A relay can detect the faults in electrical system and send the trip signal to open the circuit.By disconnecting the faulty part, the healthy part can work properly.
Types of protection relays are mainly based on their characteristic, logic, on actuating parameter and operation mechanism.
Based on operation mechanism protection relay can be categorized as electromagnetic relay, static relay and mechanical relay. Actually relay is nothing but a combination of one or more open or closed contacts. These all or some specific contacts the relay change their state when actuating parameters are applied to the relay. That means open contacts become closed and closed contacts become open. In electromagnetic relay these closing and opening of relay contacts are done by electromagnetic action of a solenoid.
In mechanical relay these closing and opening of relay contacts are done by mechanical displacement of different gear level system.
In static relay it is mainly done by semiconductor switches like thyristor. In digital relay on and off state can be referred as 1 and 0 state.
Based on Characteristic the protection relay can be categorized as-
Definite time relays
Inverse time relays with definite minimum time(IDMT)
Instantaneous relays.
IDMT with inst.
Stepped characteristic.
Programmed switches.
Voltage restraint over current relay.