A motor protection circuit breaker, or MPCB, is an electromechanical device that performs three important functions upstream of an electric motor:
MOTOR PROTECTION AGAINST OVERLOAD AND SHORT CIRCUIT,
CONTROL OF THE MOTOR (ON/OFF).
In the event of overload or short circuits, the MPCB automatically disconnects the circuit. It has internal contacts that connect the power supply line to the load. An internal coil detects the short-circuit currents (high or very high current values), while a heat-sensitive element, welded on the bimetal, detects overcurrent’s (current values slightly higher than the nominal value) which may damage the motor. In the event of overcurrent’s, the coil or the bimetal determine the opening of the contacts.
Short circuit is the most serious and dangerous fault. It can be caused by the contact between two phases or between a phase and the neutral/earth conductor. This is due to an accidental contact or an insulation loss by natural ageing or overcurrent’s damages.
Choosing the protection device, it is necessary to check that its breaking capacity is greater than the maximum short-circuit current calculated in that point of the system (it does not depend on the motor nominal current, but on the characteristics of the upstream network).
Breaking capacity: it is a characteristic of specific devices which can open an electrical circuit, such as motor protection circuit breakers, switch disconnections, fuses. It is defined as the maximum current that the device is able to “open” (or interrupt). It must be greater than the short-circuit current value.
In automatic circuit-breakers for industrial use (CEI 17-5) the breaking capacity is further classified as:
a) service short-circuit breaking capacity (Ics). Sequence test: O – t – CO – t – CO
b) ultimate short-circuit breaking capacity (Icu). Sequence test: O – t – CO
Maximum current that the device can interrupt, by ensuring the correct functioning even after the interruption.
To increase the level of their reliability, motor protection circuit breakers also often ensure protection against phase loss, which puts the motor safe from dangerous overheating of the windings or even the burning of them.