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Thermal Overload Relay: Definition, Function, Price

Thermal overload relays
Thermal overload relays

When electrical circuits experience overloads, the resulting damage to equipment can be quite big. To prevent that from happening, secondary protection in the form of thermal overload relay is often used. Continue below to learn more about this crucial protection device:

We’ll first define thermal overload relay protection then see how the device looks like in electrical drawings, its working and, finally, how much it goes.

What Is A Thermal Overload Relay?

A thermal overload relay is a specialized form of overload sensing device that protects motors and other equipment or circuits from excessive heat, especially when the load draws too much current.

You’ve likely heard of fuses and breakers, which cut off power in an overload situation. Thermal relays work similarly, but in a more delayed way. For a better understanding of this statement, consider this:

An electric motor momentarily draws a large amount of load current when it starts. This does not necessarily mean a malfunction. By delaying the tripping action, a thermal protection relay helps avoid unnecessary motor shutdowns.

Thermal overload relay symbol (NEMA)
Thermal overload relay symbol (NEMA)

Thermal Overload Relay Symbol

The thermal overload relay symbol is used schematic diagrams to indicate the presence and position of the relay. Electricians and engineers may use either a NEMA or IEC symbols to represent the relay in electrical schematics. Both NEMA and IEC are organizations that define various standards in the electrical industry.

Thermal Overload Relay Symbol (NEMA)

The NEMA thermal overload relay symbol is usually drawn in two different ways: as two opposing question marks or as something that looks like a pulse waveform. This symbol essentially represents the heating element. An example of a NEMA thermal relay symbol can be seen in the above diagram.

Thermal Overload Relay Symbol (IEC)

The IEC symbol for thermal overload relay looks pretty much like the symbol for a pulse waveform inside a rectangle. Again, electricians may use this instead of the NEMA symbol. Just like the NEMA type, this thermal overload relay electrical symbol also represents the device’s heating element.

Thermal overload relay diagram
Thermal overload relay diagram

Thermal Overload Relay Diagram

For a better understanding of the thermal overload protection relay, it’s crucial that we take a closer look at its visible parts and components. This is clearly demonstrated in the above labeled image. From the thermal overload relay diagram, we can identify these parts and features:

  • Thermal Relay Terminals: thermal overload relay connection pins that mount it to a contactor or connect to the motor. They come in two sets – T1, T2, T3 for contactor connection and L1, L2, L3 for motor connection.
  • Auxiliary Contacts: these include the normally open (labeled 97, 98) and normally closed (95, 96) contacts. The NC contacts open when the relay trips, while the NO contacts close and act as signal or alarm contacts.
  • Test Button: the thermal overload test button checks if the tripping contacts are functioning as expected.
  • Amp Setting Toggle: this is used to adjust the thermal relay’s current setting, according to the motor load.
  • Reset Button: the thermal overload relay reset button resets the relay to its normal operating position after an overload incident.
  • Auto/ Manual Reset Selector: this button allows you to select between the auto and manual reset modes.
  • Trip Indicator: this is an indicator light helps you identify when the thermal overload relay has tripped due to an overload.
The main thermal overload relay function is to prevent overheating
The main thermal overload relay function is to prevent overheating

Thermal Overload Relay Function

The main function of thermal overload relay protection is to prevent motors, transformers, and other electrical systems or circuits from becoming too hot. It does that by delaying the tripping action. The thermal overload relay function can, therefore be summarized as follows:

1. A thermal overload relay allows harmless overloads such as temporary inrush currents of inductive loads to pass through without tripping and disconnecting the power supply.

2. A thermal overload relay responds to the sustained overloads and de-energizes the contactor when a certain temperature is reached. This cuts off power to the motor or any other connected load or circuit.

3. Thermal overload relay protection ensures proper functioning of motors and other electrical systems even under extreme temperature changes or variations in load currents.

4. The thermal relay switch acts as a monitoring mechanism to alert operators of potential or existing overloading conditions.

5. A thermal overload relay ensures the safety of personnel and equipment by automatically disconnecting power during overloads, thereby preventing hazardous conditions (such as fires) from occurring.

How Does a Thermal Overload Relay Work?

The most common type of thermal overload relay is the bimetallic thermal overload relay. This uses a bi-metal mechanism to trip a circuit when the temperature of the rises to a certain predetermined limit. Below is how it works.

  • The bi-metal trip mechanism is made up of several strips made of two metals with different linear expansion rates.
  • When the temperature increases, one metal expands more than the other.
  • This causes a bending action that operates a lever to open contacts, thereby tripping the circuit.
  • Once the fault has cleared (either on its own or by the user), the bimetallic strips cool and return to their original, equal positions.
  • This closes the contacts and the circuit can be powered again, either via an automated reset or manually resetting the relay.

Application of Thermal Relay

Thermal overload relays are mostly used in the protection of industrial motors, transformers, and other devices that may overheat and burn out or even, in the worst of events, cause a fire.

In this application of thermal overload relay protection, nuisance trips are also avoided by allowing temporary overloads such as inrush currents without disconnecting the power supply.

You can also use the relay to protect home appliances and consumer electronics, such as refrigerators, air-conditioners and more. Specific thermal overload relay applications and their details are listed below:

Overload Relay for Motor

Motors require overload protection when the amperage is higher than their rated value. This can be a thermal overload relay for single phase motor protection or a thermal overload relay for a 3 phase motor.

Overload Relay for Transformer

Transformers can also overheat when the current is higher than their rated value. A thermal overload relay helps in protecting the transformer from prolonged, excessive currents, that may cause damage or start a fire..

Overload Relay for Fridge

Refrigerators and other cooling appliances have a thermal overload relay that monitors the temperature of their components. The overload relay for refrigerator specifically protects the compressor motor from any damage that could occur due to an excessive increase in temperatures.

Overload Relay for Air Conditioner

A thermal relay is also installed to monitor the current draw of HVAC motors. Just like in the other thermal overload relay applications, the aim is to protect the device from any damage that could occur due to current overload.

Thermal overload relay models
Thermal overload relay models

Thermal Overload Relay Price

A thermal overload relay price list would show significant differences in cost depending on the type of relay, the manufacturer, and the included features. For example, an electronic thermal relay will cost more than mechanical types. Prices for basic models will also be lower than those of high-end models with extra settings and features.

Generally speaking, the thermal overload relay price starts at around $60 but can range up to $200 depending on the features and size. For example, a relay that comes with an auto reset feature is usually more expensive than one without.

Overall, thermal overload relays remain an affordable solution for protecting electrical motors and other equipment or power systems from overheating. Besides the low thermal overload relay cost, these devices are simple to use, which further reduces your expenses.


Thermal overload relays provide crucial protection to motors and other circuits that may otherwise be damaged by electrical overloads. They are easy to install and can be used in a wide range of applications, from industrial motors to household appliances. With their ability to detect potential rise in temperature, these relays are an invaluable safety device.

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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.


    C’est très bien détaillé

    Please, also tell us how the thermal relay works in protetecting equipment from overheating.

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