Overload relays are designed to imitate (on a much smaller scale) the heating patterns of the electric motor it is installed in, and interrupt the current once the heat sensing mechanism in the relay reaches a certain temperature.
An overload relay is made up of a heater paired with normally-closed contacts that open once the heater gets too hot. The overload contacts are connected in series and located between the contactor and the motor itself to prevent the motor from restarting once the overload trips.See our full product selection.
In a wiring diagram, the symbol for overload contacts may look like two opposed question marks or an S.
Though there are different types of overload relays, the most common type is the bimetallic thermal overload relay. This type of overload relay uses two different types of metal strips that are bonded together and expand at different rates when heated. When heated to a certain temperature the bimetallic strip will bend far enough to break the circuit.
When there is more current going to the motor than what the overload heaters are rated for, the overload trips after several seconds. There are three classes of overload relays depending on how long it takes the relay to trip. Class 10, 20, and 30 overload relays will trip after 10, 20, and 30 seconds respectively.
One key safety characteristic of an overload relay is that it prevents the motor from immediately restarting. For example, once the overload relay trips in a bimetallic relay, the normally-closed bimetallic contacts open the circuit and keep the circuit open until the strip cools down. Even if someone were to push the start button to close the contactor switches, the motor would not turn on.
Why do I need one?
An overload relay essentially protects an electric motor from overheating. In fact, you don't have a motor starter until you attach an overload relay to a contactor, which goes to show how important it is to have adequate motor protection.
Do I still need a circuit breaker if I use a overload relay for an electric motor?
Yes. You need to have both overload relays and circuit breakers or fuses to safely operate an electric motor.
Circuit breakers and fuses protect the circuit. Overload relays protect the motor. More specifically, circuit breakers and fuses are designed to detect when there is too much current in the circuit, while overload relays are designed to detect if a motor is overheating and will open the circuit if the motor gets too hot. For example, an overload relay can trip without a circuit breaker tripping. One does not replace the other.