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What is a Thermal Overload Relay?

Paul Scott
Paul Scott

A thermal overload relay is an electrical protection device designed to disconnect a machine, appliance, or installation from its power supply should damage or overload occur. This protection is achieved by the inclusion of various types of heat sensitive elements to cut the power supply under overload conditions. These thermal elements react to the current the protected component draws during its operation. Should the component experience excessive increases in load, physical wear, or damage, the amount of current drawn will increase correspondingly. This will cause a thermal cut-out and disconnect the main feed to the protected component.

The flow of current through an electrical device is the part of the electrical supply dynamic which generates heat. An increase in current flow causes a proportional increase in the amount of heat generated. Current flow through an electrical appliance is a product of the load to which the appliance is subjected. Again, an increase in load sees an increase in current and generated heat. If the this load increases to a point which exceeds the design specifications of the appliance, it will overheat and eventually be destroyed.

Man with a drill
Man with a drill

Thermal overload relays are designed to prevent damage or destruction of electrical machinery by reacting to increases in current induced temperatures. Should the temperature rise above the rating of the relay, it will activate and disconnect the primary supply and prevent equipment damage. This deactivation is achieved via either a mechanical or electrical interlock between the relay and the main power supply. The temperature sensitive component of a thermal overload relay may be a simple bi-metal strip or a more sophisticated electronic sensor or probe.

The Bi-metal strip in a thermal overload relay is made up of two dissimilar metals fused together. The different metal characteristics mean they heat at different rates causing the strip to bend. This bending motion then activates a cut-out device. The electronic thermal overload relay uses a sensor or probe to read the current generated temperature. A microprocessor then dictates when the circuit will open and cut the main supply depending on preset parameters.

Overload protection relays are either installed in the control circuits of large machines or as a direct link in the supply of smaller appliances. Control circuit relays consist of a heat sensitive element and a set of contact points. The control circuit for the protected machine passes through the relay's contact points. Should the machine reach overload current levels, the relay's heat sensor switches the thermal overload relay off which, in turn, cuts the main supply to the machine. Smaller appliances generally have their power supply pass directly through the heat sensitive element which bends under overload conditions and switches the machine off.

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