Hello to all Breaktime members —
I’m a new member and this is my first post, prompted by an electrical outlet mystery that occurred in my Seattle house this weekend —
A standard Leviton 15 amp duplex outlet in our finished attic, in the middle of the night, shorted out (apparently), sending sparks flying and nearly starting a fire. The circuit breaker kicked in and interrupted power.
Here’s the mystery: there was nothing plugged in to the outlet, it was drawing no current, and when I took the darn thing apart to replace it, there was absolutely no evidence of improper wiring, loose wires, cracked insulation, etc. The damage appeared to be internal, inside the bottom outlet.
So here’s my question: can properly wired outlets spontaneously fail internally, showering sparks out to whatever is nearby?
I am in the process of replacing all of the outlets on that floor, but after 10 of them, I’ve found nothing wrong… Some of them are probably 30-40 years old, but appear to be in perfect shape…
Also, as a note, we DO NOT have any aluminum wiring in the house…
Any bright answers to this perplexing mystery would be much appreciated!
Edwardo from Seattle
Replies
First question. How many wires were attached? Was it feeding through to something else? Current draw farther down could have caused a connection to overheat.
But I have seen outlets fail only a little differently -- when one was removing a plug (ordinary lamp) that caused some very minor arcing. Once an arc gets started the plastic carbonizes and provides a conduction path.
Another thing to consider: Any animals (invited or otherwise) in the area? A little mouse or cat pee could easily provoke this.
It was the next-to-last outlet on the run. The final was a light, that was not on. As far as animals/insects/pests, either in the attic or behind the wall, there was absolutely no evidence -- I crawled back behind the kneewall and examined the whole run of wire, and back of the boxes...
Bug commited suicide?
That would be known as insecticide, LOL
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
Whoa! That's scary. In a new house, I once had an outlet spark and spit flames the first time I plugged in a lamp. Was probably a bad outlet.
As far as a reason for the short circuit in your case, well, it sounds like a short circuit. That is, a neutral wire or the equipment grounding conductor (bare copper ground wire) came in contact with either the hot wire or the hot receptacle terminals. Or vice-versa.
How could this happen? Many possibilities. A friend had a house fire start in a receptacle outlet box, from a short circuit. A rodent had chewed the insulation off the wires, and the wires touched. Your problem could have been a loose wire that finally moved enough to cause a direct short.
How would a wire move all by itself? Gravity and vibration. Houses vibrate, or maybe I should say that houses with forced air heating vibrate--vibration of the forced air unit (blower fan) usually causes slight vibration in the entire house. If there was a loose wire in the box, or if the bare copper ground was not tucked carefully against the side of the electrical box, it's possible that over time, under the influence of gravity and vibration, the a neutral or ground wire touched the hot terminal or a bit of the hot wire. Or the hot wire was loose and toughed the neutral or ground wire. This is a direct short, and can cause an arc fault.
Now, a solid contact would produce a solid (or "bolted") fault, and the breaker would trip. To arc/spark enough to cause a fire, the short would have to be sustained, and this can occur from a grazing or light contact.
When the wires lightly touch (or a wire touches a terminal) and produce an arc, the resulting very slight arc blast can push the wires apart, interrupting the arc. The wires then go back into contact, and a sort of stuttering arc is set up. This is bad news, as it may not cause enough current to flow to trip the breaker, but it may release enough energy to set fire to the paper filler and other debris in the outlet box.
Sometimes the fault will burn itself clear after some arcing. The wire melts and so stops the stuttering contact.
This is just one possibility, based on your assessment. But it's based on direct knowledge. I've seen the results of stuttering arc faults on 120v circuits, and it's amazing how long they can be sustained before the breaker trips.
Once I saw a case where a sewing needle had fallen across the hot and neutral blades of a plug that wasn't completely inserted into the receptacle. The needle had chattered and arced, and had cut halfway down through the blades of the plug! The cover plate and wall above had a bit of soot on 'em. Finally the breaker tripped. This was in a bedroom where two little girls slept; it gave me the cold sweats thinking about the consequences of a fire.
Would an arc-fault breaker have protected against this sort of fault? Probably not--arc-fault breakers are designed to protect against series arc faults, and not parallel arc faults.
This just goes to show you that the devil is in the details on electrical work. Was the installation done properly--no excess bare hot wire, the ground wire routed properly, terminals tight, and all excess paper filler from the cable and other crap removed from the box?
Were the wires connected to the receptacle using the terminal screws, or a clamp system, instead of using the feeble poke-in backwire hole connections? The poke-in backwire connections are UL-listed, but they can come loose over time, especially if an outlet sees heavy loads (like a vacuum cleaner). Then you've got a loose wire in a box, and the makings of a fire.
Consider yourself lucky. I was hired by one client to go through the entire electrical system of his house after there was a minor electrical fire (due to a bad splice in a bathroom vent fan). I found a few outlets with reversed polarity and some bad backwire connections. He felt it was money well-spent. I was happy to save him, at best, the inconvenience of a dead outlet, or at worst, another fire.
Cliff
Cap - thanks for the detailed reply --In regard to the wiring in the box that had the short, when I pulled it out to look at it, there was nothing wrong that I could see. The wires were all properly attached (screw-attached, not poke-in), and they were not loose AT ALL. --Very disturbing, to say the least, to think that an outlet itself could just be defective that way, and self-destruct in a fiery manner!
"Would an arc-fault breaker have protected against this sort of fault? Probably not--arc-fault breakers are designed to protect against series arc faults, and not parallel arc faults."It has been a while since I read up on AFCI's, but don't you have it backwards.I thought that they detect parallel arc faults."How would a wire move all by itself? Gravity and vibration. Houses vibrate, or maybe I should say that houses with forced air heating vibrate--vibration of the forced air unit (blower fan) usually causes slight vibration in the entire house."Last week I noticed some water dripping on the floor under the toilet.Started checking to find the source and it was come out the bottom where the fill valve comes through. Touch the nut that holds it in place and holds in the seal and it was loose. Took 1/2 turn to get it to start touching.Hasen't been touched for 10-15 years.
Bill,
I do believe you're correct--a parallel fault of something like 50 or 70 amps. And from what I've read, the GFPE (ground fault for protection of equipment) feature of the AFCI breakers (which trips at 30mA, unlike a personnel protection GFCI, which trips at 5mA +/- 1mA), would be the most likely way that AFCI breakers will prevent fires in the building wires--an arcing fault will create enough heat to melt the wires's insulation, and a ground fault would ensue.
The AFCI breaker will also trip if there is a series fault of a certain type/current, such that the AFCI chip recognized the current signature.
The point is, AFCIs will detect certain types of arc faults, but not all--because some appliances (like some motors) can look like certain kinds of arc faults.
Anyway, sorry to pass on bad info, and thanks for catching it.
Cliff
Attic receptacles get a lot of abuse. They are baked and frozen. The tradesmen who use them are busy and in a hurry to get into a better location. Heat, cold, fiberglass, lousy lighting and a constrained space all make for grumpy workers.
Likely the plastic of the receptacle had become brittle. An extension cord plugged in and yanked to get that extra foot or a heavy boot that whacks the receptacles would also take a toll. Once cracked the receptacle could fail at any time. A common fault is the face of the receptacle falling off allowing the contacts and bars within the receptacle to contact each other.
Also if the box it metal abuse can cause the receptacle to shift enough to bind a wire or, in a few cases and if the screws are loose, allow the screws on the side of the receptacle to contact the box. I have seen such a loose contact to 'cook'. Overheating the box and possibly allowing sparks to fly.
In such isolated locations and where receptacles are likely subject to abuse I push for nylon face welded receptacles to be used. Shops, kitchens, service receptacles and behind the headboards of beds. P&S makes a nice line of these that is not much more expensive than a regular make.
Good to hear it worked out for you. A fire in an attic is, according to a firefighter friend, a fireman's nightmare. In an attic a fire can grow, spread unknown to the people below. Then, at its leisure and often all at once, descend into the living area. Enveloping the house in seconds and trapping occupants.
I have pushed for heat sensors in the attic wired into the smoke alarms. Also some care should be used when determining what will be stored in the attic.
You make some good points, but in this case there are no visible cracks, and no burn marks on the inside of the metal utility box to indicate that a terminal screw had come in contact with it. As I said, all the screws and wires were tight and properly connected. A real mystery... The only clue I have is that outlet had been occasionally used with a space heater, and I wonder if the current draw (maybe 1500 watts) was enough to damage the interior contacts on the outlet, leading to its failure....???
A poor contact on the space heater plug prongs could have overheated the outlet. This could result in carbonizing of the plastic, creating a path for current to flow. Once a path is established, it will reenforce itself until "meltdown" occurs.Another possibility is that a piece of a contact broke loose (possibly due to overheating removing it's temper) and shorted between the sides.Also, newer outlets seem to be made with thermoplastic vs thermosetting materials, and overheating could have caused the outlet to deform, allowing the contacts to touch each other.
Edited 4/5/2005 2:02 pm ET by DanH
You could be right about the overheating through the space heater plugs --As far as thermoplastic/thermoset, this particular duplex outlet (Leviton) was probably made in the 60's or 70's, not new at all-- but (from the outside) no visible defects/cracking/etc.
Consider it one of those once in a lifetime things. It probably was a manufacturering defect. Replace it with a good quality unit and relax. Unexpained things do happen but a detailed examination of the plugs insides would probably reveal the real cause. You might have to carefully disassemble or cut apart the device to find the evidence. If there was sparks and a breaker tripped, the evidence will be inside the plug.
I like your approach....now lets see your departure
I think you might be right -- I intend to carefully disassemble the outlet to try to determine the cause of the sparking/shorting -- but I may have to cut it open -- no easy way to get at the interior...sort of an electrical post-mortem autopsy (or outletopsy ??) for my own peace of mind...
Cutting it may destroy the evidence you're looking for. X-rays would be ideal if you can find someone who can do them on the side. Check it with an ohm meter, which may reveal carbon trails.
-- J.S.
Your mention of carbon tracking beings up a possibility. A high voltage surge, lightning or switching error, and possibly moisture, dirt and cracking could, in some combinations, cause carbon tracking. I also wonder if the box might feed a doorbell transformer. I have seen where one shorted out, possibly caused by a surge, and essentially vaporized a length of the fine gauge doorbell wire. Until we figured out what went on it was a bit of a poser as to where the line burned into the fiberglass bats came from.Edited for spelling.
Edited 4/5/2005 9:43 pm ET by 4LORN1
I had an outlet do something similar, though it was old and was in the garage for the garage door opener. I unplug the opener, pulled the outlet down and looked for something obvious. Nada. Tripped the breaker even with no load. I had a replacement handy so I put it in and everything is fine. And the old outlet looks like a decent quality one too, not a 39 cent special.