TABLE OF CONTENTS Very high current levels in electrical power systems are usually caused by faults on the system. These currents can be used to determine the presence of faults and operate protection devices, which can vary in design depending on the complexity and accuracy required. Among the more common types of protection are thermo-magnetic switches, moulded-case circuit breakers (MCCBs), fuses, and overcurrent relays. The first two types have simple operating arrangements and are principally used in the protection of low voltage equipment. Fuses are also often used at low voltages, especially for protecting lines and distribution transformers.
Overcurrent relays, which form the basis of this chapter, are the most common form of protection used to deal with excessive currents on power systems. They should not be installed purely as a means of protecting systems against overloads -which are associated with the thermal capacity of machines or lines -since overcurrent protection is primarily intended to operate only under fault conditions. However, the relay settings that are selected are often a compromise in order to cope with both overload and overcurrent conditions.
