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).
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(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.
