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5 Advantages of Thermal Relay Protection Explained

Thermal overload relay working principle
Thermal overload relay motor protection
Resource: https://youtu.be/tqlSYbpb2qg

There are many advantages of thermal relay protection that make it an essential part of electrical power systems. These range from improved safety and reliability to easy installation as you will find out shortly. We will also comparing thermal relays with the circuit breakers as well to see what advantages they offer.

What is the Function of Thermal Overload Relay?

The purpose of a thermal overload relay is to protect a motor against overloads and single phasing or phase imbalance. It does this by automatically cutting off the electric supply to an overloaded or stalled motor.

Apart from thermal overload relay motor protection other applications include transformer protection, or the protection of any system that’s prone to damage from overloads – but which should not be disconnected, unless the overload persists for a period of time.

Essentially, a thermal overload relay is designed to detect the thermal effect of excessive current in a circuit and to disconnect the electric power at an appropriate time. It’s also usually used with a contactor, as can be been in the below diagram.

TThermal overload relay protectionhermal overload relay working principle
Thermal overload relay protection
Resource: https://forums.mikeholt.com

Advantages of Thermal Relay

As earlier mentioned, the advantages of thermal relay protection are numerous. These range from improved safety and reliability to easy installation. These, together with situations where the application of thermal relay protection is recommended are listed below:

1. Simple Operation

One of the many advantages of thermal relay devices is their simple operation. This is because they are designed to operate using only a few simple components, such as heaters and bimetallic strips. This makes them easy to install and reduces the need for additional wiring and complex mechanisms.

The bimetallic strip is the main component of a thermal relay and designed to expand when the electrical current passing through it becomes too high; once this happens, the bimetal strip bends, thus disconnecting power from the motor by de-energizing a contactor.

This simple thermal relay operation is what results in the reliable and cost-effective protection of electric motors. It also makes these devices easy to use, troubleshoot, and maintain.

2. Phase Protection

Thermal overload relays typically include the ability to protect against phase imbalance or single-phasing. By detecting these conditions, the device can disconnect the motor in time to prevent it from stalling and becoming damaged.

This, when combined with the main function of thermal overload relay use, provide for a reliable and effective protection of electric motors. It also reduces the chances of costly repairs or replacements resulting from motor damage.

3. Cost-Effective Protection

The simple design and construction of the thermal relay means the devices are generally inexpensive when compared to most protection devices. This makes them ideal for applications where cost is a major factor, such as in smaller electrical systems.

In addition, thermal overload relays require no additional wiring or programming, further reducing installation costs.

In most cases, the thermal overload relay price doesn’t exceed $200, unless we are talking about models with more functions. These will also be large relays for protecting bigger systems or heavy-duty motors.

4. Manual/Auto Reset Button

The thermal overload relay rest button offers a convenient way to reset the device in case of a power interruption or failure. This button can be used manually, or it can be programmed to reset automatically after a certain period of time.

For manual resetting, all you need to do is press and hold the button until the overload relay restarts. For automatic resetting, the device will reset itself when the current flow falls below the required threshold.

This is often a feature found in more advanced types of thermal relays, such as electronic relays, as it offers greater control and convenience for users. With an automatic reset button, you don’t have to worry about resetting the device manually, plus you can easily include a remote reset control in your system.

5. Easy Installation

The simple design of thermal relays also makes them easy to install. This is especially true when compared to other motor protection devices such as contactors and circuit breakers, which require careful wiring.

Thermal relays usually come with a plug-in connection that can be easily mounted onto contactors using the provided pins. This makes them fast and easy to install in many electric systems, reducing the time and cost associated with installation.

Overall, thermal overload relays offer a range of benefits that make them an attractive option for motor protection. They are reliable, easy to use and maintain, and offer cost-effective protection in most applications.

Given these advantages of thermal relay protection, how does the device compare with other protection devices such as the circuit breaker? Let’s take a look.

Thermal overload relay
Thermal overload relay
Resource: https://youtu.be/_WACRPkY34Y

Thermal Overload Relay Vs. Circuit Breaker

The thermal overload relay and circuit breaker are both used to provide protection for electric circuits and electrical equipment or appliances. Despite that, they differ in a number of ways. So what’s the difference between overload relay and thermal overload relay?

  • In terms of working mechanisms, the thermal relay operation relies on a thermal detection element to sense overloads, while a breaker often contains both thermal and magnetic elements for overloads and overcurrents. For this reason, a thermal relay is often used with an upstream breaker for short circuits.
  • A circuit breaker is designed to shut off mains power in the event of an overload or short circuit. In contrast, a thermal overload relay doesn’t do that on its own. Instead, opens another device (contactor) that switches off the power mains.
  • A thermal overload relay easily detects small overloads that persist over time and shut off the circuit before damage occurs to the equipment. On the other hand, a circuit breaker can only see sudden overcurrents.
  • When it comes to cost, a thermal overload relay is usually more cost-effective than a circuit breaker. In addition, the simple design and construction of the former means that it is easier and faster to install than the latter.

In general, the advantage of thermal overload relay vs. circuit breaker is that the thermal relay is more forgiving when it comes to temporary overloads, and will not trip as easily – unless the excess current stays for too long as to cause damage.

This makes them ideal for situations where short-term overloads are likely, such as motors that may experience heavy loads when starting and from time to time. Here, they prevent equipment from being damaged while also avoiding nuisance trips.


There are many advantages of thermal relay protection when used to on motor circuits and other appliances or power systems. In addition to being cost-effective devices, thermal relays are also easy to install and maintain, while also providing reliable protection against overloads.

By understanding the advantages of thermal relays and how they compare to circuit breakers, installers can make better-informed decisions when selecting the best protection device for their applications.


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.

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