Any electrical equipment has the ability to sustain the overload for a definite time, depending on the severity of the overload.
The thermal relays are used to protect equipment against overload conditions, operating on the principle of the heating effect of electric current.
The characteristic of the thermal relay should match the thermal withstanding capacity of the equipment.
Thermal relay takes more time to operate compared to the overcurrent relay.
When the overload occurs in the system, it does not mean a fault is created. Instead, it implies the overload is not a fault.
Hence, it does not require to trip circuit in a concise time (millisecond). Generally, the thermal relays take more time (in terms of seconds).
The below figure shows the time-current characteristics of a thermal relay, overcurrent relay, and the thermal withstand capability of the equipment to be protected.
From the above figure, it is clear that the overcurrent relays are not suitable for the overload protection of equipment.
Because overcurrent relays operate very fast, such fast operation does not require an overload condition.
The schematic arrangement of thermal relays is shown below figure.
The thermal relay consists of a bimetallic strip made up of nickel alloyed steel.
A heating coil heats the bimetallic strip. The heating coil absorbs the output current from a current transformer in a power circuit.
An insulated arm (lever) is provided with trip contacts at the strip’s end and connected to the spring.
The spring provides a tension force against the closing of trip contacts.
The heater element and bimetallic strip is chosen from the approximation of the heating curve of the equipment to be protected.
The bimetallic strip remains straight against the action of spring tension in normal conditions.
When the load increases from a specific value, the bimetallic strip bends, allowing the trip contact to energize the trip circuit.
The thermal relays are used in conjunction with instantaneous short-circuit relays of high setting or suitability graded time limit fuses.
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