Design Guidelines for Safer Low-Voltage Distribution Systems

It is a challenge to design low-voltage distribution systems for sensitive arcing current detection while maintaining selective coordination. This paper provides recommendations for maximum low-voltage feeder-breaker current ratings that allow a unit-substation secondary-main circuit-breaker short-time or instantaneous trip to be sensitive enough to detect and trip on arcing currents, while retaining coordination with the feeder-breaker settings. The recommendations are organized according to transformer kilovolt-ampere rating. In addition, discussed are other variables, such as primary-source impedance, instantaneous-pickup tolerance of the feeder, and extended transformer ratings.