PAGE 01 > 02 > 03 > 04 > 05 > 06 > 07

What a motor protection circuit breaker is

A motor protection circuit breaker, or MPCB, is an electromechanical device that performs three important functions upstream of an electric motor:

  • isolation,
  • 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 overcurrents (current values slightly higher than the nominal value) which may damage the motor. In the event of overcurrents, the coil or the bimetal determine the opening of the contacts.

Protection against overload

Overloading is an unprogrammed operating condition which overheats the motor, causing it to draw a higher than nominal current, which may occur when:

  • the resistant torque is too high for the motor in question
  • the rotor jams
  • the motor is starting too frequently
  • one of the three power phases is lacking
  • the starting time is too long

The overload protection device must be sized based on the motor rated current, and in choosing it, pay attention to the minimum motor start-up times against which it is necessary to identify one of the intervention classes identified by the IEC standard: 2 – 3 – 5 – 10 – 10A – 20 – 30 – 40. The most common intervention classes are 10 – 10A, which corresponds to an intervention within 10 s for an overcurrent equal to 7.2 In

Thermal relay adjustment

The built-in thermal relay has the possibility, like the normal thermal relays, to regulate the tripping current, so as to cover a range of motors with a nominal current from 0.1 A to 100 A depending on the chosen motor protection. The calibration of the thermal releases takes place by means of an adjustment screw, located on the front of the device and equipped with a graduated scale in amperes, as shown in the figure. In the accessories it is possible to choose a transparent sealable cover, placed to protect the screw after its adjustment. In this way, the value of the thermal release is always visible, but at the same time an accidental or voluntary tampering is not possible.

Protection against short circuit

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 overcurrents 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 disconnectors, 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.

Protection against phase loss

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. The SM … series  implements this important feature.

Download all the technical information on the motor protection circuit breakers.
Find a training course on motor starting.


Laboratori tecnologici ed esercitazioni (Carlo Ferrari, Ed. San Marco)

Impianti elettrici industriali (M. Barezzi, Ed. San Marco)