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Thermal Relay Types, Working and Applications

Thermal relay types
Thermal relay types
Resource: https://www.oemmagazine.org

Different types of thermal overload relays use different mechanisms to work. These thermal relay types also offer varying benefits when used in different situations. To give you a wider overview of their construction and operation, here are the three types of this device explained.

Thermal Overload Relay Definition

A thermal overload relay is defined as a device that functions as an automatic switch and provides protection against overloads in the electrical appliance or circuit.

A thermal relay switch is usually used to protect motors from thermal damage by automatically disconnecting them when the motor temperature exceeds a certain predetermined level.

In addition to protecting a motor, thermal overload relay protection is also applied in the power systems of other devices such as generators and transformers – or even for the protection of cables.

Thermal relay protection in an electrical system
Thermal relay protection in an electrical system
Resource: https://youtu.be/yfU8wRv-Fno

Thermal Relay Types

Thermal overload relays are mainly categorized based on their working principle. Other classification criteria such as number of poles or phases and trip class may also be used. The 3 types of thermal relays by working principle are bimetallic, electronic, and melting alloy thermal overload relay.

Bimetallic thermal overload relay
Bimetallic thermal overload relay
Resource: https://www.youtube.com/watch?v=zJzq2X47bbk

Bimetallic Thermal Overload Relay

The bimetallic thermal overload relay is the most used type. It’s also one of the simplest, both in terms of construction and operation. As its name, it uses two different metals that expand at different rates when heated. The difference in expansion rate creates a lever arm, which can bend to open or close contact points.

Note that the bimetals in these types of thermal relay mechanisms are indirectly heated by coils. The coils connect to the motor circuit, which enables them to match the motor current and, therefore, temperature rise.

When the temperature reaches a certain level, the bimetallic element bends and trips the switch to disconnect power to the equipment.

The bi-metal overload relay is often used for motor protection and other applications that require monitoring of temperature variations. Advantages of thermal relay types that use bimetallic mechanisms are:

  • Allows for remote reset for extra control and protection
  • Allows for the incorporation of ambient temperature compensation
  • Low cost than solid-state thermal relays.

The bimetallic thermal overload relay is the most used type of overload relay. It’s well suited for continuous operation in areas with high temperature variations like dwellings, factories, and motor control centers.

Electronic thermal overload relay
Electronic thermal overload relay
Resource: https://www.youtube.com/watch?v=xzk61ufhg8k

Electronic Thermal Overload Relay

These electronic thermal overload relay uses electronic components (transistors, diodes, etc.) and current transformer, rather than the traditional mechanical components found in bimetallic relays.

These types of thermal relays work by monitoring the temperature of motor windings or other equipment, usually using a thermistor or other temperature sensitive element.

When the temperature reaches a set level, the electronic components trip to open the circuit and protect it from damage. This type of thermal relay offers these benefits:

  • More precise temperature control
  • Lower power losses
  • Faster response times than bimetallic relays
  • Can be used with a wider range of equipment including motors that will frequently stop and start
  • Ability to incorporate remote monitoring and control

Despite these advantages, electronic thermal relays are more expensive than bimetallic ones and require more complex, especially when their operation involves the installation of a sensor in the motor windings.

Melting alloy thermal overload relay
Melting alloy thermal overload relay
Resource: https://www.youtube.com/watch?v=IZm4LCredPA

Melting Alloy Thermal Overload Relay

The melting alloy thermal overload relay uses a tin-lead alloy (solder) as its sensing element. Basically, it consists of solder inside a tube, a ratchet wheel held under tension by a spring, heaters, and contacts to open or close. It’s also called a eutectic alloy overload relay. The solder binds the wheel in normal conditions and it cannot turn.

During an overload condition, the excess current causes heat which melts the solder/alloy. This results in an almost instantaneous release and turning of the ratchet wheel. The NC relay contacts open and trip the circuit.

Before a melting alloy thermal overload relay can reset, the molten solder must be allowed to cool to a solid. The ratchet wheel is then reset and the NC contacts allowed to close again. Note that the solder must solidify before resetting, or the mechanism will not reset. Advantages of these thermal relay types include:

  • Ability to resist shock and vibration since the solder firmly bonds the moving parts
  • Modular mechanism allows for precise selection of tripping current
  • Low cost compared to electronic relays

Despite its advantages, the melting alloy or eutectic overload relay is will not allow for convenience features such as auto reset after a trip, or even control features such as ambient temperature compensation. They also offer tighter ranges for current adjustments.

Thermal overload relay trip curve
Thermal overload relay trip curve
Resource: https://www.researchgate.net

Thermal Overload Relay Class

Thermal overload relays are also classified according to their trip curves or tripping time levels. This is referred to as “trip class” and usually used by electricians when selecting the right thermal overload relay for specific applications.

A trip curve refers to the time it takes for a thermal overload relay to trip once it has been overloaded plotted against the current flowing through it. There are four thermal overload relay class levels in use today, Class 10A, Class 10, Class 20, and Class 30

Thermal relay class 10A trips the fastest, in less than 1.5 seconds, while thermal relay class 30 provides the slowest protection with a trip time of over 5 seconds. The overload relay tripping classes are summarized below.

  • Thermal overload relay class 10A: 2-4 seconds
  • Thermal overload relay class 10: 4-10 seconds
  • Thermal overload relay class 20: 6-20 seconds
  • Thermal overload relay class 30: 9-30 seconds

Single Phase Vs. 3 Phase Thermal Overload Relay

The different types of thermal overload relays are also specified by their number of poles or if they’re single phase, or three phase. Options here include:

  • Single phase thermal overload relay– This type of thermal overload relay contains one phase and is suitable for single phase motors.
  • 3 phase thermal overload relay– These thermal relay types contain three phases and are suitable for three-phase motors.

Finally, thermal overload relays are also rated for certain voltage and power requirements. These should be taken into account when selecting a thermal overload relay for your application.


Different thermal relay types are available on the market for overload protection devices today, each providing different advantages and applications. The bimetallic thermal overload relay is the most used type of this protective device. It’s simple in construction, easy to install, and many other things such affordability and reliability.

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