Relays are classified into six useful categories:
Protection relay
Detect broken wiring, faulty equipment, or other dangerous or intolerable conditions. These relays typically trip one or more electrical fuses, but can also be used to trigger an alarm.
Monitoring relay
Check the condition of the power system or protection system. These relays include fault detectors, alarm units, channel monitoring relays, synchronization checking and network phasing. Power system conditions that do not involve circuit breakers during faults are monitored by check relays.
Reconnection of relays
Define a closing sequence for an electrical fuse after the protection relays trip.
control relay
The range unit is activated as soon as an associated operating parameter deviates from planned limit values. Control relays are activated through additional instruments to bring the quantity back to prescribed limits.
Auxiliary relay
Acting in response to the opening or closing of the operating circuit to supplement another relay or device. These include timers, contact multiplier relays, protection units, analysis relays, opposition relays, closing relays and trip relays.