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

The amount of incident energy personnel may become subject to can vary
based on two specific variables; the time duration of the fault and the voltage level of
the equipment subject to the fault. This chapter will discuss how each of these
specific variables affects the potential incident energy.
4.2.1 50/51 Application
The application of a 50/51 relay on a bus or line for fault protection uses either
an instantaneous trip, the 50 contact, or a time delayed trip based on a relay curve, the
51 contact. Either application is acceptable but choosing one over the other can
greatly affect your level of arc-flash hazard present at the fault location. In Appendix
E.1, a test system has been built to emphasize this point. In System 1, initially the 51
contact is used to protect both lines of the system. Under this configuration, Bus 2 has
an arc-flash category level 2 of 5.5 cal/cm 2 and Bus 3 has an arc-flash category level 0
of 1.01 cal/cm 2 . By using the 50 contact, which will reduce the total clearing time to
essentially the time needed to operate the breaker, reduces the Bus 2 and 3 arc-flash
categories 0 and 0, 0.88 and 0.62 cal/cm 2 respectively. The full details of the four
different results can be seen in Appendix E.2 and E.3.
4.2.2 Discontinuity Due to Voltage Levels
The second variable to be evaluated is the voltage level of the equipment. As
mentioned earlier in section 3.2, 1 kV is a break point for which equation to use in
calculating the arcing current and 15 kV is a break point at which empirical equations
are not to be used in analysis but instead the Lee equation. On a typical transmission
or distribution system, voltages can range from 0.95 to 1.05 per unit on an intact
system. For a 1 kV system the voltage could run between 0.95 to 1.05 kV and for a 15
kV system the voltage range could run between 14.25 and 15.75 kV. This results in
two different set of equations to be considered when evaluating one piece of
equipment. If the incorrect equation is used, lower levels of incident energy may be
calculated in turn causing lower levels of PPE worn by the employee which may not
properly protect the individual for an arc-flash incident.
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