Relay protection is essential to ensure the stability, reliability, and safety of electrical power systems. For example, unselective protection operation during a medium voltage network fault will cause an outage for an unnecessarily large number of consumers. While this is bad, It's not a. A practical guide to how protective relays detect faults, trip circuit breakers, coordinate protection zones, and improve power system reliability. But without electricity the lights go out, computers shut down and fridges, TV sets, and air-conditioning stop working. And so do factories, data centers and hospitals.