Thermal Overload

This is a topic that we tend to overlook at times and that is the functioning of the Thermal Overload. We tend to remember to wire them into the circuit without paying too much attention to them at all. We will start by having a quick look at the basic operation of the unit.

The thermal overload has a bi-metal strip that will heat up because of the current flowing through it. Depending on the setting and the amount of current drawn, the strip will bend and then create the “trip” condition once it bends far enough. This unit have been around for a very long time, and serves a very good purpose in protecting your motor. The question is whether it is enough?

Consider the time it takes before the strip heats up enough to trip. Now also consider the age of the unit, how accurate is it after heating and cooling during the lifespan of the unit and lastly the set point of the unit.

There are a number of alternatives available these days with the electronic overload units coming to the market, but they are rather expensive. So, you need to understand that there is a time required for the thermal unit to be hot enough to trip and it is your task to ensure these units are set correctly to match the current of the motor they are protecting. You may not believe this but I have seen these units adjusted to a higher setting because they trip! I thought that was why we install them in the first place. If we keep moving the set point, how is that going to protect the motor?

So how do we know if the unit is still accurate? This is where you would need some rather sophisticated test equipment. I know these units as “injection testers” and the principal of operation is to remove the unit you want to test from service, set it up on the tester and perform the test as per the operation manual of the tester. You will then establish whether the unit still trips at the correct current as well as whether it is able to maintain operation without so called nuisance tripping. This sort of tripping can be very frustrating when you are doing fault finding on an electrical circuit

When we consider what current rating the unit needs to be set at, there is again more than one method used in the industry. You will often hear that some people will set them at 110% of the maximum rating of the motor. I prefer not to do that. I will set the overload to match the maximum rating of the motor and then proceed to take a current reading from the circuit when it is operating under full load. Once I have the maximum current the motor draws under load, I will change the setting on my overload to 15% higher providing that value does not exceed the maximum current rating of the motor

So, the next time you hear a Maintenance Electrician telling you he has nothing to do because all his equipment is running fine, ask him when last he checked all the overload settings on his equipment, when last were these units tested and does he have a register of the size and settings of all the overloads in the plant. Bet you that more than 90% will feel it is not needed. Point is, do you agree with them?

And now for the usual, always ensure the equipment you are working on is properly locked and tagged and even then, test before you touch. Never trust the switch unless you have tested. The visual test is simply not enough. Stay Safe and be proud of your work!

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