Basically, there are two types of over-current protective devices applied in switchgear for medium-voltage distribution systems. They are fuses and circuit breakers with their associated relaying.
The function of the protective device in the operation of a power system is to prevent or limit damage during faults or overloads, and to minimize their effect on the remainder of the system. In addition to its protective function, a circuit breaker is also used for circuit switching under normal conditions.
Protective device selection is based upon determining interrupting and load current carrying requirements, nominal rated voltage, and selecting the proper device for each location.
