ABSTRACT
Overcurrent protection or short-circuit protection is very important on any electrical power system, and the distribution system is no exception. Short-circuit protection is the selection of equipment, placement of equipment, selection of settings, and coordination of devices to efficiently isolate and clear faults with as little impact on customers as possible. The philosophies of distribution protection differ from transmission-system protection and industrial protection. When an electrical distribution system energizes, components draw a high, shortlived inrush; the largest component magnetizes the magnetic material in distribution transformers. The system impedance relative to the transformer size is an important concept since it limits the peak inrush for larger transformers and larger numbers of transformers. Transformers are not the only elements that draw inrush; others include resistive lighting and heating elements and motors. Distribution protective devices tend to have steep time—overcurrent characteristics, meaning that they operate much faster for higher currents.
