Electrical Fire Questions and a Thank you

First, I’d like to thank those who responded to my inquiries about plugs being left slightly, well, ajar in the the outlet (that is, half inserted) and their potential to cause fire. I now know that the risk is minimal at best and that is only with high loads (1000W+). Reading the responses about it had greatly reduced my OCD on the matter to a point where it doesn’t really obstruct my life. That being said, I still am curious about something related to it. Here goes:

When I was freaking out about it I did google searches on it and absolutely could NOT find any statistics about how fires could be caused from a plug being halfway inserted, I looked everywhere and there was no mention of fires caused by it. I know that it is possible.
Is it really that miniscule of a risk that it is not even mentioned? or is it that it may be lumped into a category of other electrical fires/home fires?

I figured that some insurance company somewhere would have statistics on it but the only thing related was if the outlet was wired incorrectly. Any insight into this would be nice. Also I am curious, do AFCI breakers trip if you, lets say have a vacuum plugged in and pull the plug while it’s still ‘on’ and it creates a little tiny blue spark at the outlet from the load?

Thanks again. All of you are the best.

You really need to let this go.

IMHO, it’s probably very, very hard for a fire investigator to determine how far out of the socket a plug was when it’s been in a fire. They would most likely just lump it into a generalized “overloaded circuit” category. As for AFCI, they have a microcontroller inside of them which is supposed to be able to tell the difference between normal plug-disconnect sparks and real faults. Like any device, there is nuisance tripping.

I think it is going to depend a lot on the condition of the outlet, and what is plugged into it.

Average household items, most likely will just ruin the plug and outlet from constant arching.
If the outlet fits good, possibly nothing happens, if its an old worn loose outlet, it might arc and burn up the contacts.

Best practice if to plug them all the way in, then they dont fall out or get bent or arc

I do not know if unplugging the vacuum will trip an AFCI protected outlet, i suppose it could being that an electric motor will usually make a decent little arc if it is unplugged while running.

I don’t think this is correct. Arcing produces very distinct patterns on metal contacts. The patterns typically survive the fire because the temperature of a house fire is usually lower than the temperature necessary to melt the metal. There is a very good chance that a fire investigator could tell that a plug was not properly inserted because they would find the arc points, and the arc points would be at the very tip of the contacts. If the plug and socket weren’t the cause of the fire they won’t show signs of arcing. And a properly inserted plug isn’t likely to undergo arcing even if it is subject to overload (something else will fail first) but even if it is, the arcing would be mid-contact, not at the very tips.

Well, here’s one alleged example: Electrical Plugs Should be Plugged In All The Way | Fire 9 Prevention

I’m not familiar with the previous thread mentioned and I’m sure there is some good information in there, but in general plugging something in halfway certainly could create the potential for fire.

According to this 50-75% of all home electrical fires in the US are caused by arc faults. Obviously these are usually from more serious and permanent arcs than caused by a jiggling a halfway-connected power cable but there is that potential anytime you ever see a spark.

They classify electrical fires by the appliance they started in and not by the actual cause so this isn’t obvious by viewing reports and statistics.

Exactly this, I must have somehow missed that article but it is pretty concise, even states there is overlap on the conditions that can cause it. Thanks Crazyhorse

Beowulff I agree, I am pretty much over it, but the fact that it was never directly attributed, or at least in writing from what I’ve found, was bothering me. Glad to know about the controller that can tell the difference, Thanks!

Weisshund Thanks, I’ve yet to have it happen, but I just want to know if it will ever become a nuisance, I suppose I could use the excercise to run up and down to hit the breaker, but I hope not. Thank you!

I know that the general consensus here is that a half inserted plug isn’t a great fire risk(especially with nothing combustible around), I got that from my previous posts too. Even if nominally past the OCD with the subject, I still have curiosity about it.

Well, as someone who does this for a living… :wink:

A plug that is making a poor connection would be creating a series arc. While the temperature of the arc is high, the energy is fairly low, and the telltale signatures of arcing (arc beads, material transfer, etc.) can be difficult to detect. And that is assuming nothing else “bad” has happened. But a series arc usually causes a parallel arc, and thereby destroys all the evidence of the series arc.

This is what normally happens:

  1. Series arcing due to a poor contact. This is usually caused by a loose connection where a copper wire wraps around the underside of a screw head. Though it could also be caused by a poor plug/socket connection.

  2. A glowing contact might be created by the series arc.

  3. Things around the arc (mainly polymers) get very hot.

  4. Things around the arc will burn, melt, or char depending on their chemistry.

  5. The melting and charing will often eventually create a parallel arc (and thereby plasma).

  6. A parallel arc is very high energy. There will be positive feedback and all hell will break loose. (And the circuit breaker may or may not trip at this point.)

The innards of the receptacle will likely look like a piece of charcoal. :frowning:

So yea, it’s possible a failure could occur “just right” such that there are nice arc beads and evidence of metal transfer at the plug/receptacle interface. But more often-than-not it’s just a big, molten mess.

Thank you, that is exactly the explanation I was looking for, and I can assume from it being a molten mess that its why there are no statistics specifically looking for such a thing as a plug being half out of the socket. But even then the risk is only really there if its a high load device, correct?



That type of failure is probably going to be lumped under plug/socket failures in the statistics.

A plug that is half out of the socket is probably still going to make good enough contact that it’s not a problem, even at high loads. You’re only going to have a problem with plugs that are almost completely out of the socket and are just barely hanging on. You have to reduce the contact area between the plug and socket significantly in order to have a potential problem.

I personally have never seen this be a problem. I have however seen connection problems with old (as in 50 to 60 years old or older) sockets where metal fatigue and corrosion have caused them to make poor contact with the plug. If your home has old two-wire sockets and they are original to the house, you should probably replace them (IMHO).

And yes, you are correct that generally you need a significant amount of current flowing though the poor connection in order to get a significant amount of heat generated. Technically, a small amount of current going through a very tiny point of contact can also get pretty hot, but with that you don’t have as much overall heat and it’s not anywhere near as likely to cause a fire.

What’s a parallel arc? Do you simply mean arcing in several spots at once?

Yeah, thats what I figured with it being high load and poor connection, and yeah I figured as much that it would be lumped into the same category as other fires. Thanks! My curiosity has been fulfilled.

A parallel arc is in parallel with the load/switch. It’s not limited by the load / switch. It’s a short circuit. The energy dissipated in the arc and wiring is much greater than the energy you can get when it’s in series with something plugged into the outlet.

I would agree that it’s pretty rare for a failure to occur at the spring contacts. With receptacles and switches, the primarily problem is the copper wire that is compressed under the screw head (as I noted above). There is no mechanical spring, and you’re relying on compression forces. A loose connection will cause arcing and a glowing contact. (Google “glowing contacts” and “Eaton” for more info.)

While heat generated due to insufficient contact area can certainly lead to problems, the most serious issue with contacts is arcing.


As a root cause of a failure, series arcing is much more common than parallel arcing. Furthermore, a standard circuit breaker couldn’t care less about series arcing; it won’t trip. Meanwhile, the series arc is at an extremely high temperature (thousands of degrees), and things around it will vaporize, burn, melt, char, etc. When hot things vaporize a plasma is formed, and a plasma is electrically conductive. And when things char they also tend to become conductive. These “conductive things” bridge the gap between hot and neutral/ground, and eventually intermittent shorts occur between hot and neutral/ground, often accompanied by lots of sputtering & parallel arcing.

Quick question: I observed that trimming the neutral wire not connected to a fixture, when (key info here) the hot wire for said fixture is disconnected at the junction box, caused the AFCI breaker to trip. What type of fault was it detecting?

Interesting. I have quite a lot of experience with vehicle fires, and I suppose the style of wiring (single wire, vehicle body as earth) and the voltage being much lower makes things different.

I’m just a lower level repair tech and historian, so I have more basic concerns about this.

I see that no one mentioned the most obvious concern about things plugged in half way: the plug tips are contacting the live socket points, and carrying the voltages and potential current out to where things in the room could contact it. It’s essentially like having a pair of bare, live wires hanging out of the socket, instead of being recessed. Still not likely to cause fires, unless something conductive touches them.

Another rather basic thing to realize, is that electricity doesn’t flow like water. It doesn’t spill out of the socket like water from a spigot. It has to be actively drawn out, so to speak. Something has to ground the hot wire,for current to flow. Even an exposed hot wire wont cause any problems, as long as nothing grounds it.

I was taught when installing outlets, to put them in so that the ground socket (the round one) is on top. The idea is to add a small extra help, in case someone DOES pull a plug half way out, and a metal object chances to fall onto the exposed plug contacts. With the ground on top, the object would fall off, rather than close the circuit between the two lower live wires.

Due to the design, we don’t have a problem with plugs half out of the socket, but I did once have a problem with a bad connection.

My dishwasher is under the counter, and it is plugged into a socket above the counter. All domestic appliances here come with a moulded plug attached, but I cut that off as I had to thread the cable through a small hole. I fitted a new plug and the machine worked fine.

The socket was a dual type and one day I was plugging something else in and burned my hand on the dishwasher plug. I turned it off and opened it up, to fund that the clown (me) who had wired the plug had not tightened the screw holding the hot wire . This was making a poor contact and was, presumably, glowing red hot when under load. The plug is made of some fire retardant plastic, but the inside was quite melted.

All else being equal, a DC arc is much nastier than an AC arc.

Having said that, I’m pretty sure electrical arcing in an automobile’s electrical system is not much of a fire risk as I do not think an arc can be sustained at 12 V to 14 V. This paper seems to agree, assuming I am interpreting it correctly.

A couple of things (or both) could have occured:

  1. A reduced contact area will increase the current density at the contact area. This will cause the temperature of the contact area to become very high due to I[sup]2[/sup]R heating. In some cases, however, the problem automatically “fixes” itself, at least temporarily. This is because the conductor materials (usually copper) will soften when the get hot, thereby increasing the contact area and thus reducing the temperature. It’s negative feedback. But it should be stated that this only occurs when there is a force compressing the terminals together.

  2. A poor contact can become a glowing contact. When this happens, a red-hot, moving filament will be created between the two terminals. Eaton Corporation has done quite a bit of research on this topic.