Selection of a motor circuit breaker

Motor switch

The motor circuit breaker (called a thermal switch by some) is a device used to connect and separate current circuits operating under load. However, its main task is to protect the motor against the effects of overloads, short circuits and phase failure.

Engine power

Most often motor circuit breakers are used to protect low power motors – up to about 4kW. In exceptional situations, circuit breakers are also used to connect motors with higher power.
 

Motor breaker functions

An improperly working engine is exposed to damage and may cause a fire – it is therefore necessary to protect it (and the entire installation) against the effects of:

• short circuits (an electromagnetic release is responsible for instant disconnection – the activation time is in the order of milliseconds)
• power phase loss (phase unbalance)
• overloads and blocked starting (responsible for this is the thermal release)

Selection of motor circuit breaker settings

Motor circuit breakers are equipped with a knob for setting the disconnection current – it is set according to a simple formula:

Inas = In * 1.1, where
In - motor rated current
Inas - circuit breaker setting current

For example, to protect a motor with a rated current of 10A, the knob should be set to 11A (10*1.1).
 

Installation of the motor circuit breaker

The key is always to install a circuit breaker before the contactor – this will protect the contactor against possible short-circuits.
 

 

Other information

Short-circuit current is also an important consideration when choosing a motor starter protector. This parameter indicates the maximum current that can flow during a short-circuit without permanently damaging the circuit breaker.
In installations using multiple motors operating in parallel, remember to protect each motor with a separate circuit breaker – this will prevent failure if one of the motors is under more load than the others.