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I noticed a burning plastic smell coming from my 15-year-old AC unit pictured below (this is in the attic, the heater/blower unit, the type that has heating strips).

What causes this? Why the circuit breaker did not flip? What is a protection against such an event when circuit breaker is not sufficient?

Does replacing the breaker unit resolve the issue?

(I have since disconnected the panel breaker for obvious safety and fire hazard issues.)

enter image description here

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    Could be from just a loose connection at the breaker(best case), but also a bad breaker(not tripping) plus problem/s with heater. Should have the circuit and heater checked out before turning on a new breaker(that one is toast).
    – crip659
    Feb 11, 2022 at 19:56
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    That's a huge fire hazard. If you use the existing wire, be sure to sand off the oxidation from the extreme heat. Otherwise it'll reduce conductivity and end up overheating too. Definitely consult with an electrician what the problem is. He/she may have insights that you don't realize.
    – pmont
    Feb 11, 2022 at 20:20
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    A circuit breaker usually trips if more than the rated amperage goes though it. 60 amp breaker will trip over 60 amps, but your case probably using less. Don't need that many amps to heat something red hot, 15 amp toasters/30 amp dryers. Expensive just to replace breaker, cheap to check everything out.
    – crip659
    Feb 11, 2022 at 22:28
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    Presumably you noticed it from the smell of burning plastic, which it will to some extent from now on. Taking a $20 disconnect out, cleaning the contacts, squirting some goo at it, and putting it back where it was once half way to being on fire is not going to be on a docket for my attic. Also, a vacuuming is in order. And check the ground lug.
    – Mazura
    Feb 12, 2022 at 6:06
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    @pmont This is more than a fire hazard. This is a fire almost started. I would turn it off first and photograph second.
    – fraxinus
    Feb 14, 2022 at 18:36

1 Answer 1

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Oh crap, turn it off. If that is the feeder side turn it off at the main.

You have a really bad connection. Let it cool down prior to doing anything!

It is possible and highly probable it is just loose and not arcing yet. If this is the case it can be fixed really easily.

Once cool loosen the screw and remove the wire. Is it all there or burned off?

Next run the screw down and make sure it is ok. Loosen the screw and remove any debris. A touch of an anti oxide compound like noalox or deox on the wires would be a good idea.

Put the wire under the clamp and tighten (of course you should be using a torque driver). I usually tighten, loosen, and tighten 3 times. Then wiggle and pull the wire--it should be secure.

If the wire was damaged from arcing I usually replace the breaker as the clamp is also damaged. Cut off and strip, then jump up to putting the wire in and tighten.

I see this quite often and most of the times it will be fine. I am not really concerned about the breaker for 2 reasons: it’s a square D QO and it is a double pole. The same amount of current is going through the right side but it’s tight.

If there is physical damage to the breaker it should be replaced but from what I see it’s just loose and not arcing.

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    You're not concerned that any of the soft components of the breaker have deformed, or that the heat has oxidized interior connection parts? I'd want the breaker replaced.
    – isherwood
    Feb 11, 2022 at 21:27
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    Gotta totally agree with Ed's answer... my first reaction was about the same "holy crap"! It's most likely a very bad connection and when resistance heating (aux heat) turned it revealed the bad connection. In this case, we don't know if the breaker was damaged or not and they aren't expensive. So I'd recommend replacing the breaker, cutting a inch or so off the wire, re-strip it, install the new breaker, insert the wire and torque to specs. It looks like copper but if AL, be sure but the "goop" (Noalox) on it first. Feb 11, 2022 at 22:30
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    Am I concerned, with what I see and have seen many hundreds of times in residential, commercial and industrial is a loose connection will cause this every time, I have seen up to 500 mcm or kcmill the same size different name based on how many years in the trade. If the wires have not started arcing they are good I mentioned how to check. Can things deform sure then if loose the arcing starts the wire burns off and the breaker is damaged, this is more likely with higher voltage but can happen with 120/240, a little noalox or deox and re torquing has never failed me if the clamp was not arced
    – Ed Beal
    Feb 12, 2022 at 2:55
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    There IS physical damage. That breaker should 150% be THROWN OUT and replaced. The contacts are not rated for 700C operation, nor is anything else in there. It is no longer reliable and should absolutely not continue to be used.
    – J...
    Feb 12, 2022 at 12:32
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    Regardless of whether the breaker is visibly damaged, it's been exposed to extremely high temperatures for an extended period of time. Replace the breaker.
    – bwDraco
    Feb 12, 2022 at 16:07

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