In January ’99 Readers’ Quiz, B.B.B. states the main fuse “did exactly what it was designed to do,” by taking “the overload from an incoming problem.” However, fuses only respond to problems on their load or outgoing side.

In another response, E.D. states: “The maximum current-handling ability of standard nonrenewable cartridge fuses is some 10,000A—before it’s destroyed…If the main were increased to one with a higher short-circuit ampacity, then the branch circuit fuse would blow, while the short circuit flows through the main without interruption.”

This implies the trip current for a fuse exposed to a fault is equal to its Interrupt Current rating (e.g. 10,000 A.I.C). This comment approaches total electrical nonsense. Such notions are not only erroneous, but could also be life threatening.

Unfortunately, as a young project engineer for a major medical center, my BSEE program never covered power distribution and protection coordination. Fortunately, I learned much about power systems through other sources, notably EC&M. At that time, none of our electricians new the meaning of the interrupt rating of protective devices. In a medical center, coordination is essential to prevent minor faults from placing 20 surgeries and critical care areas in total powerless blackness. For those who rely on EC&M for “continuing education,” such obvious errors as those presented above should be edited out of this and other columns.

Ralph Crawford, P.E.
Forensic Electrical Engineer
and Electrical Contractor,
Palo Alto, Calif.

Our Response

Mr. Crawford brings up some good points, as they relate to proper terminology or lack thereof.

E.D.’s statement implies overcurrent performance, as opposed to short-circuit current interrupting performance, which should be obvious. Nevertheless, we assumed the readers’ understanding of the difference between the two. We should have inserted the words “short-circuit current” for clarification.

Yet, E.D.’s latter statement, about replacing main fuses with one having “increased short-circuit ampacity” actually clarifies his first statement.

Yes, there is a difference between overload and short-circuit current. An overload is the result of too much load current through a given load or in a given circuit. A short circuit is the result of a line-to-line or line-to-ground contact initiated by an accident or a breakdown of insulation. The latter results in a higher magnitude of current flow. Basically, E.D.’s contention is the problem stems from a fault condition, and series-rated devices would prevent the problem from happening. Of course, this is based on the limited information in R.A.M.’s question.

B.B.B.’s statement, about the main fuse “took the overload from an incoming problem,” also seems to be based on the limited information of the original question. Crawford is correct: “fuses only respond to ‘problems’ on their load or ‘outgoing’ side.” But, the Readers’ Quiz column is just that: responses from readers to questions posed by other readers. We do include a disclaimer stating: “Answers shown here do not necessarily express the point of view of EC&M editors.”

John DeDad, Editor-in-Chief