The thermal protection relay (bimetallic relay) switches off the three-phase motors in case of overheating using the built-in auxiliary switch (1C + 1P) through a contactor, for example 3RT10. Inside the apparatus, three bimetallic elements and heating coils register the motor current. Since the heating of the motor is determined by its current, this is a current-dependent protection.
Current-dependent protective devices are: overload relay and circuit breaker. For short-circuit protection when using an overload relay, fuses or circuit breakers are required for starting combinations.
Short-circuit protection is already integrated in the motor circuit breaker.
In addition to current-dependent protection devices, there are also thermal-dependent devices, such as the 3UN2 thermistor motor protection relay.