Protective Relays:

Those are designed to cause the prompt removal of any element of a power system which may cause damage or interfere along with the effective and continuous operation of the rest of the system. That  are  aided  in  this  task  through  circuit  breakers  which  are  capable  of disconnecting faulty components or subsystems.

Protective relays could be used for categories of protection other than short circuit or overcurrent.  The relays could be designed to protect generating equipment and electrical circuits from any undesirable condition, like as underfrequency, under voltage, or interlocking system lineups.

There are only two operating principles for protective relays that are as follows: (1) electromagnetic attraction and (2) electromagnetic induction. Electromagnetic will attraction relays operate through a plunger being drawn up into a solenoid or an armature which is attracted to the poles of an electromagnet.  That category of relay could be actuated through either DC or AC systems.  The Electromagnetic induction relays operate on the induction motor principle whereby torque is developed through induction in a rotor. That category of relay could be used only in AC circuits.