Basic motor protection scheme: circuit-breaker + contactor + thermal relay

From Electrical Installation Guide

Among the many possible methods of protecting a motor, the association of a circuit breaker + contactor + thermal relay (1) provides many advantages

The combination of these devices facilitates installation work, as well as operation and maintenance, by:

• The reduction of the maintenance work load: the circuit-breaker avoids the need to replace blown fuses and the necessity of maintaining a stock (of different sizes and types)
• Better continuity performance: the installation can be re-energized immediately following the elimination of a fault and after checking of the starter
• Additional complementary devices sometimes required on a motor circuit are easily accommodated
• Tripping of all three phases is assured (thereby avoiding the possibility of “single phasing”)
• Full load current switching possibility (by circuit-breaker) in the event of contactor failure, e.g. contact welding
• Interlocking
• Diverse remote indications
• Better protection for the starter in case of over-current and in particular for impedant short-circuit (2) corresponding to currents up to about 30 times In of motor (see Fig. N67)
• Possibility of adding RCD:

  - Prevention of risk of fire (sensitivity 500 mA)
  - Protection against destruction of the motor (short-circuit of laminations) by the early detection of earth fault currents (sensitivity 300
    mA to 30 A).

(1) The combination of a contactor with a thermal relay is commonly referred to as a «discontactor». (2) In the majority of cases, short circuit faults occur at the motor, so that the current is limited by the cable and the wiring of starter     and are called impedant short-circuits.

Fig. N70: Tripping characteristics of a circuit-breaker + contactor + thermal relay

The combination of a circuit-breaker + contactor + thermal relay for the control and protection of motor circuits is eminently appropriate when:

• The maintenance service for an installation is reduced, which is generally the case in tertiary and small and medium sized industrial sites
• The job specification calls for complementary functions
• There is an operational requirement for a load breaking facility in the event of need of maintenance.

The Selection of a motor protection association is often done using coordination tables provided by manufacturers, however digital tools now available make this selection much easier, quicker and reduces drastically possible selection mistakes. As an example, you might check the LV motor starter solution guide provided by Schneider Electric.