Design guidelines for safer low voltage distribution systems

Guidance is provided to designers of low voltage distribution systems on the maximum low voltage feeder rating of a unit substation that will allow the secondary main circuit breaker short time or instantaneous function to be sensitive enough to detect and trip on arcing currents and still coordinate with the feeder breaker settings. Recommendations are presented based on the transformer kVA rating, along with some variance for the primary source impedance and the instantaneous pickup tolerance of the feeder, based on it being either a molded case circuit breaker, insulated case circuit breaker or a low voltage power circuit breaker.