Overload relays are electrical devices designed to offer reliable protection for motors in the event of overload or phase failure.
These types of relays are used to protect against a running overcurrent that would cause overheating in a motor. Instead of instantaneous trip characteristics found in circuit breakers, fuses and overcurrent relays, overload relays monitor current over time.
Thermal overload relays have a bimetallic strip that bends when heated to a set temperature. When excessive current heats the relay to that setpoint, the contactor de-energizes and interrupts power to the motor.
Electronic overload relays use electronics to constantly monitor current, instead of relying on heat to trip a contact coil. These relays often offer adjustable current ranges, protect against phase loss, and can automatically reset.
Overload relays are marked by Class (Class 10, 20 and 30), which indicates the response time of the relay.