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My dishwasher was plugged into this socket, and then this happened, tripping the circuit breaker.

enter image description here enter image description here

So I assume there was a short in the outlet, but how could that have happened? I replaced the outlet and plugged the dishwasher back in, and all seems to be working well.

There was no sign of water in or near the receptacle, and I can't imagine a piece of metal or wire somehow got into the wrong place. I'm now concerned it might happen again, but I have no idea where to check.

  • Not an answer, but when you replace this, shorten the in-wall leads a little, if there's enough spare.... the bare bits shouldn't be reused. And consider wrapping a plastic water diverter around the appliance's lead in case water ever does track down the outside. – Criggie Aug 14 at 0:27
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    Remember to check that the plug is clean and not burned, otherwise you should replace it also. – jpa Aug 14 at 9:37
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    It witnessed the interior of the Arc of the Covenant. ;) If it weren't for the text of the question giving the picture context, I'd have thought that this was some sort of "what happened in this internet meme" question. – nick012000 Aug 14 at 12:41
  • Where were the wires connected - to the screws, or were they stuck into the push-in fittings at the back? – J... Aug 14 at 20:50
  • A bad connection somewhere. – Hot Licks Aug 14 at 22:04
33

The most likely explanation is that the screws were loose causing a poor connection, and arcing under load - evidenced by the fact that the base material has gone brown where it's been hot over a long period. What happens in this case is that the contact surfaces slowly carbonize (so become higher resistance, and get hotter), and it gets progressively worse

You can see that the right-hand screw-terminal has actually moved because the plastic melted. I assume that it moved enough for the contacts inside to either come into contact or arc between themselves, and that's what tripped your breaker.

It's unlikely this will repeat itself, provided the new outlet has been wired correctly, but it wouldn't hurt to keep an eye on it for a few weeks, just to be sure. Also, as mentioned in the comments above, check the plug and replace if there's any evidence of damage.

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    It might be worth ensuring that the dishwasher isn't placed in such a manner as to transmit continuous vibration to the receptacle. – chrylis -cautiouslyoptimistic- Aug 13 at 22:24
  • @chrylis-cautiouslyoptimistic- Dishwashers generate hardly any vibration, and what they do certainly isn't going to be transmitted to the outlet via the flex. – SiHa Aug 14 at 6:05
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    Via the flex, no, but if the dishwasher is pressed up against the cord, compressing the plug? – chrylis -cautiouslyoptimistic- Aug 14 at 6:12
  • As I say, dishwashers generate virtually no vibration, other than noise, but I see your point. – SiHa Aug 14 at 7:32
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    OP didn't say, but it might have also been poked into the backstabs... that's usually a recipe for magic smoke. – J... Aug 14 at 20:50
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Another thing to look for is aluminum wiring - I don't know if that's "a thing" in your area/house.

AL wiring gets a mostly deserved bad rap, but the worst thing is to connect it to a non-AL-compliant (wrong type) socket and then make it even worse by drawing lots of current through it.

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    Everything is copper here. – Patrick Szalapski Aug 13 at 13:44
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    You don't need lots of current for aluminum to copper direct connection to overheat. As much as couple of amperes in often enough. Yes, tens of amperes get it burnt faster. – sharptooth Aug 13 at 15:03
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At this point, being disconnected from any wiring, it's difficult to impossible to tell what happened. Clearly there was excess heat.

My first inclination is that there is a problem in the dishwasher itself where it's developed a short circuit and is drawing excessive current. That might account for the damage to the outlet if it was improperly installed or just defective.

I'd check the dishwasher itself carefully before plugging it back in. Chances are there is similar damage in it somewhere.

Also, is it possible you have a circuit breaker on this line that is too large? You should check that too.

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  • Dishwasher is working great now. How could there be a short in the dishwasher if it is working fine? Maybe if it's intermittent... – Patrick Szalapski Aug 13 at 13:45
  • The OP should also examine the plug itself. If the prongs are not shiny metal, then make it so. – Mike Waters Aug 14 at 2:51
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This is a classic signature of arcing. This is what arc-fault breakers detect. It happened in one of two ways:

  • Improperly torqued screw.
  • The spring which holds the plug blade got sloppy, probably because this is a cheapie ($1.00) receptacle

As to screw torque

EC&M Magazine made a test rig and took it to four trade shows. Their data was interesting. They tallied under-torqued, torqued within 20% of spec, and over-torqued - and whether a person was an electrician or not. Both groups turned in exactly the same scores.

it was obvious during the test that people who had never made an electrical connection before had as much chance of getting it within 20% as an electrician with decades of experience.

And given the broad tolerance (+/- 20%), it's as likely that the 25% correct were simply attributable to luck.

Cheap outlets

The presence of "backstab" holes reveals this recep to be a cheapie "Builder grade". They are made to be cheap and fast to assemble, so the builder can get to closing, collect a signature and get paid. When a maintainer has a long-term stake in the reliability of the outlet, they tend to go for "spec grade" or "industrial-grade" outlets. These are in the $2.50 to $10 range, and have better everything - including deletion of the "backstab" connection often replaced by a screw-and-clamp arrangement.

The one thing going for backstabs is you can't screw up the torque setting. But they have so many other problems they are not worth using.

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6

We can only guess, but another reason that happens is simple long-term use by a high-current device. A space heater that draws just under the current limit for extended periods, maybe with a worn outlet, results in slow but significant heat buildup.

I've seen the same thing when extension cords are used for such devices. The plug ends will actually get hot to the touch or melt but the breaker won't trip.

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    and a dish washer uses high current to heat the water. – Jasen Aug 13 at 12:51
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    When I was a kid, the socket for our fridge was very brown and crusty. My dad was an electronics engineer and really should have known better. – SiHa Aug 13 at 13:02
  • This could be, as the dishwasher worked fine here for 6 years. – Patrick Szalapski Aug 13 at 13:44
  • @SiHa assuming it wasn't just dirt. Who cleans behind their fridge? – user253751 Aug 14 at 16:58
1

It could be that suds, not water, overflowed the machine and made it's way to the outlet.

Did someone put in the wrong kind of detergent?

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Were the wires under the screws or in the push-in “backstab” connections on the back of the receptacle? Backstabs are notorious for failing exactly like this - the contact area can be much smaller than a proper screw connection, leading to heating and meltdown.

Make sure that your new receptacle is using the screw connections with the wire bent into a hook shape that wraps all the way around the screw, and under the screw all the way around - not squeezed out to the side anywhere (unless it’s the clamp type, then keep the wire straight).

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    Yes, they were using the screw connections, and seemed all properly connected and tight to me. – Patrick Szalapski Aug 13 at 16:28

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