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How to Test Thermal Overload Relay, Maintenance Tips

Thermal overload relay testing during installation
Thermal overload relay testing during installation

Knowing how to test a thermal overload relay correctly can help determine if the electrical device is functioning properly. Here, we outline the different ways to do so. We’ve also included maintenance tips to help keep it functioning properly and a troubleshooting guide if you happen to find a problem.

Thermal Overload Relay Testing

Thermal overload relay testing means checking the relay for various functions, such as operation, tripping time, and other parameters. A thermal relay test is also used to check the integrity of contacts and internal circuits, so you can determine if the unit is in good enough condition to provide reliable protection.

Thermal overload relay testing should be periodic, using the manufacturer recommended intervals. Alternatively, you may want to test the relay if you suspect there is a problem. So how do you go about carrying out the test? Let’s take a look at the three thermal relay testing methods.

Thermal overload relay testing
Thermal overload relay testing

How to Test Thermal Overload Relay

You can test a thermal relay with three different methods- using the testing button, using a testing kit or machine, and with a multimeter. Each overload relay testing method has it procedure, as explained in the below guides.

Thermal Overload Relay Test Button

This is the simplest testing method. The thermal overload relay test button (or switch) is located on the front of the relay. It essentially simulates an overload situation by opening the NC contacts and closing the NO contacts. Note that you’ll need a multimeter for this process. Here are the steps:

  • Locate the test button or switch on the relay.
  • Set the multimeter so it reads resistance in ohms.
  • Connect multimeter leads to the 95 and 96 terminals of the relay: it should read continuity
  • With the multimeter probes still connected to the 95 and 96 contacts, press or slide the test button to simulate a trip.
  • This will open the NC relay contacts and close the NO contacts. The multimeter should also now read 0 to show continuity.
  • Reset the unit by pushing the reset button.

Thermal Overload Relay Testing Kit

A thermal overload relay testing kit is a sophisticated machine that tests the relay’s operation and tripping accuracy, among other parameters. It works by injecting a very high (but controlled) current into the relay to simulate a situation where the relay should trip. Here’s how it’s used.

  • Turn off all power sources to the relay and disconnect it from its circuitry.
  • Connect the thermal relay testing kit as per the user manual instructions, making sure that all the terminals are properly connected.
  • Turn on the power in a few seconds; you should see readings for both current and voltage displayed on the tester’s screen.
  • Make sure the tested parameters are within the recommended values.
  • The results of the test will help you determine if the relay is operating properly or not and if it needs replacing.

How to Thermal Overload Relay with Multimeter

A multimeter is another device you can use to test a thermal overload relay. The process to use it is also simple enough. Here is how to check a thermal overload relay with multimeter:

  • Start by switching off power and disconnecting the thermal overload relay. Then:
  • Select the resistance mode on your multimeter so that it displays Ohms.
  • Connect one end of the multimeter probe to L1 and T1. Usually, there should be continuity and the multimeter should read 0 Ohms. If it shows an infinite reading (OL), then there is a problem with the thermal overload relay.
  • Next, connect the probes to these contacts: 97, 98. These contacts are NO and should read an open circuit, so the multimeter should read OL. If there’s continuity, the relay is no longer working properly.
Thermal overload relay inspection
Thermal overload relay inspection

Thermal Overload Relay Maintenance

One of the most important aspects of electrical maintenance is keeping electrical equipment in condition. Apart from knowing how to test a thermal overload relay, you should also be aware of its maintenance requirements. This involves the following:

Cleaning: Be sure to keep everything clean and free from dirt or dust buildup, as this can affect the unit’s performance. A thermal relay often picks up dust and dirt from the environment, so it’s important to keep it clean and free from obstructions.

Tightening: Make sure all the screws and terminals are tight, as loose connections can affect the accuracy of the thermal overload relay. Note that any loose connections can also cause a false trip, leading to unnecessary downtime

Inspection: Once in a while, inspect the thermal overload relay unit for any signs of wear or damage. This includes checking for damaged terminals and loose connections or other parts. These can indicate a problem that needs to be addressed immediately.

Checking a bimetallic thermal overload relay
Checking a bimetallic thermal overload relay

Thermal Overload Relay Troubleshooting

Should you encounter any issues while testing a thermal overload relay, then it’s important to troubleshoot them as soon as can be possible. A few common problems include incorrect current settings, damaged terminals, and loose connections.

If the relay cannot be fixed, then you may need to replace it with a new one. Be sure to buy a replacement unit that meets the specifications of the original.

In addition to testing and maintenance, be sure to observe all the safety precautions needed when working with electrical equipment. Always make sure that all power sources are disconnected before beginning any work on the unit and wear safety gear.


Knowing how to test thermal overload relay tripping accuracy is one of the ways you can ensure the safety of your electrical system. It’s also important to maintain and troubleshoot the relay, should any problems arise.

The last thing you want is for the relay to fail in an emergency situation, when it’s needed the most. With proper testing and maintenance, you can prevent such a scenario from happening.

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.


    So happy too see your blog , it is really helpful for me!

    voltage stabilizer working principle

    me agrada la informacion

    My machine’s motor has an exterior thermal overload protector. Isn’t it conclusive proof positive that my overload malfunctioned if my motor “burned up”, which it did?

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