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Thread: Allowable temperature rise in Breakers

  1. #1
    Join Date
    Feb 2006
    Posts
    2

    Allowable temperature rise in Breakers

    I work in a factory maintenance department.
    Recently a thermal imaging contractor came in and scanned all of our panels for hot spots.
    They came up with several breakers that they say are having problems.
    In each of these cases they are 20 ampere 120 volt breakers that have a continuous load of 11 to 15 amperes on them for lighting or some other load.
    I have checked the wire and breakers for any discoloration and nothing is apparent.
    My thoughts on this are there is going to be some heat generated in the breaker and wire with usage, because everything has resistance to some degree.
    What is a normal temperature rise in a breaker under continuous use? The breaker is marked 40 C but I can’t find any data on what that stands for.
    The temperature rise the thermal imaging people documented is from 9 C to 27 C above ambient.
    Thanks for your help.

  2. #2
    Join Date
    Feb 2005
    Location
    Charlotte, NC
    Posts
    6,357

    Re: Allowable temperature rise in Breakers

    A 9 degree (C) rise above ambient is not much to worry about, 27 degree rise may be a problem, they should have given you recommended corrective actions for each scan and a reference for the recommendations.

    I would compare similar loaded breakers and investigate any major differences, are the hot spots from the contacts or connections?

  3. #3
    Join Date
    Feb 2006
    Posts
    2

    Re: Allowable temperature rise in Breakers

    Their recommendation was to burnish, clean, and tighten the connections. I inspected and tightened the connections and they were already tight and looked clean with no discoloration. I even replaced one breaker just to see if it still gets warm. As of yet, I haven't been back to check it. The thermal imaging picture seems to indicate the heat is coming from inside the breaker, but I couldn't say for sure.
    I would assume there has to be a allowable temperature rise given by breaker manufacturers, but so far I haven't found it.
    Thanks.

  4. #4
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    19,139

    Re: Allowable temperature rise in Breakers

    The following is from SquareD (type in temperature and select circuit breaker in the "drop down")
    What are the allowable temperature rise limits for circuit breaker parts? Customer is doing thermal imaging.

    Answer UL Surface Temperature Rise Limits
    Non-metallic 60 deg. C
    Metallic 35 deg. C
    UL Terminal Temperature Rise
    80% Rated CB 50 deg. C
    100% Rated CB 60 deg. C
    Another question on the same site says that the rise is over a 40 deg. C ambient.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  5. #5
    Join Date
    Feb 2003
    Location
    Florida
    Posts
    1,127

    Re: Allowable temperature rise in Breakers

    i have been doing infrared thermography since 1989. much of my knowledge was self taught. yes i went to the school when i bought the camera --- but little was taught concerning the electrical system use of the camera --my class was full of roofing contractors --so thats what was taught!! originally they taught us that that 20 to 25 degree rise on that 20 amp breaker was an upcoming problem. guess what? the next time we did that building --yearly or bi-yearly -- that same 20 amp breaker was bad again!!! no there was no problem -- a breaker loaded to near 75 per cent of it's rated load is going to be hot!! and if it is an a/c load -- the compressor may have been running for hours!!

    the real purpose of the infrared scan isn't to look at low level branch circuits -- this thermographer should be consentrating the efforts on equipment that will cause the building to shut down!!! i've been through this myself.

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