Thermal overload relays & motor protection circuit breakers

bg triangle top left

What’s the difference between a motor protection circuit breaker and a thermal overload relay? What’s an electronic motor protection relay? Read on to find out more.

Australian Standards require two types of protection for motor circuits – short circuit protection, and overload protection. Short circuit protection must operate very quickly in the event of a high current, whereas overload protection is typically delayed to allow a motor to draw a higher current while starting. Let’s take a quick look at the difference between the two:

Short circuit protection uses an electro-magnetic mechanism to trip the circuit instantly when the current reaches a certain threshold. It’s generally set to trip at ten or more times the normal motor running current, to avoid tripping during normal motor starting.

Overload (thermal) protection works using a bimetallic strip which bends when heated to a set temperature by the current flowing to the motor. This indicates the motor is continuously drawing excessive current over time – it may be jammed, stalled or under too much load.

Thermal overload relay

A thermal overload relay, commonly called a ‘thermal overload’, is a device which is usually connected directly to a motor contactor. It offers overload protection only, so must be coupled with an upstream magnetic circuit breaker for short circuit protection. Thermal overloads are generally cost-effective, reliable and simple to install and use so are widely used for motor control.

Motor protection circuit breaker

Also known as a motor circuit breaker, a motor protection circuit breaker is a switchgear device which includes both overload and short circuit protection in a single compact frame. It differs from a standard MCB (miniature circuit breaker) in that it allows the user to preset the exact motor sizing for accurate protection. Motor circuit breakers are very common in industrial motor starting applications.

Electronic motor protection relay

An electronic motor protection relay, or electronic overload relay, is a sophisticated electronic device which uses electronics to continually monitor the motor current and protect against overload scenarios. Most models don’t offer short circuit protection, so must be also coupled with a circuit breaker. A large range of additional communications options are also available, including Ethernet or Serial connections to PLC’s and other automation hardware. An electronic overload offers greater accuracy and flexibility when compared to a standard thermal overload, but is much more expensive. Their use is mostly limited to applications in mining and critical infrastructure where the additional visibility and flexibility is required.