Overcurrent Protection Fundamentals

Relay protection against high current was the earliest relay protection mechanism to develop. From this basic method, the graded overcurrent relay protection system, a discriminative short circuit protection, has been formulated. This should not be mixed with ‘overload’ relay protection, which typically utilizes relays that function in a time related in some degree to the thermal capacity of the equipment to be protected. On the contrary, overcurrent relay protection is completely directed to the clearance of short circuits, even though with the settings typically assumed some measure of overload relay protection may be obtained.