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

If we refer to Figure 4.2 were we have two radial feeders connected to the
system. In our case the load at bus 2 is 350 amps and bus 3 is 100 amps giving a total
current at bus 1 to be 450 amps. Since bus 1 is the closest to the system it will have
the highest fault current available with bus 2 being the second largest and bus 3 is the
lowest on the feeder. When using inverse time overcurrent curves like those in Figure
4.6 and applying them to the fault currents on each bus down the feeder, we see that
the largest incident energy will be at bus 1 and reduces as we move down the feeder
towards bus 3 in the same fashion as Figure 4.8 depicts.
If a 50/51 relay was to be used instead of just a 51 relay, then the feeder would
have two points where the incident energy would need to be calculated; one at the
feeder source and one at the position were the 50 relay would stop operating and the
51 relay would start operating.
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