Some units include time-delay settings to prevent nuisance trips from short blips. Digital/microprocessor relays offer precise adjustment, logging, and self-tests. Protective Relay Definition: A protective relay is an automatic device that senses abnormal conditions in electrical circuits and triggers actions to isolate faults. Undervoltage relay: This relay watches for voltage dips, and if things drop too low, it cuts off power to avoid stressing motors and electronics. The terminals of the relay mainly include; common, coil, NO (normally open) & NC (normally closed).