Thermal Relay : Construction, Circuit, Working & Its Applications

A relay is an electrically operated switch that is used to open & close the circuits or to make or break electrical connections by simply getting electrical signals from external sources. These are required whenever electrical isolation is mandatory in control circuits, otherwise when various circuits need to be controlled with a single signal. There are different types of relays available in the market which are utilized depending on the application. So, the thermal relay is one of the types of the relay, used to provide complete safety against single phasing, unbalanced voltages & overloads. Thermal relays are the perfect solution for providing protection to motors which provides the most precise tripping for the electric motor during single phasing and overload. This article discusses an overview of a thermal relay – working with applications.

What is a Thermal Relay?

Thermal relay definition is; the relay which is used to provide electromechanical protection to electric motors from overloading and also drawing extreme input current is known as a thermal relay. These relays provide huge protection from stable electrical damages throughout electrical anomalies like overvoltages & phase failure. The thermal relay symbol is shown below.


Construction of Thermal Relay

The thermal relay construction is quite simple. This relay is constructed with important parts like bimetallic strips, heating coils & the CT (current transformer).

The current transformer (CT) in this relay simply supplies the flow of current to the heater coils. So the heater coil’s thermal energy will heat the bimetallic strips where these strips are made using different materials like steel & the alloy of nickel. These materials have maximum steel resistivity & they are also free from aging of thermal.

Thermal Relay Construction
Thermal Relay Construction

In the above relay, an insulated liver arm is simply connected to the trip coil through the bimetallic strips & the spring. The strain of the spring is changed with the help of the sector-model plate.
Once the system is in normal operating condition, then the spring will remain straight. So when any fault takes place on the system, then the bimetallic spring will be heated & bends. The strain of the spring will release to trip the contacts of the relay. So the relay contact will energize the trip circuit die to which the circuit breaker contacts close. Therefore, the system remains secure.

Working Principle of Thermal Relay

The thermal relay working principle is that whenever a bimetallic strip in the thermal relay is heated up through a heating coil then it bends & makes normally open (NO) contacts.


Once the motor works normally, then the thermal relay’s thermal element will not produce sufficient heat to make the protection function operate & its normally closed (NC) contact will keep closed condition. Once the motor is overloaded, the thermal element in the relay will produce sufficient heat to make the protection function perform & its normally closed (NC) contact will be broken to make the electric motor lose power throughout the control circuit to guard the electrical motor. Once troubleshooting is done, then this relay must be reset before the electric motor is restarted.

Generally, the thermal relay has two reset forms automatic and manual reset. These two reset forms conversion is simply completed by simply changing the reset screw. Once the thermal relay is designed, the usually manufacturer sets it to the automatic reset condition. While using, whether the relay is set to automatic reset or manual reset state condition mainly depends on the particular condition of the control circuit.

Types of Thermal Relay

Thermal relays are available in three types bimetallic thermal, solid state, and temperature control..

Bimetallic Thermal

A bimetallic thermal relay uses a bimetallic strip for opening the contacts mechanically. This strip includes two metal conjoined pieces which increase at different rates once they are exposed to heat. Once they are heated the bimetallic strip will bend. In this relay, the bimetallic strip is connected to contact by a spring. Once excess heat causes the strip to bend from the overcurrent & the spring is pulled, then contacts in the relay are pulled apart & the circuit is broken. Once the strip gets cooled then comes back to its actual shape.

Bimetallic Thermal Relay
Bimetallic Thermal Relay

Solid State Relay

Solid-state relays have no mechanical or moving parts. This relay simply calculates the information of the Thermal overload relay and the normal motor temperature by simply monitoring its beginning & running currents. These relays are faster as compared to electromechanical relays and also include trip times & adjustable set points because they are not capable of generating a spark, so used in unstable environments.

Solid State Type
Solid State Type

Temperature Control Relays

These types of relays are used to detect the motor’s temperature directly using a resistance thermal device probe & thermistor fixed in the winding of the motor. Once the nominal temperature of the RTD probe is attained, then its resistance rapidly increases. After that, this increase is detected through a threshold circuit, that opens the contacts of the relay.

Temperature Control Relay
Temperature Control Relay

Melting Alloy Relay

A melting alloy thermal relay includes a heater coil, a eutectic alloy & a mechanism to break the circuit. By using this heater coil, then this relay will measure the temperature of the motor by simply monitoring the current drawn.

Melting Alloy
Melting Alloy

Thermal Relay Circuit Diagram & Working

A thermal relay circuit for overload protection is shown below which is used to avoid the failure occurring in the motor. This overload protection circuit comprises a fuse, contactor, thermal relay, start button, and stop button.

Thermal Relay Circuit
                            Thermal Relay Circuit

When the thermal relay is used to protect the motor from overload, the thermal element of the relay is simply connected in series to the motor’s stator winding. The thermal relay’s normally closed contact is connected simply in series with the control circuit of the AC contactor

If the electric motor is overloaded, then the flow of current within the winding will be increased & the flow of current within the thermal element of the relay also be increased, and the temperature of the bimetallic sheet increases higher & the bending level increase. After that, it pushes the NC contact to disconnect and disconnects the AC contactor coil circuit, so that this contactor disconnects the power supply of the electric motor, Thus, the electric motor will be protected by stopping.

Thus, the AC power contactor coil is turned off then the main contact is off to stop the electric motor M. Finally, the overload breakdown of the motor winding burn will be eliminated effectively. Once the overload failure is removed the thermal relay’s Reset button will be pushed & the starting button ST so the motor starts working again.

How to Select a Thermal Relay?

The function of the thermal relay is to guard the electric motor from overload. To make sure that the electric motor can attain both sufficient & necessary overload protection, it is required to completely know the motor’s performance & allocate it with an appropriate thermal relay to achieve the required settings. In general, the related conditions of the motor are the starting current, working environment, working system, load nature, permissible overload capacity, etc.

The proper selection of this relay is very much related to the motor working. Once the thermal relay is utilized to guard the motor for long-term, then it is selected based on the motor’s rated current. For instance, the thermal relay’s setting value may be equivalent to 0.95-1.05 times the motor’s rated current otherwise the median value of the set current of the relay is equivalent to the motor’s rated current & after that adjusts.

Once this relay is utilized to guard a motor that is operated frequently for a little time, then this relay has simply a certain range of flexibility. If there are several operations for each hour, then a thermal relay with a current transformer with speed saturation should be preferred.

For particular motors working with frequent forward & reverse phases ON & OFF, it is not suitable to utilize these relays like overload protection devices. As an alternative, temperature relays or thermistors are used in the windings of motors to guard them.

This relay has the capacity of low overloading, so it is mainly designed to work under 6 – 7 times extra than the full load current.

This relay is not utilized in short-circuit conditions. When the short circuit current enhances the temperature of the bimetallic strip then the relay contacts will be closed. So this relay is mainly used by the short circuit relay with the time limit fuse only.


The advantages of thermal relays include the following.

  • Thermal relays have greater accuracy.
  • They protect electrical motors from overheating eventually. So they can be used conveniently in 1 & 3 θ motors.
  • These relays are installed easily.
  • They can be mounted directly to contractors and otherwise easily mounted to the operation panel with rail adaptors.
  • Some relay models are simply equipped through internal trip class selection buttons.
  • These relays are available with automatic & manual reset functions for simple operations.
  • They include an internal test button used for troubleshooting.
  • These are very active on a wide & adjustable range of current.
  • They have a trip-free mechanism used for optimal operation.
  • They include temperature compensation features used for precise functioning.
  • These can be used readily anywhere.


The disadvantages of thermal relays include the following.

  • Thermal relays do not come with short circuit protection although they offer electrical protection.
  • Most thermal relay-based devices’ operation is slow.
  • These are not designed by direct breaking functions but they need to be utilized with other electrical protection & switching devices for detaching a live circuit.
  • They optimally work against low-resistance circuits.
  • When they are used in heavy-duty circuits then they do not perform well always.
  • These are not able to withstand vibrations and electrical shocks.
  • These relays do not available with a high switching frequency, so they frequently need time to become cold when they have tripped & overheated.


The applications of thermal relays include the following.

  • The thermal relay is used in overload protection of the motor.
  • This is a protective device mainly designed to cut the power once the electric motor utilizes extra current for an extended period of time.
  • These relays are helpful in protecting electrical devices, motors & transformers from overheating.
  • This relay is mainly designed for current-dependent applications protection by normal start-up conditions against impermissibly high increases in temperature as a result of phase or overload failure.
  • These are protective electrical appliances mainly used for overload protection of electrical circuits and devices.
  • This is used mainly in low-output rating DC motors & low voltage-based squirrel cage induction motors.
  • These relays are utilized in motor starter circuits to avoid the motor from using extreme current that is very dangerous to the insulation of the motor.
  • These relays avoid motor damage & also keep the equipment working for a very long time.
  • This relay is used in a DC motor with a low output rating & squirrel cage induction motor with low voltage.

Thus, this is an overview of a thermal relay – working with applications. These relays are protective electrical devices mainly used for overload protection of electric motors, electrical equipment & electrical circuits. Here is a question for you, what is the function of a relay?