Electrical Schematic – Motor Protection – Against Overloads – Bimetal Thermal Overload Relays

These are linked to a contactor to protect the motor, the power supply and the equipment against low prolonged overload.

They must be protected from strong over currents by a circuit breaker or fuses.

The operating principle of a thermal overload relay is based on the
distortion of its bimetal strips heated by the current that crosses them.


As the current crosses them, the strips distort and, depending on the setting, cause the relay contact to open suddenly.
The relay can only be reset when the bimetal strips have adequately cooled down.

It does however have the disadvantages of imprecision with regard to the thermal status of the motor and sensitivity to the thermal conditions where it is installed (housing ventilation, etc.).