Electrical Schematic – Motor Protection – Against Short Circuit – Motor Circuit Breakers

This device is a thermal and a magnetic circuit breaker in the same package which protects a motor against short circuits and overload by rapidly opening the faulty circuit. It is a combination of a magnetic circuit breaker and overload relays It complies with the IEC 60947-2 and 60947-4-1 standards.


The thermal elements (protection against overload) are compensated for fluctuations of the ambient temperature. The thermal protection threshold can be adjusted on the front of the unit. Its value must correspond to the rated current of the motor to be protected.

The insulation distance (between contacts) in most of these units is adequate to ensure isolation.

Tripping curves

A motor trip switch is characterised by its tripping curve, which represents the time it takes to trip based on the current (multiple of Ir).
This curve is divided into four zones:

  • lc normal operating zone 1. As long as I < Ir, there is no tripping
  • thermal overload zone 2. Tripping is ensured by the “thermal” feature; the greater the overload, the less time it takes to trip. The standards refer to this as “inverse time”
  • strong high current zone 3, monitored by the “instant magnetic” or “short-circuit” feature which works instantaneously (less than 5ms)
  • on some circuit breakers (electronic), an intermediate zone 4 monitored by a “timed-delay magnetic” feature with a delay function (0 to 300ms). The standards refer to this as “definite time-lag”


Their limits are:

  • Ir: setting current for protection against overload; should correspond to the rated current value (In) of the motor to be protected
  • Im: tripping current of timed magnetic protection
  • Iinst: tripping current of instant magnetic protection. This can range from 3 to 17 times Ir but is usually close to 10 Ir
  • Ics: service rated breaking capacity in short circuit
  • Icu: ultimate (maximum) breaking capacity in short circuit