Thermal Relay: Protection against overload
As the magnetic relay, the thermal relay is used to protect the equipment against damages due to an overload.
It contains three bimetal strips together with a trip mechanism in a housing made of insulating material. The bimetal strips are heated by the motor current, causing them to bend and activating the trip mechanism after a certain travel which depends on the current-setting of the relay
The release mechanism actuates an auxiliary switch that breaks the coil circuit of the motor contactor (Figure 1). A switching position indicator signals the condition “tripped”.
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Definition : Symbol
4.4. Class of the thermal relay
Method : Choice of thermal relay
The thermal relay is chosen depending on the class and the rated current of the load to be protected.
The thermal relay doesn't open the power circuit, it detect the overload and through its control contact act on the control circuit to switch off the equipment in fault.
Example : Example
A thermal relay protects an Induction motor with the following specifications: Pa=15 kW,cos〖ρ=0.8〗 power supply 3*400V, control circuit voltage 24V ac. Chose the thermal relay. It would be Class 10A
Thermal relay: LRD 32, setting at 27 A
