typically perform system coordination studies during the
design of the system and again periodically and after any significant
change to the system (Tr. 1030-1031). If no initial or periodic studies
take place, the system owner risks having a fault on one part of the
system cause an outage over an extended portion of the system instead
of having the fault confined to the affected circuit. (See, for
example, 269-Exs. 8-15, 8-16, 8-17, 8-20, 8-21, 8-22.) As required by
existing Sec.  1910.269(n)(4)(i), employers must ensure that a similar
engineering analysis is performed to determine the appropriate ampacity
for protective grounding equipment; this provision specifies that
protective grounding equipment must be ``capable of conducting the
maximum fault current that could flow at the point of grounding for the
time necessary to clear the fault.'' As noted by Mr. James Tomaseski of
IBEW: ``For . . . employees to install personal protective grounds on a
circuit, they need to establish what level of . . . fault currents are
available, and that will decide what size grounds they will install''
(Tr. 960). Consequently, OSHA concludes that employers are likely to
have information that the Agency can verify about the system voltage,
fault current, and clearing times. OSHA will deem any manipulation of
these parameters for purposes of estimating heat energy under final
paragraph (g)(2) to result in an unreasonable estimate of incident
energy in violation of the standard.
    Table 8 in proposed Appendix F presented estimates of available
energy for different parts of an electrical system operating at 4 to 46
kilovolts. Table 9 of proposed Appendix F presented similar estimates
for systems operating at voltages of 46.1 to 800 kilovolts. These
tables were for open-air, phase-to-ground (that is, single-phase)
electric-arc exposures typical for overhead systems operating at these
voltages. Table 8 and Table 9 of proposed Appendix F provided
information on what OSHA would consider as reasonable estimates of arc
length and the distance from the arc to the employee, as described
later in this section of the preamble. OSHA revised these tables as
described later in this section of the preamble and included them in
the final rule as Table 6 and Table 7 of Appendix E. OSHA will consider
it reasonable for an employer to use the Table 6 and Table 7 estimates
of arc length and the distance from the arc to the employee--for
single-phase arcs in open air--for purposes of the calculations
required by final paragraph (g)(2). IEEE Std 1584a-2004 also provides
guidance on these parameters (Ex. 0425).
    Reasonable estimates of the arc gap (arc length). As noted earlier,
the exposures covered by Table 6 and Table 7 of Appendix E of final
subpart V, that is single-phase arcs in open air, typically occur
during overhead line work. In this case, the arc will almost always
occur when an energized conductor approaches too close to ground. Thus,
employers can determine the arc gap, or arc length, for these exposures
by the dielectric strength of air and the voltage on the line (Exs.
0041, 0533).\327\ The dielectric strength of air is approximately 10
kilovolts for every 25 millimeters (1 inch) (Ex. 0041), with a minimum
arc gap of 51 millimeters (1 inch). For example, at 50 kilovolts, the
arc gap would be 50 / 10 x 25, or 125 millimeters (5 inches). Although
OSHA is providing this guidance in the final rule, as discussed later
in this section of the preamble, employers may use other estimates of
the arc gap for single-phase arcs in open air if the estimates
reasonably resemble the actual exposures faced by employees.
---------------------------------------------------------------------------

    \327\ Table 6 of Appendix E of final subpart V uses a more
conservative arc gap that equals the electrical component of the
minimum approach distance rather than a value corresponding to the
dielectric strength of air for the system voltage. (See the summary
and explanation for final Sec.  1926.960(c)(1), earlier in this
section of the preamble, and Appendix B to final Subpart V for
additional information on determining the electrical component of
the minimum approach distance based on the maximum transient
overvoltage for a system and determining the dielectric strength of
air for the maximum phase-to-ground system voltage.) OSHA used the
electrical component of the MAD to create Table 6 in final Appendix
E for consistency with the approach used in similar tables in the
2007 NESC (Ex. 0533) and the 2012 NESC.
---------------------------------------------------------------------------

    For three-phase arcs in open air and in enclosures, the IEEE 1584
method provides guidance (Ex. 0425). That method does not require the
user to input an arc gap (id.). Instead, it incorporates the arc gap
into its calculations based on the class of equipment involved. The
user selects the type of equipment involved (for example, 600-volt
switchgear). It then uses the appropriate bus or conductor spacings in
that equipment as the arc gap in the calculation of incident energy.
For a three-phase arc to occur, current must arc between all of the
phases. Such arcs typically occur when a conductive object drops across
the phases or when there is an internal fault in the equipment;
therefore, OSHA concludes that it is reasonable to use the bus or
conductor spacing as the arc gap. Notably, neither the NFPA 70E Annex D
nor the Doughty, Neal, and Floyd method require users to input an arc
gap.
    Reasonable estimates of the distance from the employee to the arc.
All of the acceptable methods of estimating incident energy require the
user to input the distance from the arc to the employee. This approach
requires some judgment by the employer. However, the hazard assessment
required by final paragraph (g)(1) will provide information that the
employer can use to assess where arcs are reasonably likely to occur in
relation to the employee. To determine employee exposure to hazards
from electric arcs as required by final paragraph (g)(1), the employer
must determine where an employee is reasonably likely to be when an arc
occurs (in addition to whether there is a reasonable likelihood that an
arc could occur in the first place).
    In Appendix E to final subpart V, OSHA provides guidance on
distance assumptions it will consider reasonable for estimating
incident energy for exposures involving single-phase arcs in open air.
As noted earlier, work on overhead power lines typically exposes
employees to single-phase arcs in open air. Employees performing this
type of work handle conductors; and these conductors can contact a
grounded object, or a grounded conductor (such as a guy or grounding
jumper) can contact a phase conductor (Ex. 0004 \328\).
---------------------------------------------------------------------------

    \328\ See, for example, the six accidents described at http://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=170805238&id=200021004&id=170070981&id=201791803&id=14291868&id=170178370.

---------------------------------------------------------------------------

    As noted under the summary and explanation for final paragraph
(c)(1), earlier in this section of the preamble, much of the work
performed on energized parts operating at 46 kilovolts and less is done
by employees using rubber insulating gloves.\329\ Working in a
comfortable position with elbows bent, an employee would be
approximately 380 millimeters (15 inches) from the energized conductor
on which he or she is working, measured from the employee's chest.\330\
Thus, OSHA used a distance of 380 millimeters (15 inches) to calculate
the incident-energy values in Table 8 in proposed Appendix F (Table 6
in final Appendix E) and will deem that a reasonable estimate for
employers to use when performing incident-energy calculations for
single-phase open-air exposures on voltages of 46 kilovolts and less.
Employers may use other distances if those distances reasonably
resemble the actual exposures faced by employees.
---------------------------------------------------------------------------

    \329\ Work is not performed on energized parts in the 46.1- to
72.5-kilovolt range using rubber insulating gloves. The maximum
voltage rating for rubber insulating gloves is 36 kilovolts. (See
Table E-4 to final Sec.  1926.97.) The phase-to-ground voltage on a
72.5-kilovolt circuit is 41.8 kilovolts, which is above the maximum
use voltage for rubber gloves. Minimum approach distances are set
for the 46.1- to 72.5-kilovolt range based on the rubber insulating
glove work technique because rubber insulating glove work is
performed close to energized parts in this voltage range. For the
purposes of estimating incident-energy levels, the Agency believes
that the most likely electric arc will generally involve live parts
the employee will be handling, which will be energized at 46
kilovolts or less.
    \330\ Rubber insulating gloves with leather protectors and
rubber insulating sleeves normally cover the employee's arms. This
equipment provides protection against incident heat energy (Exs.
0373, 0466; Tr. 434).
---------------------------------------------------------------------------

    TVA maintained that the 380-millimeter (15-inch) distance
assumption for these exposures was too small, commenting:

    OSHA states that an employee's chest will be about 380
millimeters (15 in.) from an energized conductor during rubber glove
work on that conductor. A review of anthropometric estimates
(``Anthropometry, Ergonomics, and the Design of Work'' by S.
Pheasant) for British adults (19 to 65 years old) shows that the
elbow to finger tip length for the 5th percentile is 440 mm (17.3
inches) for men and 400 mm (15.75 inches) for women. After adding a
distance of 51 mm (2 inches) for the arms to move toward the front
of the body and into a working position, the distance from the chest
to the potential arc point will be 451 mm (17.76 inches) for women
and 491 mm (19.33 inches) for men. Based [on] this data, the default
distance from the worker to the arc point should be 451 mm (17.76
inches) or about 18 inches. The 15-inch distance proposed by [OSHA]
will increase the calculated arc flash incident energy, which means
that employees will have to wear heavier protection within the area
of the arc flash boundary. This heavier protection is not warranted
based on anthropometric data. IEEE 1584 states that a typical
distance is 455 mm (17.91 inches) to the arc for cable work and low
voltage panelboards and motor control centers. It is recommended
that the final rule adopt 457 mm (18 inches) as the default distance
to the arcing point. [Ex. 0213]

    OSHA does not dispute the anthropometric data described by TVA.
However, the Agency does not agree with TVA's application of this data
to rubber glove work. An employee working in a comfortable position on
a conductor will have his or her upper and lower arms at an angle of
about 60 degrees (269-Ex. 8-5). This position forms an equilateral
triangle with the sides produced by the upper arm, the lower arm, and
the distance between the employee's chest and the conductor. Therefore,
the distance from the energized part to the worker's chest is the same
as the distance between the energized part and the worker's elbow.
Although the 95th percentile distance between the elbow and the
fingertip may be 440 millimeters (17.3 inches), the conductor will be
closer than that distance because it will originate at the crotch
between the thumb and the palm rather than at the fingertip (id.).
Subtracting 60 millimeters (2.4 inches) from the length of the lower
arm, which is a conservative approximation of the distance between the
middle fingertip and the crotch between the thumb and the palm, yields
a distance of 380 millimeters (15 inches). This is the approximate
distance between an employee using rubber gloves on an energized
conductor and the live part, which also is the same distance as the
estimated distance TVA was challenging.\331\ OSHA does not dispute the
IEEE Std 1584 distance mentioned by TVA; however, the IEEE distances
are for cables and enclosed equipment, not for open conductors in air
(which involve the use of rubber insulating gloves). The Agency
concludes that the distance from the arc to the employee should be
different for these exposures, as explained later. Consequently, OSHA
concludes that 380 millimeters (15 inches) is a reasonable distance to
assume between the employee and the arc for work by employees using
rubber gloves involving exposures to single-phase arcs of up to 46
kilovolts in open air.
---------------------------------------------------------------------------

    \331\ OSHA's approach is identical to the approach taken by the
2007 NESC in Table 410-1 (Ex. 0533). (The 2012 NESC retains this
approach in Table 410-2.)
---------------------------------------------------------------------------

    At voltages higher than 46 kilovolts, employees must use live-line
tools or the live-line barehand technique to handle energized
parts.\332\ For this work, OSHA considers it reasonable to calculate
incident-energy exposures for single-phase open-air arcs using a
distance from the employee to the arc that is equal to the applicable
minimum approach distance minus twice the arc length. In this case, the
employee would be at the minimum approach distance from the energized
part,\333\ where OSHA assumes the arc occurs, and subtracting twice the
arc length from that distance accounts for movement of the arc \334\
and for small errors in judging and maintaining the minimum approach
distance. There is no evidence on the record that this distance is
unreasonable, and the Agency received no adverse comments on that
assumption. Therefore, OSHA concludes that, for exposures involving
single-phase arcs in open air when employees perform work using live-
line tools, a reasonable estimate of the distance from the arc to the
employee is the minimum approach distance minus twice the arc length.
---------------------------------------------------------------------------

    \332\ Although the rest of this discussion relates to work
performed using live-line tools, an employer can use the same
technique to reasonably estimate the distance from the employee to
the electric arc when the employee is performing live-line barehand
work. An employee performing live-line barehand work is at the
potential of the conductor and is maintaining the applicable minimum
approach distance from ground. From the worker's perspective, the
dangerous potential is ground, not the conductor to which he or she
is bonded. In that case, the employer can reasonably assume that the
arc, if one occurs, will be close to objects at ground potential as,
for example, if an energized conductor drops onto a grounded tower
leg, or at the potential of other phase conductors as, for example,
if a phase conductor drops on another phase conductor below.
    \333\ The design of the live-line tool keeps the employee at a
distance from the energized part equal to, or greater than, the
applicable minimum approach distance.
    \334\ When the arc initiates, the worker is likely to react by
pulling the live-line tool away from the energized part and toward
himself or herself. This action would pull the arc toward the
worker. If the worker reacts in the opposite direction, then he or
she would get closer to the arc.
---------------------------------------------------------------------------

    Table 9 in proposed Appendix F only covered work on systems
operating at more than 46 kilovolts. The Agency recognizes that some
employers require their employees to use live-line tools on voltages of
46.0 kilovolts and less. (See, for example, Exs. 0125, 0127, 0159.)
Therefore, the Agency is extending Table 7 in final Appendix E to cover
these lower voltages as well. Table 7 applies whenever employees use
live-line tools, irrespective of voltage, because OSHA based the table
on the work method, not on the voltage. OSHA also revised the titles of
Table 6 and Table 7 in final Appendix E to indicate that they are
applicable to work using

rubber insulating gloves and live-line tools, respectively, rather than
work on systems based on voltage as proposed.
    One mechanism for reducing estimated incident energy is to move the
employee farther away from the electric arc. One way to accomplish this
objective is to use live-line tool work methods with a larger minimum
approach distance than the minimum distance required by paragraph
(c)(1) of final Sec.  1926.960. OSHA encourages employers to use such
methods to reduce incident-energy levels. If an employer requires an
employee to maintain a minimum approach distance greater than the
minimum distance required by paragraph (c)(1), OSHA would deem it
reasonable for the employer to use an estimate of the distance from the
employee to the arc that reflects the employer-imposed minimum approach
distance rather than the minimum approach distance required by the
standard.
    Work that exposes employees to three-phase arcs in open air, or
single-phase or three-phase arcs in enclosures, typically involves the
employee working at a greater distance from energized parts than is the
case when an employee is working on a single phase conductor of an
overhead line. For example, employees typically perform work on
energized equipment using insulating tools or test equipment on the
energized parts or by operating the equipment or removing covers. In
the first two cases, that is, using insulated tools or test equipment
on energized parts, the employee will be working with arms extended. In
the latter two cases, that is, operating the equipment or removing
covers, employees would be working with their hands near the outside of
equipment. OSHA believes that, in all four cases, it is reasonable to
assume that the employee is working at a greater distance from the
energized parts than an employee working with rubber insulating gloves
on energized overhead line conductors. IEEE Std 1584a-2004 uses
distances based, at least in part, on the dimensions of the equipment
enclosure (Ex. 0425). Because IEEE designed that standard to address a
wide range of equipment, OSHA believes that the IEEE approach is
broadly applicable to work on energized equipment. The IEEE approach is
explained in Section 4.8 of that standard as follows:

    Arc-flash protection is always based on the incident energy
level on the person's face and body at the working distance, not the
incident energy on the hands or arms. The degree of injury in a burn
depends on the percentage of a person's skin that is burned. The
head and body are a large percentage of total skin surface area and
injury to these areas is much more life threatening than burns on
the extremities. Typical working distances are shown in [the
following table:]

------------------------------------------------------------------------
                                                        Typical working
                Classes of equipment                   distance \a\ (mm)
                                                           [inches]
------------------------------------------------------------------------
15 kV switchgear....................................            910 [36]
5 kV switchgear.....................................            910 [36]
Low-voltage switchgear..............................            610 [24]
Low-voltage MCCs \[\\335\\]\ and panelboards........            455 [18]
Cable...............................................            455 [18]

                              * * * * * * *
------------------------------------------------------------------------
\a\ Typical working distance is the sum of the distance between the
  worker standing in front of the equipment, and from the front of the
  equipment to the potential arc source inside the equipment. [id.\336\]
IEEE Std 1584a-2004--IEEE Guide for Performing Arc-Flash Hazard
  Calculations--Amendment 1--Reprinted with permission from IEEE--
  Copyright 2004, by IEEE. (Table revised from original).

    There is no evidence on the record that the distances in IEEE Std
1584-2004 for three-phase arcs in open air or single-phase or three-
phase arcs in enclosures are unreasonable. Therefore, OSHA concludes
that the distances in IEEE Std 1584-2004 described earlier are
reasonable estimates for the distance from the employee to the electric
arc for three-phase arcs in open air, and single-phase and three-phase
arcs in enclosures, for voltages up to 15 kilovolts. Above that
voltage, employers must consider equipment enclosure size and the
working distance to the employee in selecting a distance from the
employee to the arc. The Agency will consider a distance reasonable
when the employer bases it on equipment size and working distance.
---------------------------------------------------------------------------

    \335\ Motor control center.
    \336\ IEEE Std 1584b-2011 makes editorial changes to the quoted
paragraph and adds a column with English units to the table. The
metric distances in the table remain unchanged.
---------------------------------------------------------------------------

    Summary and discussion of general issues related to incident-energy
calculation methods. Table 12, Table 13, and Table 14 in this preamble
summarize OSHA's findings related to methods employers can use to
estimate incident heat energy as required by final paragraph (g)(2).
OSHA included these tables in Appendix E to Subpart V in the final rule
to enable employers to readily select incident-energy calculation
methods and input parameters that OSHA will consider reasonable and
acceptable for compliance with paragraph (g)(2) of final Sec.
1926.960.

                                         Table 12--Selecting a Reasonable Incident-Energy Calculation Method\1\
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                     600 V and less \2\            601 V to 15 kV \2\              More than 15 kV
              Incident-energy calculation method               -----------------------------------------------------------------------------------------
                                                                 1[Phi]    3[Phi]a   3[Phi]b   1[Phi]    3[Phi]a   3[Phi]b   1[Phi]    3[Phi]a   3[Phi]b
--------------------------------------------------------------------------------------------------------------------------------------------------------
NFPA 70E-2004 Annex D (Lee equation) \3\......................       Y-C        Y         N        Y-C       Y-C        N      N\4\      N\4\      N\4\
Doughty, Neal, and Floyd......................................       Y-C        Y         Y         N         N         N         N         N         N
IEEE Std 1584-2004 \5\........................................        Y         Y         Y         Y         Y         Y         N         N         N
ARCPRO........................................................        Y         N         N         Y         N         N         Y      Y\6\      Y\6\
--------------------------------------------------------------------------------------------------------------------------------------------------------
Key:
1[Phi]: Single-phase arc in open air

3[Phi]a: Three-phase arc in open air
3[Phi]b: Three-phase arc in an enclosure (box)
Y: Acceptable; produces a reasonable estimate of incident heat energy from this type of electric arc
N: Not acceptable; does not produce a reasonable estimate of incident heat energy from this type of electric arc
Y-C: Acceptable; produces a reasonable, but conservative, estimate of incident heat energy from this type of electric arc.
Notes:
\1\ Although OSHA will consider these methods reasonable for enforcement purposes when employers use the methods in accordance with this table,
  employers should be aware that the listed methods do not necessarily result in estimates that will provide full protection from internal faults in
  transformers and similar equipment or from arcs in underground manholes or vaults.
\2\ At these voltages, the arc is presumed to be three-phase unless the employer can demonstrate that only one phase is present or that the spacing of
  the phases is sufficient to prevent a multiphase arc from occurring.
\3\ The entries for NFPA 70E-2004 Annex D (Lee equation) apply equally to NFPA 70E-2012, and the comparable table in Appendix E refers to NFPA 70E-2012
  Annex D (Lee equation).
\4\ Although OSHA will consider this method acceptable for purposes of assessing whether incident energy exceeds 2.0 cal/cm\2\, the results at voltages
  of more than 15 kilovolts are extremely conservative and unrealistic.
\5\ The entries for IEEE Std 1584-2004 apply equally to IEEE 1584-2011, and the comparable table in Appendix E refers to IEEE Std 1584 with this latest
  amendment.
\6\ OSHA will deem the results of this method reasonable when the employer adjusts them using the conversion factors for three-phase arcs in open air or
  in an enclosure, as indicated in the program's instructions.

                                    Table 13--Selecting a Reasonable Arc Gap
----------------------------------------------------------------------------------------------------------------
                                           Single-phase arc mm
         Class of equipment                     (inches)                  Three-phase arc mm \1\  (inches)
----------------------------------------------------------------------------------------------------------------
Cable...............................  NA \2\......................  13 (0.5)
Low voltage MCCs and panelboards....  NA..........................  25 (1.0)
Low-voltage switchgear..............  NA..........................  32 (1.25)
5-kV switchgear.....................  NA..........................  104 (4.0)
15-kV switchgear....................  NA..........................  152 (6.0)
Single conductors in air, 15 kV and   51 (2.0) \3\................  Phase conductor spacing.
 less.
Single conductor in air, more than    Voltage in kV times 2.54      Phase conductor spacing.
 15 kV.                                (0.1), but no less than 51
                                       mm (2 inches) \3\.
----------------------------------------------------------------------------------------------------------------
\1\ Source: IEEE Std 1584a-2004.
\2\ ``NA'' = not applicable.
\3\ Table 6 of Appendix E of final Subpart V uses a more conservative arc gap that equals the electrical
  component of the minimum approach distance rather than a value corresponding to the dielectric strength of air
  for the system voltage, which forms the basis for the values in this table.

 Table 14--Selecting a Reasonable Distance From the Employee to the Arc
------------------------------------------------------------------------
                                Single-phase arc mm   Three-phase arc mm
      Class of equipment              (inches)             (inches)
------------------------------------------------------------------------
Cable........................  NA\*\................            455 (18)
Low voltage MCCs and           NA...................            455 (18)
 panelboards.
Low-voltage switchgear.......  NA...................            610 (24)
5-kV switchgear..............  NA...................            910 (36)
15-kV switchgear.............  NA...................            910 (36)
Single conductors in air (up   380 (15).............                  NA
 to 46 kilovolts), work with
 rubber insulating gloves.
Single conductors in air,      MAD - (2 x kV x 2.54)                  NA
 work with live-line tools     (MAD - (2 x kV /
 and live-line barehand work.   10))[dagger].
------------------------------------------------------------------------
* ``NA'' = not applicable.
[dagger] The terms in this equation are:
MAD = The applicable minimum approach distance, and
kV = The system voltage in kilovolts.

    With the guidance provided here and in Appendix E to final subpart
V, OSHA believes that employers will be able to reasonably estimate
incident-energy levels as required by final paragraph (g)(2). The
Agency expects that, upon inspection, it will be able to detect any
manipulation of input parameters designed to undermine the purpose and
requirements of this final rule.
    In enforcing paragraph (g)(2) of the final rule, the Agency will
accept as reasonable any estimates made following the guidance in the
preamble and in Appendix E. Employers may depart from this guidance as
long as the methods and variables used to calculate incident heat
energy relate reasonably to the electric-arc exposures actually faced
by employees. Duke Energy pointed out that ``standard writing
committees . . . are continuing to address the electric-arc hazards,
specifically NFPA 70E, IEEE Std 1584-2002, and technical papers written
by the IEEE/ESMOL[337] committee'' (Ex. 0201). These efforts
may result in additional sources of information for employers to use in
estimating incident heat energy for purposes of final paragraph (g)(2).
---------------------------------------------------------------------------

    \337\ Electrical Safety and Maintenance of Lines.
---------------------------------------------------------------------------

    Several rulemaking participants noted that IEEE and NFPA are
undertaking a joint research effort to address issues related to
methods of calculating incident heat energy from electric arcs. (See,
for example, Exs. 0177, 0201, 0227; Tr. 1095, 1128-1129.) These
rulemaking participants recommended that OSHA delay the rulemaking
pending the results of this research. For example, Ms. Kathy Wilmer,
testifying on behalf of EEI, stated:

    In 2005, IEEE and NFPA sponsored a joint task force whose charge
was to develop a research and test plan intended to address
technical issues, including those raised by the calculation methods.
It will be several years, however, before the results of the IEEE/
NFPA Research and Test Plan Committee are available to employers.
[Tr. 1095]

    EEI recommended that ``OSHA wait for NFPA and IEEE to answer some
of [the] questions'' related to the calculation methods (Tr. 1129).

    As noted by Ms. Wilmer, the results of any research conducted as a
result of the IEEE-NFPA joint effort may be years away. Today, the
final results of this research are not available. OSHA concludes that
there is sufficient information in the rulemaking record to determine
that existing calculation methods can reasonably estimate incident heat
energy from electric arcs. Therefore, the Agency does not believe that
it is necessary to wait for IEEE and NFPA to complete the research. In
the future, this research may result in additional sources of
information for employers estimating incident heat energy for the
purposes of final paragraph (g)(2).
    Note 2 to paragraph (g)(2), which is being adopted without
substantive change from the proposal, explains that paragraph (g)(2)
does not require the employer to estimate the heat-energy exposure for
every job task performed by each employee. The note indicates that the
employer may make broad estimates that cover multiple system areas
provided that: (1) The employer uses reasonable assumptions about the
energy-exposure distribution throughout the system, and (2) the
estimates represent the maximum exposure for those areas.
    Proposed Appendix F explained that the employer could use the
maximum fault current and clearing time to cover several system areas
at once.
    NIOSH expressed concern that, following this guidance, an employer
could estimate incident energy based on the maximum available fault
current, even though a higher incident-energy level is possible with a
lower fault current (Ex. 0130). NIOSH explained:

    [Proposed Note 2 to paragraph (g)(2) and proposed Appendix F]
suggest that the point in a power system that has the highest
available fault current will also have the maximum heat energy
hazard in the event of an arcing-fault. [T]he heat energy released
during an arcing-fault is a function of both current and duration
(clearing time). The maximum heat energy hazard may be at a point in
the system where available fault current is less than the system
maximum and may consequently have a longer clearing time. This
longer clearing time is due to the inverse-time characteristic of
many circuit protection components such as fuses and relays (the
higher the fault current, the more quickly the circuit protection
components will clear the fault). [Id.]

NIOSH recommended ``providing a more detailed explanation of the
interdependence of current and clearing time with respect to arcing-
fault hazards,'' and indicated that ``NFPA 70E-2004 provides an example
of such an explanation'' (id.).

    OSHA recognizes that fault current lower than the maximum available
fault current can produce a higher incident energy. The maximum fault
current, also known as the bolted-fault current, occurs when the fault
has no impedance,\338\ as if the two conductors were bolted together.
The current in an electric arc is never as high as the maximum
available fault current because the arc itself has some impedance, and
this lowers the fault current. All of the incident-energy calculation
methods, except ARCPRO, account for this reduction (Exs. 0134, 0425,
0430, 0469).
---------------------------------------------------------------------------

    \338\ Impedance is the effective resistance of an electric
circuit to alternating current. It includes the combined effects of
ohmic resistance and reactance.
---------------------------------------------------------------------------

    As NIOSH notes, when the current is less than the maximum available
fault current, the protective devices for the circuit may take longer
to clear the fault, resulting in longer clearing times. IEEE Std 1584a-
2004 accounts for this difference in clearing times and for variations
in arc current with arc voltage in the formulas it uses to calculate
incident energy (Ex. 0425). The other methods use the clearing time
corresponding to the fault current used to calculate the incident
energy.
    However, the fault current and the clearing times used to calculate
incident energy in these calculations are only approximations of the
values that might occur in an actual fault. Like the distance from the
employee to the arc and, in some cases, the arc length, the fault
current and clearing time in an actual fault likely will be different
from the fault current and clearing time used to calculate incident
energy. The final rule requires that the employer's estimate of
incident energy be reasonable, not that it be a precise estimate of the
maximum possible incident energy. Lower fault current may produce a
higher incident energy, but so would exposures with the employee closer
to the arc. Other variations, such as short clearing times (which can
occur if the arc self-extinguishes) or longer distances between the
employee and the arc, could lead to lower incident energy. Considering
the evidence in the record as a whole, the Agency believes that using
maximum fault current in estimating incident energy will produce
reasonable estimates of the exposures faced by employees.
    Mr. John Vocke with Pacific Gas and Electric Company stated that
his company conducted testing to verify the values in Table 8 and Table
9 in proposed Appendix F (Ex. 0185). He maintained that the incident-
energy values provided in those tables may be inaccurate.
    As noted earlier, the Agency concluded that the ARCPRO method, on
which OSHA based the incident-energy values in proposed Table 8 and
Table 9, reasonably estimates incident energy from single-phase arcs in
open air on systems of more than 600 volts. Mr. Vocke did not provide
the parameters used in, or the results of, Pacific Gas and Electric
Company's testing. For example, it is not clear from Mr. Vocke's
comment whether the testing was with single-phase arcs in open air. If
not, then the Agency would expect their results to differ from the
values in proposed Table 8 and Table 9.
    As described earlier, OSHA based Table 8 and Table 9 in proposed
Appendix F on calculations using ARCPRO and designed those tables to
cover a wide range of exposures faced by employees performing overhead
line work. TVA noted that these tables had little application and
expressed concern that employers would misuse the tables, commenting:

    We believe the use of tables, e.g., * * * proposed Tables 8 & 9,
have limited application for estimating heat energy for electrical
circuits common to the electric utility industry. The footnotes to
these tables instruct users to use other methods if the circuit
assumptions in the tables are not applicable to the circuit being
analyzed. Our concern is that many companies will not understand the
limitations of these tables or choose to ignore the instruction to
use other methods. Either of these actions could result in under
estimating the arc flash hazard.
* * * * *
    [W]e do not agree with the ``table'' method approach. We believe
that for many exposures in generating and transmission facilities
OSHA's proposed Tables 8 and 9 will not be useful to employers for
selecting arc flash protection. The tables are misleading because in
reality there are too many circuits with parameters that do not meet
the table use criteria. OSHA states in [proposed Appendix F] that
employers will need to use other methods in situations not addressed
by Table 8 or Table 9. We believe that an accepted method should be
used to calculate arc flash incident energies and recommend that the
final rule not include tables like proposed Table 8 and Table 9 for
selecting arc flash protection. [Ex. 0213]

    OSHA believes that Table 8 and Table 9 from proposed Appendix F
(Table 6 and Table 7 in final Appendix E, which OSHA revised as
described elsewhere in this section of the preamble) serve as
relatively simple ways for employers to estimate incident energy. The
SBREFA Panel Report specifically recommended that OSHA consider
including such tables in the standard (Ex. 0019). The National
Electrical Safety Code committee adopted provisions on protection from
electric arcs that included tables similar to the ones in the proposal
(Ex. 0480). Mr. James Tomaseski of IBEW supported the proposed tables
and stated that the values in those tables represent ``common exposures
out on distribution lines'' (Tr. 939--940). Mr. Brian Erga with ESCI
also supported proposed Table 8 and Table 9, testifying:

    ESCI fully supports the table 8 and table 9 in the appendix of
this proposal as a way of providing a method of choosing some FR
clothing for workers or small companies.
    It will allow a company to figure out, take their fault current,
their clearing time, go into a table, and find . . . some clothing
that might be appropriate, buy that for them, and feel . . . assured
that they were doing what they could do and . . . what OSHA would
require. [Tr. 1246-1247]

The Agency concludes that Table 8 and Table 9 in proposed Appendix F
will assist employers in complying with the requirement in final
paragraph (g)(2) to estimate incident heat energy and that the tables
reasonably represent exposures in electric distribution systems, as
noted by Mr. Tomaseski, if not transmission systems.\339\ (See, also,
Mr. Erga's testimony at Tr. 1247: ``I passed table 8 and table 9 around
to my customers. All of them feel it looks very good and looks very
straightforward for them to follow. And they feel pretty comfortable
that they would be willing to get into an FR program using [those]
table[s] . . . .'') Consequently, OSHA is including the tables in final
Appendix E, with revisions as described elsewhere in this section of
the preamble. OSHA agrees with TVA that it is important for employers
to heed the notes to these tables, which limit their application to
rubber insulating glove work (Table 6) and live-line tool work (Table
7) involving exposure to single-phase arcs in open air. OSHA further
agrees that these tables are of little, if any, use in electric power
generating plants, where most of the exposures come from three-phase
arcs. Nevertheless, the Agency believes that many employers, especially
small ones, will find these tables useful.
---------------------------------------------------------------------------

    \339\ Although there is nothing in the record that states
explicitly that Table 9 represents actual exposures for employees
working on transmission systems, the existence of similar tables in
the 2007 NESC (Ex. 0533) and the 2012 NESC strongly suggests that
Table 9 does reasonably represent transmission exposures. (Table 8
of proposed Appendix F covers only distribution voltages.)
---------------------------------------------------------------------------

    Mr. Tom Chappell of Southern Company suggested that the final rule
not require incident-energy estimates for voltages of 600 volts and
less, arguing that these systems do not pose the same risk as higher
voltage systems:

    This proposed language would require that the employer make
estimates of the maximum available heat energy to which employees
are exposed to at 600 volts and below as well as those above 600
volts. We do not believe this to be reasonable. Even OSHA recognizes
that the risks of exposures at 600 volts and below do not carry the
same risk as those above 600 volts since the proposed regulations do
not require flame resistant clothing at voltages 600 volts and
below. Additionally, Note 2 suggests making broad estimates that
cover multiple system areas, and further gives an example of how
that may be done for distribution circuits. Both of these suggest
that the OSHA's intent was not to cover systems operating at 600
volts or less where such broad estimates are meaningless and not
possible. We recommend that estimates of heat energy not be required
for systems operating at 600 volts and below and that engineering
controls and work practices be used for these systems so that
contact is avoided. This recommendation would be consistent with
NESC proposed language. [Ex. 0212]

    Mr. Chappell misunderstood the rationale behind OSHA's final rule.
First, Note 2 to proposed paragraph (g)(2), which OSHA is adopting
without substantive change, contained an example, clearly identified as
such, of how to estimate incident heat energy over a wide area. There
are other possible circuits that might be suitable for wide estimates.
In addition, the note only addresses circuits that are far-ranging,
such as transmission and distribution circuits. Circuits that operate
at 600 volts and less are found normally as services or as feeder or
branch circuits inside electric power generation plants. (See, for
example, 269-Exs. 8-5, 8-17, 8-20, 8-21, 8-22.) These circuits do not
normally extend for miles; each of them usually serves a single
facility. Second, OSHA does not agree that 600-volt systems produce
lower amounts of incident energy or pose a lower risk of burn injury to
employees than higher voltage systems. The rationale behind the
requirement in final Sec.  1926.960(g)(4)(i) that employees exposed to
contact with circuit parts operating at more than 600 volts wear flame-
resistant clothing relates to the reduced likelihood that contact with
a circuit part energized at lower voltages would produce an electric
arc through, and ignite, the clothing. As noted under the summary and
explanation for final paragraph (g)(4)(i), many commenters noted that
systems operating at 600 volts and less are capable of producing
extremely high levels of incident energy, sometimes even higher than
systems operating at higher voltages. For example, Mr. Paul Hamer
stated, ``Many systems and equipment operating at 600 volts and below
have severe arc-flash hazards . . .'' (Ex. 0166). In addition, TVA
noted:

    The magnitude of the heat energy in 480 V arc flash accidents is
greater [than at voltages higher than 600 volts] because of the
following: 1. The single phase fault typically propagates to three
phase fault. 2. The clearing times in generating plants are
typically longer. 3. The arc flash energy is typically forced into
one direction (arc in a box). [Ex. 0213]

Therefore, while there may not be an ignition hazard from contact at
the lower voltages, burn hazards at these voltages may still be serious
and require arc-rated protective equipment.

    For these reasons, OSHA is not adopting Mr. Chappell's
recommendation. The Agency believes that it is just as important to
estimate incident-energy levels for systems operating at 600 volts and
less as it is for systems of higher voltages. Without an estimate of
incident energy, an employer would not be able to select appropriate
arc-rated protective equipment for employees exposed to these voltages
in accordance with final Sec.  1926.960(g)(5).
    Some rulemaking participants maintained that incident-heat-energy
exposures change over time. (See, for example, Exs. 0126, 0163; Tr.
404-405.) For instance, Ms. Susan O'Connor with Siemens Power
Generation commented that ``if new equipment is added or the available
fault current to the plant from the utility changes, the entire
calculations change. The arc faults become a moving target'' (Ex.
0163). Noting that fault current can change hourly, Mr. James Shill
with ElectriCities of North Carolina testified:

    [I]n one of my first assignments in the power company I was in
charge of coordinating the equipment, and fault currents change
hourly. [I]t depends on where your source of energy comes from. [Tr.
404]

    The final rule does not require employers to estimate incident-
energy levels on a moment-by-moment basis. As indicated by Note 2 to
paragraph (g)(2), the final rule permits employers to make broad
estimates of incident-energy exposure, provided those

estimates represent the reasonably expected maximum exposures. There
would be no need to perform additional calculations when changes to the
system would lower incident energy. In addition, as long as the
protective clothing and other protective equipment selected by the
employer will protect against the incident energy, including any
increase caused by changes to the system, the final rule does not
require the employer to reconduct the incident-energy estimates
required by paragraph (g)(2).
    The Agency believes that employers will select arc-rated protective
equipment, not on the basis of estimates for individual circuits, but
on the basis of what levels will provide protection for broad areas of
the employers' systems. For instance, an employer could select a base
clothing outfit rated at 8 cal/cm.\2\ This clothing would be acceptable
as long as the estimated energy levels are less than that value.
Accordingly, OSHA believes that an employer can take measures to
minimize the number of times it must perform additional calculations.
For example, an employer using Table 6 or Table 7 in final Appendix E,
can select an incident-energy estimate for a maximum number of cycles
at a given level of fault current on a particular circuit. As long as
any change to the circuit does not increase the fault current or
clearing time beyond the fault current and clearing time used in
selecting a value from the table, the employer would not have to make
additional estimates. The employer then would know that as long as
relay settings (which affect clearing time) and transformer kilovolt-
ampere ratings (which affect maximum fault current) stay below the
values on which the employer bases the selection of incident-energy
level, then employees would remain safe, and the employer would remain
in compliance. Thus, the employer could avoid having to reestimate
incident-energy levels simply by limiting the types of changes that
could be made to a circuit or by selecting protective clothing and
other protective equipment that accommodates any changes that will be
made. As Mr. Donald Hartley of IBEW testified: ``[If] you don't find
that [the fault current and clearing times] are substantially different
[then] you may not have to change what it is you were doing'' (Tr.
1031-1032). On the other hand, it is possible that employers that do
not adequately plan changes to their systems will need to reestimate
incident heat energy for some of their circuits.
    OSHA does not expect employers to account for unanticipated changes
to their systems in estimating incident-energy levels. As Mr. Shill
noted, it is possible that an unanticipated system change could
increase incident energy. For example, an unidentified faulty relay
could substantially increase the clearing time and, thus, an employee's
potential incident-energy exposure. However, final paragraph (g)(2)
does not require employers to anticipate such events. The estimates
required by this paragraph are for normal operating conditions.
    For these reasons, OSHA concludes that concerns that employers
would need to constantly update their incident-energy estimates are
largely baseless. To the extent that employers must update these
estimates, the Agency's regulatory analysis fully accounts for periodic
updates. (See Section VI, Final Economic Analysis and Regulatory
Flexibility Analysis, later in the preamble.)
    Some commenters maintained that employers would need to hire
consultants to perform the incident-energy calculations required by
final paragraph (g)(2). (See, for example, Exs. 0163, 0178; Tr. 375-
376, 563.) Mr. James Shill of ElectriCities of North Carolina
testified: ``Even if professional engineers know the method to use in
calculating maximum available heat energy, small electric utilities
often do not have such qualified personnel on staff. Instead, small
utility businesses will be faced with hiring outside consultants to
perform this work for each job at each workplace, and for each
employee'' (Tr. 375-376).
    OSHA agrees with these commenters that small employers may need to
hire consultants to perform or assist in the preparation of incident-
energy calculations. Even some larger utilities hire consultants to
help perform incident-energy calculations (Tr. 1197). The Agency
understands that estimating incident heat energy demands some
electrical engineering expertise. OSHA believes that most employers
that work on electric power generation, transmission, and distribution
systems have such engineering expertise available. As noted by some
witnesses, these estimates require much of the same knowledge and skill
as other assessments needed to operate, maintain, and work on electric
power generation, transmission, and distribution systems (Tr. 1030-
1032). In any event, OSHA's estimate of the costs associated with
complying with paragraph (g)(2) in the final rule accounts for the
possibility that, in some instances, consultants will perform the
required estimates. (See Section VI, Final Economic Analysis and
Regulatory Flexibility Analysis, later in this preamble.)
    Some rulemaking participants suggested that contractors would have
difficulty estimating incident energy or would not be able to perform
the estimates at all. (See, for example, Exs. 0162, 0169, 0234, 0501;
Tr. 1326-1327, 1335-1336.) For instance, Quanta Services noted that
utility operators frequently do not know the maximum fault current on
their systems, making it ``difficult [for contractors] to determine the
maximum fault current'' (Ex. 0234). The Davis H. Elliot Construction
Company suggested that utilities might provide worst-case estimates to
their contractors because of potential liability concerns (Exs. 0156,
0206, 0231).
    OSHA understands that contractors may face challenges in estimating
incident heat energy as required by paragraph (g)(2) in the final rule.
The requirements in final Sec.  1926.950(c)(1), which specifies that
host employers provide information about their systems to contract
employers, should ensure that contractors have the information they
need to estimate incident energy. Paragraph (c)(1)(iii) of final Sec.
1926.950 specifically requires host employers to provide information to
enable contract employers to perform the assessments required by the
final rule. This would include information contractors need to estimate
incident heat energy as required in final Sec.  1926.960(g)(2).\340\ In
any case in which the host employer does not provide the contractor
with necessary information and, therefore, violates this final rule,
contractors can use other (albeit less certain) means of estimating the
system parameters needed to perform incident-energy calculations.
Contractors can estimate fault currents through the ratings of the
transformers supplying the circuit \341\ and clearing times from the
type of overcurrent devices protecting the circuit \342\ (Ex. 0425;
269-Ex. 8-15). The Agency assumes that, when utilities are

not providing this information, contractors already are using these
methods when determining the size of grounds necessary under existing
Sec.  1910.269(n)(4)(i) (``Protective grounding equipment shall be
capable of conducting the maximum fault current that could flow at the
point of grounding for the time necessary to clear the fault.'') There
is no evidence in the record that utilities are currently providing
unduly conservative estimates of fault current or clearing times to
contractors for the purposes of existing Sec.  1910.269(n)(4)(i), and
it seems unlikely that they would provide different estimates after
this final rule becomes effective. Consequently, the Agency concludes
that the concerns specific to contractors are baseless.
---------------------------------------------------------------------------

    \340\ In the economic analysis, OSHA assumes that costs related
to estimating incident energy will be borne only by host employers.
The Agency anticipates that, for economic reasons, host employers
will provide the results of their estimates to contract employers
even though the final rule does not require them to do so. See
Section VI, Final Economic Analysis and Regulatory Flexibility
Analysis, later in the preamble.
    \341\ For example, a contractor can estimate the fault current
on the secondary side of a transformer on a radial system by
calculating the fault current at the transformer, which is equal to
the transformer rating divided by the product of the per-unit
impedance and the voltage (Ex. 0134).
    \342\ IEEE Std 1584a-2004 gives the clearing times for a wide
range of circuit protective devices (Ex. 0425). Contractors also can
try to obtain clearing times from a number of other sources,
including the manufacturer.
---------------------------------------------------------------------------

    Several commenters suggested that proposed paragraph (g)(2) was too
vague. (See, for example, Exs. 0126, 0152, 0227; Tr. 1095-1097.) For
instance, Ms. Jean Thrasher with Community Electric Cooperative
commented: ``With undefined terms in the equation and no firm
guidelines from OSHA the employer has the potential to be cited even
though they performed a good faith appraisal but the inspector
disagreed with the values chosen'' (Ex. 0152).
    OSHA made it clear in this preamble and in Appendix E to final
Subpart V that the employer is free to choose any method for estimating
incident energy that results in a reasonable estimate of incident heat
energy to which the employee would be exposed. Appendix E provides
guidance on how to estimate incident heat energy and information on
approaches that OSHA will recognize as reasonable for performing these
estimates. In the final rule, OSHA revised Note 1 to paragraph (g)(2)