Chapter 7 of the NEC can be confusing unless you understand the
difference between emergency systems, legally required standby
systems, and optional standby systems. Once you sort these out,
you may still be scratching your head if you have systems vital
to public safety (including systems that protect against
environmental releases of toxins or waste).  This is where Annex
F becomes tremendously helpful, despite being only two pages
long. The first thing it does is walk you through how to
determine:    • Availability. The percentage of time a system is
available to perform its functions   • Reliability. The inverse
of the probability and frequency of failures. The key components
of these two metrics are:   • MTBF. Mean time between failures.
• MTTF. Mean time to failure.   • MTTR. Mean time to repair.  A
common way of expressing availability is the number of point-
nines. Annex F provides a chart showing hours of downtime
expressed on the scale of point-nines. For example, an
availability of 0.9 translates