Arc Flash - Incident Energy Calcs

[RonShap]

When determining available fault current for a system with current limiting fuses and current limiting circuit breakers, we cannot use the "up-over-down" method on the let-through curves of the device to consider fault current limiting by the device, since we cannot be sure of the operation of other devices "seeing" the current (dynamic impedance).
Why can we then use the current limiting effect of the device when performing incident energy calculations? Seems that the thought process should be the same.

 

[dpc]

IEEE-1584 has equations for some low voltage current limiting fuses that are based on actual incident energy test for arcing faults. These equations are the basis for reduction of incident energy, not the "up-over-down" curves provided by fuse manufacturers. The problem is that these CLF equations in IEEE-1584 cover only a subset of available fuses.

In reality, it is the very fast (1/2 cycle) fault clearing time of the fuses **in their current-limiting range** that provides the dramatic reduction of incident energy.

If the fault current is low enough that the fuse clearing time is much longer than 1 or 2 cycles, the fuse really isn't doing much current-limiting.

If you compute incident energy using basic equations in IEEE-1584 and ignore the current-limiting effect, the resulting energy will be extremely low (< 2 cal/cm2) in most cases involving smaller fuses. This is due to the fast clearing time.

 

[RonShap]

Thanks, for the reply.
I probably should have included my best guess at the reason for the difference, to get more replies.
I think the reason is to get the worst case. During fault calcs, we don't consider the current limiting to result in more conservative AIC ratings.
During arc flash calcs, the lower fault current, by considering current limiting nature of the OCPD, will result in higher incident energy, thus more conservative PPE selection.

 

[dpc]

The only way a fuse can limit current is to melt. So if a fuse acting to limit fault current, it is going to melt and interrupt the fault within 1/2 cycle.

So, if you consider the current-limiting effect of a fuse, the fault duration cannot exceed this 1/2-cycle duration. This is not really a worst-case for incident energy and will not result in a more conservation PPE selection, I don't think.

Also, lower fault currents do not always create higher incident energy. It depends on the time vs. current response of the upstream device.