I'm doing an arc flash hazard analysis using SKM for a large solar facility. The utility has conveniently provided both maximum and minimum available utility fault currents (which one pertains depends on which substation is feeding the site at the moment). I've modeled the system and coordinated the protective devices, including the 4000A main breaker in the 600V switchboard, current-limiting fuses in the 600V/34.5 kV transformer, and an SEL relay in the MV recloser. Using SKM/IEEE 1584, I've also calculated the arcing fault current from the available bolted fault current and estimated the incident energy at the switchboard for its dimensions and configuration, for both maximum and minimum utility fault-current scenarios.

Here's my question: is it possible for a resistive fault current to occur that is lower than the arcing current calculated from the minimum available utility fault current, say if a worker drops a tool in the switchboard? Since the minimum current needed to sustain an arc at 600V is only 500A, it seems that a lower resistive fault current that trips the breaker more slowly (maybe even up to 2 s) could be a true worst-case scenario for incident energy and worth considering. Or would that be overly conservative?

C.C. Russell, PE