Thermal Overload Relay is mainly used in low voltage squirrel cage induction motor and in low output rating DC motor, it is designed to operate under 6 to 7 times more than the full load current.
When the system is in normal operating condition, the spring remains straight. When the fault occurs on the system, the bimetallic spring is heated and getting bent, the tension of the spring becomes releases which trip the relay contacts.
The contacts of the relay energizes the trip circuit because of which the contacts of the circuit breaker close. Thus, the system remains safe.