Circuit Breaker Temperature

[zog]

Scott,
What am I missing when I read the NETA spec you quoted and the information in the SquareD document that I linked to in an earlier post says?

It seems to me that UL is saying a rise of up to 50 ?C at the termination points is permitted by the standard.

Few things here, 1st the Ul is a max spec, part of the design basis, the delta T's I posted are for IR scanning and recommended actions (Apples and Oranges).

IR scanning wont give you the real temp, just surface temp, the actual conductor core temp will be higher.

 

[don_resqcapt19]

This is how I read that article.

Underwriter Laboratories Inc. (UL) standards require that molded-case
circuit breakers rated at 125 amperes or less be marked with the conductor
insulation-temperature rating. Table 1 contains a listing of wire temperature
ratings for Square D circuit breakers. The wire temperature rating is
determined by testing the circuit breaker under full-load current with
conductors sized for the appropriate temperature rating?60?C or 75?C. The
temperature rise at the circuit breaker terminals must not exceed 50?C above
ambient per UL Standard 489.

Yes, the temperature rise must not exceed 50?C above ambient, but the testing standard says that you have a serious problem 29?C below that. I see that as a contradiction.

 

[don_resqcapt19]

Few things here, 1st the Ul is a max spec, part of the design basis, the delta T's I posted are for IR scanning and recommended actions (Apples and Oranges).

IR scanning wont give you the real temp, just surface temp, the actual conductor core temp will be higher.

I don't see how that is apples and oranges. If the standard says 50?C is the maximum design operating temperature, I don't see why a temperature below that should require corrective action.

 

[zog]

I don't see how that is apples and oranges. If the standard says 50?C is the maximum design operating temperature, I don't see why a temperature below that should require corrective action.

Because the 50?C is the failure temp (Slightly above), the actual temps should never get near that and with IR scanning you are seeing a cooler temp than the actual conductors temp.

 

[ELA]

I believe if the difference has to do with whether you are measuring "spot" temperatures or overall average surface temperatures?

When the word "IR" scan is used I take it to mean a more averaged temperature, whereas with "IR Thermography" you can present the temperature gradient and show peak spot temperatures.

A normal operating breaker will have a high peak temperature directly at the thermal overload as it is intended to get hot.

http://www.flickr.com/photos/cbcthermal/1475767562/

 

[don_resqcapt19]

Because the 50?C is the failure temp (Slightly above), the actual temps should never get near that and with IR scanning you are seeing a cooler temp than the actual conductors temp.

I can't imagine a standard that has a maximum permitted temperature that is at or above the failure point. That makes no sense to me.
I agree that I would not want to see a termination operating at 50?C rise over the permitted ambient of 40?C, but to me the standard says that is ok.