Arc Flash Calculations at Sub Panels

[nayjay67]

When calculating the arc flash hazard at a sub panel (has two CBs between it and the Xformer), is the bolted fault current still set using the transformer rating, or is it adjusted due the upstream circuit breakers? Also, when considering the clearing time, with CBs in series, would you use the shortest or longest? Thanks.

 

[ron]

For an arc flash analysis, using the infinite primary calculation from an upstream transformer will result in artificially high fault current and thus artificially low PPE (since the breaker opens faster for higher fault current).

You need to calculate the worst case lowest fault current considering all impedances. The upstream circuit breakers will not impact fault current since they do not have impedance, just the feeders are significant.