melted receptacle mystery

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charles2

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The attached photos show the plug of a radiant space heater and the Cooper receptacle it was plugged into. The receptacle was backstabbed and the wire on the burnt side was loose, but I don't know if the looseness was a cause or an effect of the problem. I think also that the polarity of the wires was reversed by the installer. What's weird is that the circuit is protected by a Siemens AFCI breaker that didn't trip. When tested at the service panel 2 out of 3 AFCI breakers seem slow to react.

So what's to blame? The installer, the space heater, the receptacle or the breaker?
 

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The plug blade damage is definitely indicative of arcing. Back-stabbing is definitely inferior to screw terminating. The heater definitely caused much current to flow. High current eventually causes the weakest link to rear its head.

It's hard to definitively say what occurred first, but we must conclude that the weak link was the back-stabbing, where small contact area and poor contact pressure combined with the high current was the proximate cause.
 
charles2 said:
What's weird is that the circuit is protected by a Siemens AFCI breaker that didn't trip. When tested at the service panel 2 out of 3 AFCI breakers seem slow to react.
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AFCIs look for arcing. The majority of that thermal damage was not caused by arcing but by joule heating.



Because your dealing with 120 volts, you aren't going to get a sustained arc unless there is heavy carbonization.
 
My vote is for a bad connection, either at the backstab, or between the plug blade and the receptacle contacts. Simple overheating by I squared R heating.

No arcing or overcurrent is necessary, and therefore it could go undetected by any circuit breaker.

Dang, in a few more years there is probably going to be a code requirement for ground fault, arc fault, overcurrent sensing, and loose connection detecting circuit breakers.

Edit: Are there any backstab receptacles where you have to tighten the screws to properly hold the wire in the backstabs?
 
steve66 said:
My vote is for a bad connection, either at the backstab, or between the plug blade and the receptacle contacts. Simple overheating by I squared R heating.

No arcing or overcurrent is necessary, and therefore it could go undetected by any circuit breaker.

Dang, in a few more years there is probably going to be a code requirement for ground fault, arc fault, overcurrent sensing, and loose connection detecting circuit breakers.

Edit: Are there any backstab receptacles where you have to tighten the screws to properly hold the wire in the backstabs?
Click to expand...

Yes kind of, pressure pad. #5362 ?
 
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I would say the receptacle. Look how the top receptacles contacts cover the whole slot but the bottom contacts only come down about 30% of the way. I have seen similar burns when the neutral wire was not properly tightened down on the terminal screw. It could be loose quickwire terminals but I'm thinking it's just a poorly designed receptacle. Remind me not to use Cooper receptacles.
 
One of the important design considerations is assuring that a spring never carries current. Only a little bit of overcurrent heating will cause a loss of spring tension, which will then cascade into more heating, then more loss of tension, then still more heating, et cetera.

I don't know whether that was the case here, but I've seen it often in cheap devices. Could you disassemble it (further than it has already self-disassembled) and let us know?
Whatever the case, I'm disappointed a plug & receptacle failed while used within their published specifications. (I'm assuming the heater was no more than a 12-amp load?)
 
I had a case where a receptacle with nothing lugged into it caught fire, and took a bed with it, due to a heater plugged into another receptacle on the same circuit. Like I said, high current exposes the weak link.

Coppersmith said:
I love the title of this thread. It sounds like a Nancy Drew book. :)
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I wold have said The Hardy Boys, but that's just me. :D
 
LarryFine said:
I had a case where a receptacle with nothing lugged into it caught fire, and took a bed with it, due to a heater plugged into another receptacle on the same circuit. Like I said, high current exposes the weak link.
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I have seen happen first hand. It was a feed through.
 
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charles2 said:
What's weird is that the circuit is protected by a Siemens AFCI breaker that didn't trip. When tested at the service panel 2 out of 3 AFCI breakers seem slow to react.
Click to expand...

Just something you need to understand here because it sounds like you have some kind of faith in AFCI. AFCI breakers are useless and about the only thing they do is transfer money from your wallet to the manufacturers bank account. What you witnessed is a perfect example that they don't work. No mystery or weirdness at all.

-Hal
 
Sparks4All said:
I would say the receptacle. Look how the top receptacles contacts cover the whole slot but the bottom contacts only come down about 30% of the way. I have seen similar burns when the neutral wire was not properly tightened down on the terminal screw. It could be loose quickwire terminals but I'm thinking it's just a poorly designed receptacle. Remind me not to use Cooper receptacles.
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That's interesting.

Is it just me, or does it also look like all the receptacles in K8MHZ's collection were all burnt on the bottom half?
 
LarryFine said:
I had a case where a receptacle with nothing lugged into it caught fire, and took a bed with it, due to a heater plugged into another receptacle on the same circuit. Like I said, high current exposes the weak link.


I wold have said The Hardy Boys, but that's just me. :D
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I wonder how many of these events progress into full blown house fires?
 
ptonsparky said:
While I’ve had the backstabbed receptacle on the list, I agree those heaters are the common denominator and IDC if it cost $24.95 or $249.50. Guilty!
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When end I was younger and living at home, before I ever thought about getting in the trades, I can’t tell you how many hours I would be at home and leave a heater plugged in and running in my bathroom during the winter...1970s wire... looking back I bet I really put a stress on the wire insulation and connections.
 
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