How to Select Overcurrent Relay Characteristics

One table herein lists induction overcurrent relays, available from the various manufacturers, along with equivalent time versus current characteristics. Another table gives relay types and current ranges suitable for phase and ground protection of 12 of the most commonly encountered industrial plant distribution and motor circuits. Recommendations result from detailed considerations of a hypothetical industrial power system. Use of these tables can be instrumental in good relay co-ordination and protection in the majority of applications when time does not permit a detailed study.