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What are Hot and Cold State of Thermal Overload Relays

09/03/2021 2:28 PM

Thermal Overload Relays have a hot and a cold state represented by their respective tripping curves. I have never been able to understand these despite going through pages after pages on google. There is no clear definition for either.

Can anyone define in easy terms when is the relay considered hot and when is it considered cold and when to follow which curve.

Thanks a lot

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#1

Re: What are Hot & Cold State of Thermal Overload Relays

09/03/2021 2:42 PM

https://download.schneider-electric.com/files?p_enDocType=Tripping+curves&p_File_Name=30068-index.pdf&p_Doc_Ref=30068-index

You will find your information here,but you will have to read it to the end.

It is not a simple one-size-fits all solution.

There are many variables to consider.

And specs vary from one manufacturer to another.

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#2

Re: What are Hot & Cold State of Thermal Overload Relays

09/03/2021 10:58 PM

Hot = motor has been running.

Cold = motor has been off for a period equal to or greater than its cooling time constant, which is different for every motor.

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#3

Re: What are Hot & Cold State of Thermal Overload Relays

09/04/2021 1:01 AM

The motor protection relay constantly calculating the thermal content in the motor using the motor parameters inputted such as cooling & heating time constants, motor starting current and time, hot and cold withstand times, allowable number of successive starts per hour, Thermal Overload threshold setting etc.

When the thermal content falls to <25%, the motor is considered cold in case of a motor rated with two hot and three cold starts per hour.

Above I have taken from Siemens relay manuals.

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#4
In reply to #2

Re: What are Hot & Cold State of Thermal Overload Relays

09/04/2021 2:58 AM

Understood.

Now, what we do as part annual preventive maintenance is that we remove the thermal overload relay and test it by injecting a current of 1.5x the dial setting. It trips after a certain time, usually in under 2mins. We verify the tripping time using the Cold state curve. Then after 2-3mins, when the relay has cooled down & reset, we test it again now with current 2x the dial setting. So should we follow the Cold curve or the Hot curve for the second test.

By your definition, results of both tests should be verified with the Cold curve as HOT is only for motor running scenario.

Please correct me if I`m wrong.

Thanks

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#5
In reply to #4

Re: What are Hot & Cold State of Thermal Overload Relays

09/04/2021 6:24 PM

I would expect a bimetal overload to come close to cold condition in 2-3 minutes, maybe, perhaps eutectic alloy heating elements might take longer.

I think for your test, it is best to use the cold curve for both tests, but perhaps expect a little premature opening for the 2nd test.

Also, there exist ambient and non-ambient compensated relays. So your testing ambient temperature should match the conditions of your installation.

This is very thorough testing in my experience. If we ever tested, it was to see if they worked at all. In one case, we had new equipment, maybe 20% of the bimetal relays did not operate at all(!). If we operated high value motors, we would use a comprehensive motor protection package with RTDs such as MultiLin or similar

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#6
In reply to #4

Re: What are Hot & Cold State of Thermal Overload Relays

09/12/2021 6:49 AM

One test should be enough to verify control of termal rele. If it is electromehanic construction you should make links between all three phases and with one current test, usualy with 2 In, deside if it is OK. If you test just one phase time for rele should be 10% longer.

Good luck

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#7
In reply to #6

Re: What are Hot & Cold State of Thermal Overload Relays

09/12/2021 12:26 PM

You bring up another very good point, testing 3 at once or individually. Some overload relays such as Siemens models have a lower trip characteristic for unbalanced line currents, to provide excellent single phasing protection. This could be more valuable than accurate motor overload replica, for protection for loose connections from starter to motor.

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