It is the part of electrical power engineering that deals with protecting the electrical power system from faults by isolating the faulty part from the rest of the network.

The protection system is made up — mainly — from 4 parts:

1- Current Transformers and Voltage Transformers These devices will step down the high voltages and currents of the electrical power system into convenient levels for the relays to deal with.

2- Relays: These devices will sense the fault and intiate a trip, or disconnection, order.

3- Circuit Breakers: These devices will open/close the system based on the relay commands.

4- Batteries: To provide power in case of power dissconnection in the system.

For small systems, fuses are capable of both sensing and disconnecting faults.