Over current protection includes short circuit protection. Short circuits can be phase faults, earth faults and three phase faults. Fast fault clearance is always required on short circuits. Normally short circuit currents are generally several times (5 to 20) of full load current.
In electromechanical relays fast operation was achieved by electromagnetic attraction principle. These are normally attracted armature relays. In numerical relays this is achieved through micro processor.
Whenever over current relays are used in the system, correct relay coordination is achieved either by time or by over current or by both. In discrimination by time method appropriate time interval is given by each of the relays to ensure that the breaker nearest to the fault opens first. This is known as “Differential Time Over Current Relay”.
Grading by current method relies on the fact that the fault current varies with the position of the fault. Therefore the relays controlling the various circuit breakers are set to operate the nearest one to the fault.
Both the methods, that is grading by time and grading by current have fundamental disadvantages.
In case of grading by time, the disadvantage is due to the fact that more severe faults are cleared in longest time. Current grading can be used where there is appreciable impedance between the two circuit breakers concerned.
