arc-flash analysis of utility power systems - Michigan Technological ...

Chapter 4: Issues of Concern
In this chapter, two areas of concern will be discussed, both of which are main
factors affecting the incident energy of an arc flash. The first of these concerns is the
type of fault used to conduct an arc-flash analysis. Assumptions of the fault may or
may not be correct based on the equipment or location of the fault analyzed. Second,
the amount of energy exposure personnel could be exposed could potentially differ
based on assumptions of the protective equipment used to clear faults. Each of these
points is addressed below.
4.1 Faults
During all the research for and implementation of an arc-flash analysis it was
determined that there is some discontinuity for the type of fault to be used on different
type of equipment. If we look at typical power system there is a correlation to the type
of fault personnel could be exposed to depending on the equipment they may be
working on. Out in a switch yard working on a 345 kV gang operated disconnect
switch, a three phase fault is rarely heard of. On the other hand, a three phase fault
inside of some switch gear in a motor control panel is very plausible.
If you look at the IEEE standard a three phase fault current is used based on
any voltage level or type of equipment at the point where the analysis is being
conducted. For voltage levels up to 15 kV this is justifiable because those equations
are based on three phase fault lab test results. With the equation for voltage levels
greater than 15 kV it is assumed the fault is a three phase fault. But if you look at
Table 3-2 shown above, which is provided by the NESC gives a quick reference lookup
table for arc-flash values, a phase to ground fault is used to develop the table.
Looking at Table 3-3, NESC doesn’t state the type of fault used to develop the table.
An example of this discontinuity can be seen on the test system used to
benchmark the ASPEN® software in Figure 4.1.
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