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Thermal Overload Relay Working Principle Explained

Thermal overload relay working principle and construction
Thermal overload relay working principle and construction
Resource: https://youtu.be/u7rs9ZZsaoU

The thermal overload relay working principle depends on the type of mechanism use. This is typically a bimetallic type, but can also be electronic or melting alloy. Because the bimetal relay is the most used type of this device, this post will focus on that.

What is Thermal Overload Protection?

Before we can look at the thermal overload relay working process, let’s answer the question – what is thermal relay protection? In simple terms, it’s a type of overload protection that monitors and controls the temperature within an electrical circuit.

If the temperature exceeds a given level, the relay trips to open the circuit and disconnect the affected piece of equipment. An electric motor, for example can experience overheating due to an overload caused by the following situations:

The thermal overload relay operation can help to protect the motor from these overloads by sensing and responding to changes in temperature as simulated by the sensing element.

Thermal overload relay parts
Thermal overload relay parts
Resource: https://www.youtube.com/watch?v=iZ9Q_oB9lzU

Thermal Overload Relay Working Principle

The thermal overload relay working principle is based on the amount of current that flows in the motor or circuit it’s connected to. The idea is to use this current to recreate the heat that would be generated in the appliance and use that to determine when an overload can cause damage. The exact thermal relay working – and parts – are discussed below.

Thermal Overload Relay Parts

A thermal relay is composed of several different parts, some of which are moving components. The main parts of a thermal overload relay that are involved in temperature sensing and the tripping action include the following:

  • Temperature sensing element – This is usually a bimetallic strip, which consists of two strips of different metals. The amount by which the strip expands will depend on how much current is flowing through it or how hot the environment is.
  • Tripping mechanism – This is the section of that contains levers and slides that trips off the relay when the temperature sensing element senses a prolonged overload. The parts include trip lever, contact lever, trip slide, and springs.
  • Contacts – These overload relay parts open and close to allow current flow or to disrupt it when tripped. A typical thermal relay will have two sets of contacts; one set is normally closed and the other normally open. When an overload is detected, the contacts switch positions to disrupt current flow.

Other crucial thermal overload relay parts include the setting and control components such as: reset knob and mechanism, current adjuster cam, test feature, trip indicator, temperature compensation feature, disconnect button, and housing.

Thermal Overload Relay Operation

Let’s now take a look at how a thermal overload relay works. This will help you understand what role each part plays and what happens before tripping can happen. The thermal overload relay operation, based on the parts we’ve just discussed, is as follows.

  • As current flows through the bimetal heating coil, it generates heat. Depending on the set rating, a sustained overload reaches a point where it causes the bimetallic strip to expand and bend.
  • This will press against the trip lever and the relay’s normally closed contacts open while the normally open contacts close. The action disconnects the affected piece of equipment from the power supply.
  • When this happens, a tripping indicator is activated to show that the device has been disconnected. You can then reset the tripped the tripped relay as necessary or when the specific fault has cleared.

Note that it’s common practice to use a thermal relay in combination with a contactor. That’s because it cannot disconnect power on its own. To facilitate this overload relay and contactor connection, the relay usually comes equipped with a set of pins.

Thermal overload relay connection diagram
Thermal overload relay connection diagram
Resource: https://youtu.be/LpphvWSrejQ

Thermal Overload Relay Connection Diagram

How do you connect a thermal overload relay? As can be seen from the above example of a thermal overload relay connection diagram, it’s quite simple. The following steps explain how to go about it, specifically when using the thermal relay for motor protection.

The relay’s connection pins are inserted into the contactor and screws used to tighten the two devices together. After that, the following steps for typical thermal overload relay wiring are followed:

The input terminals of the relay are wired to the output of the contactor, and the relay output terminals to the motor. The power supply is then connected to the relay input. For control, the NC contacts of the relay are wired o the contactor’s coil circuit.

Thermal Overload Relay Setting

Thermal overload relay setting is the process of adjusting the relay so that it responds with the appropriate tripping action to an overload. This is necessary because thermal relays respond differently, depending on their ratings and setting range or application.

Thermal Overload Relay Range

Every thermal overload relay range for current, ambient temperature and other specs is typically specific to it, mainly depending on its mechanism and intended application. Some relays may have extended range settings than others based on their specifications.

When selecting a thermal overload relay, it’s recommended to choose one that has a range suitable for the application. That is, the current and temperature ratings must be high enough to handle the expected load types and load conditions.

How to Set Thermal Overload Relay

The main steps of setting thermal overload relay features include selecting between manually or automatic reset for the device, adjusting current setting and so on. For example, you can set the relay current to match the motor’s full-load current.

It’s important to note that incorrect overload relay setting will result in frequent tripping or even the failure of the contactor to trip in case of an overload. Understanding how to set thermal overload relay parameters is thus essential for proper operation and protection against faults.


The thermal overload relay working principle explained in this article should help you understand not only how these devices work but also why they are important when it comes to protecting motors and electric circuits from overloads. That way, you can ensure that any unexpected current surges won’t cause damage to your equipment.

Picture of William

I am William, Electrical Engineering Author. Dedicated to writing technical articles on Timer Relay, Monitoring Relay, Surge Protection Device and other electrical devices. With 7 years of writing experience, I am committed to providing accurate and in-depth expertise to my readers.


    A good and important notes to users of electrical motors.
    Iam using single-phase motor of posho mills
    Please keep me informed

    Which brand of thermal overload relay is good for 7.5hp motor

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