Complex sentence: The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, over-voltage, reverse power flow, over- and under-frequency.
Split into simpler sentences: The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions. Protection relays respond to such conditions as over-current, over-voltage, reverse power flow, over- and under-frequency.