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Let me start by saying I have ceased the use of this outlet, and will be contacting maintenance for a replacement.

I have a grounded outlet in my apartment that has had a power strip plugged into it for nearly 2 years. The plug on the strip is 3 prong without the wider blade of a polarized plug. I believe I may have bumped the cord when I was running some cables nearby as the power strip had lost power. I checked the plug to find it ever so slightly tilted out of the socket (tipped down, pivoting on the ground prong if that matters). Upon tipping it back to be fully seated I heard the telltale sound of an arc happening.

I pulled the plug out and found a scorch mark on a very specific spot on the left prong. Upon re-seating the plug I heard it again. I shut off the strip and moved it to the plug above on the same outlet. I figured I had a bad connection in that particular socket, so I plugged in my lamp which has a polarized 2 prong plug. No issue. Not even when wiggling it.

So now I thought, well maybe only a non-polarized plug will have a bad connection due to the size. So I plugged in a simple night light. It has 2 prongs and is not polarized. Again, no issue. Regardless of the orientation or any wiggling.

So that leaves me wondering, what is so special about the power strip that makes it arc? Is it that it's grounded? Is it the amount of power being drawn?

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  • There is nothing special about a normal power strip with three prongs. A probable cause of why your power strip is arcing is that it was damage when "bumped" and now has an internal short. This short is what is causing the arcing when plugged into power.<br/>You are right to not use that outlet and until it is replaced. While it may be working, the outlet could have internal damage that cause issues later.<br/> Throw away the power strip was arcing. DO NOT plug into other outlets to see if it works. – Programmer66 Apr 11 '20 at 20:47
  • @Programmer66 I should have pointed out, the strip works just fine in the other outlet and only arcs in the one in question. The status lights on it for ground and protection are still functioning, so it doesn't have an internal short. – Logarr Apr 11 '20 at 20:54
  • Outlets are 60 cents. Or $2.50 for the good ones. – Harper - Reinstate Monica Apr 11 '20 at 22:56
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When you "bumped" the grounded cord, you bent up the grounding clamp of the outlet and made momentary contact with the neutral side of the outlet causing the arcing. When you removed the grounded plug and just used a two prong plug the ground inside the outlet didn't make contact because no ground plug was inserted.... Those outlets are very close quarters inside and can arc over with little pressure when damaged. I'd turn off the breaker for that outlet until it gets replaced.

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  • Would an arc between those connections internally cause a scorch on the right side of the left prong on my power strip? Unfortunately I can't turn off the breaker as it powers our entire living room, but I may remove the outlet and tape off the wires if it's a serious threat. – Logarr Apr 12 '20 at 0:45
  • @Logarr Yes, the left prong is the neutral and the prongs being inserted into the outlet would make the connection between ground and neutral. It is a serious threat. Turn off the breaker while you remove the plug. Contact the landlord. – JACK Apr 12 '20 at 13:49
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You don't need to do too much analysis. The response to arcing is always the same: Kill it with fire, before it kills you with fire.

Plainly from your question, you have inspected the plugs closely, and have nothing special to report about them. That narrows it down to the receptacle.

The internal springs in receptacle contacts do get fatigued, bent or broken. That fact is not anything exciting, nor worthy of any forensics unless you are bored. Since you have excluded plugs, what remains is to swap the receptacles. Receps are 50-75 cents for the cheapies or $2.50-$3.00 for the good'uns, or $10 for the hospital-grade (whatever that is).

Generally you are not allowed to do your own electrical work in a rental unit. However some jurisdictions make exception for trivial work such as swapping receptacles, light switches or lamps. (though each can be quite complicated actually).

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Arcing only happens when a significant current is running. No current, no arcing. That is a law of physics.

Some loads, like most power supplies, draw a significant current for a short moment, just when connected. If the room is dark and I plug in my laptop, I can see a tiny spark, every single time. When I connect or disconnect my vacuum cleaner, there is a spark. There is nothing wrong about that. I can't make the same happen with a regular light bulb though.

If there is a continuous arc that's a real problem. That means that there is bad connection, with a gap of air between two conductive parts and current is passing through the air via an arc. An arc can only be sustained as long as current is running.

In theory the arc could be from live (or from neutral even) to ground. But if that was the case, then the current would be very high because the only resistance in that short circuit would be the wires, which have very low resistance, and the arc itself, which also has very low resistance. And a high current would mean that a circuit breaker would trip instantly.

In your case two things must be fulfilled for a continuous arc to exist.

  1. There is a bad connection. Each of the two brass pins (live and neutral) in the plug are supposed to touch their respective brass spring blades. For one of them, it is either touching only very little or not at all.

  2. The load must draw a current high enough to sustain an arc. Under most circumstances, small loads like light bulbs could not do that, though it is possible.

Perhaps the bad connection is there because of a broken plug, perhaps because of a broken outlet, perhaps because of a combination of the two.

In my opinion, as an electrician, the safest and most correct way to deal with your situation is to replace both the plug and the outlet since both are scorched.

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Yes, it is most likely the different voltage on the contacts. With a low power (= high resistance) device as a voltage divider the resulting voltage at the contacts is too low to ensure that enough electrons are set free from the metal resp. to ensure that the electrons are accelerated to en energy level/speed where they can ionize N2 or O2 in the air before they recombine.

Arcing needs enough electrons with enough speed to start an avalanche.

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By mistake, added my answer as a comment. Re-edited and modified with new information and moved to Answer.
There is nothing special about a normal power strip with three prongs. A probable cause of why your power strip was arcing in the outlet is either the power strip or outlet was damage. With new information from OP, the outlet was damage when "bumped" and now has a poor connection or broken insulator. The connection involves the ground (round) connection in the outlet.
You are right to not use that outlet and until it is replaced. Also I would throw away the power strip that was arcing.

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  • There is absolutely nothing that suggests that there is a short in either the outlet or the power strip or anywhere else. A short would trip the circut breakers or blow a fuse. Perhaps you don't know what a short is. – Mads Skjern Apr 11 '20 at 23:24
  • Not to debate the point - There are details of arcing causing short circuits not powerful enough for the circuit breaker to detect it . Also this, breakers are devices to detect short circuits and (usually) shut the power off. – Programmer66 Apr 12 '20 at 1:46
  • To alleviate your concern of my misuse of a word, I have edited by answer. Thanks for your guidance. – Programmer66 Apr 12 '20 at 1:48

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