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When taking apart faulty outlets I found melted white wire terminals. I'm told these are the first two outlets on the circuit. The first outlet seemed to melt from both white leads equally without breaking the connection. The second outlet seemed to melt only from one white lead, and got hot enough to melt the wire off the terminal and break the connection, killing the rest of the outlets down the line.

I would blame it on a loose connection or bad plug, but two mistakes right next to each other seems to me less probable. And why only the white leads, not the live wire? Happy to offer more details if they're helpful.

First outlet in the series, white leads (melted) and black leads (fine)

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Second outlet in the series, white leads (one melted) and black leads (fine)

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The outlets are about five feet apart, if that matters.

enter image description here

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  • First I agree @Spehro pefhany to a point. Screw terminals are thousands of times better than backstab. I have seen properly wrapped and torqued terminals do this when the owner swapped out a 15 amp breaker with a 20 then called me because his outlet melted. My guess would be that both were not tightened enough and with years and being fully loaded they started arcing. As far as the patina I have seen that from overheated conductors in dry locations. – Ed Beal Mar 8 '18 at 17:32
  • Which circuits failed as a result, and which circuit breakers are those circuits powered by? Yes, I said plural. – Harper - Reinstate Monica Mar 8 '18 at 21:32
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Looks like they got loose. If the connection is loose it will arc and create a lot of heat if something that draws a lot of current is plugged in.

It also looks like they got wet. There is rust and the copper wire has a patina. That would not help. If the outlets are below a water source that leaked, that could have contributed. Or maybe someone sprayed water into the outlets.

I guess they're making receptacles even more crappy these days, I'm used to brass screws which would never rust (and are more electrically conductive than mild steel). The old bakelite (thermoset) plastic receptacles also would not melt.

Incidentally, this is why connections are made in a fire-resistant box. There are also initiatives to incorporate mandatory arc detection into fuse panels. The arcing has a characteristic signature that can be used to shut down the circuit. Arcing still (rarely, fortunately) causes house fires, but it was worse when they were installing aluminum wire.

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    +1 yup you can still get good ones, but the "home depots" of the world sell cheap crap that are attractive to weekend-sparkies who think they know what they are doing... – Trevor_G Mar 8 '18 at 15:12
  • The main thing I'm wondering is this: looks like the actual problem was an arc at the second outlet. Why did the first outlet also melt? I find it hard to believe that all three of these screws would've been loose when the rest of the outlets in the house were installed adequately. – sam Mar 8 '18 at 15:19
  • @Sam it also depends on what was plugged in and where. Either way this looks like an amateur/cowboy installation so all bets are off. – Trevor_G Mar 8 '18 at 15:31
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    @Trevor_G, re: cheap home depot outlets... That might be somewhat true, but it's all relative. The outlets I get from there and install are still a ton better than the back-stabbed crap the cheapo builder installs. – JPhi1618 Mar 8 '18 at 15:57
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    @JPhi1618 yes, unfortunately, in todays world, the cheapest bid gets the job from the general contractor, often at the expense of quality and workmanship. BUt I rather meant when visiting your hardware store and you have a choice between the $1 part and the $2.50 part, buy the good one. – Trevor_G Mar 8 '18 at 16:03
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I agree with Spehro Pefhany. This happens when the wire is not tightly held down. When that happens, and current is being drawn through the circuit to power something, an arc can form where the connection is poor. That arc acts like a switch so current continues to flow.

However the arc will reach temperatures over 1,000C melting and burning things. Further, since metals expand when heated, the arc actually makes the connection looser so it can be a run-away condition.

That effect introduces a secondary cause. A badly fitting plug used in the outlet can cause arcing where the plug touches the internal connections. The heat caused by that will heat the whole thing and can loosen the screw making the situation worse. Result, smoke, acrid smell, and a hot plug. Unfortunately, this type of plug and socket technology is very old and cheap and, for the currents they are expected to carry, rather badly under-designed. Using even cheaper imported outlets exacerbates the problem.

As Sphero also mentions, it also looks like it got wet at sometime. If this is a basement it may have been flooded at some point. Anyhow, the rusting of the screw just makes the bad connection worse and more likely to arc. Another reason why not to install cheap outlets.

You, or the previous owner, are lucky the house did not burn down. I can't believe someone did not smell or even hear it.

And why only the white leads, not the live wire?

It's about current. It does not matter which wire is loose since whatever current flows out the live wire must return through the neutral.

It also looks like whomever wired that up used cheap outlets and was not a professional electrician. You should check them all and, if you can afford it, invest a few dollars and replace them all.

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Neutrals don't have breakers

There are a number of wiring defects which can cause this to matter.

The entire concept of "not breakering neutral" is that neutrals are protected by the breakers that protect the hots. That presumes neutrals are monogamous to their partner hots.

An obvious way for this to happen is if neutrals are carelessly mixed. It is common when wiring to simply grab any handy neutral without thinking. Grab one not the partner neutral, and you're overloading it.

One defect that will cause neutrals to be overloaded is when a multi-wire branch circuit (MWBC) is incorrectly breakered. MWBCs rely on each of its hots being on different poles or phases, with the shared neutral handling only differential current (which cannot be more than any hot is breakered for). If miswired with hots on the same pole, neutral handles the sum of currents, causing overload! In the early days, MWBCs were simply put on two breakers and pole choice was done by a competent electrician. If breakers are shuffled around by the homeowner, they can land on the same pole. Today, MWBCs require common maintenance shutoff, which calls for 2-pole or handle-tied breakers which guarantee opposite poles. But people confuse 2-pole breakers with duplex/double-stuff breakers, which put both circuits on the same pole.

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I will bet that the 2nd plug had some kind of short to earth, so it drew a bunch of current in through the white wire. That current went from your fuse box, into the first plug through one white wire, out the other white wire into the second plug, and then into wherever the short was. The wires melted at the screw terminals because they had a high resistance, and power = \$I^2R\$.

The short was to earth, which has a much less resistive path than the negative terminal, so you do not see any matching melting on the return path.

Not sure why the terminals melted before your fuse blew. Maybe they were low current plugs on a high current circuit? The plugs I've seen have a junction area so the wire carrying current through does not go through the plug like that.

  • In the US the white and ground are going to have the same potential, so you're not going to get current flowing from white being shorted to ground. – JPhi1618 Mar 8 '18 at 16:10

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