Thermal Overload Relay

Thermal overload relays are protective devices. They are designed to cut power if the motor draws too much current for an extended period of time. To accomplish this, thermal overload relays contain a normally closed (NC) relay. When excessive current flows through the motor circuit, the relay opens due to increased motor temperature, relay temperature, or sensed overload current, depending on the relay type.

Thermal overload relays are similar to circuit breakers in construction and use, but most circuit breakers differ in that they interrupt the circuit if overload occurs even for an instant. Thermal overload relays are conversely designed to measure a motor's heating profile; therefore, overload must occur for an extended period before the circuit is interrupted.