Electrical Schematic – Motor Protection – Against Short Circuit

Definitions and characteristics
The main characteristics of short-circuit protection devices are:

  • breaking capacity: the highest value in the estimated short-circuit current that a protection device can break at a given voltage
  • closing capacity: the highest value a protection device can reach at its rated voltage in specified conditions. The closing value is k times the break capacity as shown in the table below:

Two protection devices are commonly used for this:

  • Fuses, which break the circuit by melting and must be replaced
  • Magnetic Circuit Breakers which automatically break the circuit and
    only require to be reset
  • Motor Circuit Breaker: this device is a thermal and a magnetic circuit breaker in the same package