The Basics of Circuit Breakers For Electrical Engineers

Circuit Breakers for Motor Protection

Continuous overload current leads to unacceptable temperature-rise, accelerated ageing of the insulating material, increased wear and ultimately faulty operation of the connected equipment.

Similar to a bimetallic thermal overload relay, a circuit breaker is suitable for:

• Starting and running under normal service conditions of – low voltage motors from 0.02 to …kW.

• With additional devices also for special starting and running conditions such as: – Motors in intermittent periodic duty or with fluctuating load – Soft starter (protection of motors with phase-controlled soft starting). – Frequency converter (speed control of three-phase motors).

1| Protection of Motors with Direct-On-Line Starting

A typical three-phase motor is sufficiently protected by the combination of a contactor and a circuit breaker with motor protective characteristics.

Left – Starter for direct-on-line starting with circuit breaker; Right – Reversing starter with circuit breaker

As the full rated current of the motor flows through the circuit breaker, the current setting of the thermal release is equal to the motor rated current.

2| Protection of Motors with Star-Delta Starting

With one circuit breaker

In the case of a star-delta starter, the upstream circuit breaker takes over the motor protective function and the two thermal overloads connected in series with the motor windings become superfluous. The cross-section of the connecting leads to the motor must be selected on the basis of the rated current of the overload protective device (current setting of the circuit breaker).

!!! Because of the circuit breaker, a smaller cross-section of the connecting wire may be taken than in the case of a fuse together with a single thermal overload relay in the motor winding circuit (over-dimensioning of the fuse is necessary to cope with the high starting current of the motors).

Starter for star-delta starting with one circuit breaker

The current setting of the thermal release of the circuit breaker is equal to the actual rated current of the motor. The calculation of the rated current for the setting of the current scale with the help of factor √3 (=1,73) is no longer necessary.

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