I am performing an arc flash study in a mobile mining equipment and found the following situation:

An 800A circuit breaker is protecting the low voltage MCC, this breaker has no instantaneous but it has a shunt trip connected to a NGR that is connected to the neutral of the auxiliary power transformer (the aux. Power transf is upstream the circuit breaker).

Not having an instantaneous on the circuit breaker increases the incident energy level and the fault clearing time.

My question is, does the shunt trip connected to the NGR covers for the absence of instantaneous protection in the circuit breaker?

Please let me know your thoughts.

Thx

The incident energy (Ei) is dependent upon the clearing time of the breaker and the 3-phase bolted fault current. If there is no Instantaneous element, then it is likely the Ei will be relatively high. The NGR does nothing to change the amount of available 3-phase fault current nor the clearing time of the breaker, therefore it does not dramatically impact the Ei.

The NGR does significantly impact the likelihood that a single L-G fault will escalate into a 3-phase fault.

Thx a lot for your help.