Why did a spark occur between my HDMI cable and metal lamp base?

Question

Edit:

• Update 1/2/22 @ 2:27 UTC: outlet tester shows that all outlets and power strip outlets being used were normal.
• Update 1/2/22 @ 21:07 UTC: performed various continuity tests which suggest to me that the monitor, HDMI cord, and PC are normal, while the lamp is faulty. Included results in a table at the end of the post. Pending someone to review the results and verify if my interpretations make sense.
• Update 1/3/22 @ 1:27 UTC: added PC case, PSU, and monitor photos, and my summary of the whole thing at the bottom.
• Update 1/3/22 @ 22:16 UTC: added photo of exposed wiring inside the disassembled lamp.

Thank you, everyone who has offered suggestions and words of caution. I acknowledge how hazardous this situation is. At the moment, my suspicion is that the lamp has a loose wire which has caused it to become live. But other possibilities include a badly wired outlet, or (disproven by outlet tester) some issue with the monitor which caused the HDMI cable to become hot (disproven by continuity testing).

I will do 3 things.

1. First, I will disassemble the lamp to inspect for faulty wiring. [done]. This is the wiring exposed to the lamp casing:
2. Second, I will use an outlet tester to check the outlet and power strip that I was using. [done, all are normal]
3. Third, I will use a multimeter to measure for voltage at the end of the HDMI cable when the monitor is plugged in. [done with a continuity test instead, see below]
4. I would also want to check the voltage of the lamp surface when it's plugged in, but at this point I would rather not plug in this lamp ever again. [done with a continuity test instead, see below]

Description of incident

I kicked an HDMI cable that was plugged into a monitor so that its free end contacted the metal base of a standing lamp, at which point a spark occurred and the circuit breaker for all the plugs in my room got tripped.

• The monitor which the HDMI cable was plugged into was plugged in, powered on, sleeping.
• The lamp was plugged in and turned on.
• The previous day, I had connected and disconnected the same monitor+HDMI to a motherboard that I was using to build a custom PC.
  • The motherboard was in one of two different states while I had plugged in the HDMI cable.
    • State #1: The motherboard was separated from the PC case, powered on by the power supply. (I was testing whether or not I could get into the BIOS with the minimal set of components.)
    • State #2: The motherboard was affixed to the PC case, powered OFF. (I was testing if I could physically fit the HDMI cable into the port after having installed the motherboard and peripheral components into the case)
  • I had disconnected the HDMI cable and left it disconnected for about 18 hours before the spark incident.

Details

Here are some details which I think are relevant.

• Here is a sketch of the lamp, monitor, and their connections at the time of the spark.

• Here is a sketch of the motherboard, monitor, and their connections when I was using the monitor to test the motherboard the previous day, in State #1 (powered on).

• Here is an image of the damage caused by the spark to the cable head and to the lamp base

• Here is an image of the lamp base, monitor, and HDMI cable

• Here is a close up of the lamp base. It has TWO burn marks. The one next to the vertical pole is NOT where the spark occurred. The one further from the vertical pole IS where the spark occurred.

• The lamp still works.

• The monitor is still able to power on.

• Not sure if the monitor+sameHDMICable still works for display (scared to test)

• Not sure if the monitor+otherHDMICable still works for display (scared to test)

Intent of asking this question

I am interested in finding out

1. what the most likely reason for the spark was
2. whether or not it is possible that it could be caused by some kind of mistake I made in assembling my PC that I was working on and had tested with the same HDMI cable.

Thank you very much.

Continuity Testing Results

Which items being tested?	Probe1 location	Probe2 location	Continuity?	Comments
MONITOR AND HDMI
	HDMI cable damaged end	HDMI cable normal end	YES
	Monitor Plug Ground Prong	Monitor cable plate where the various ports are located	YES
	Monitor Plug Ground Prong	Metal bottom of monitor stand	NO	Not sure why. Possibly the monitor stand is not connected to the monitor frame via metal, or there is some insulation? Here is a closer pic of the monitor
	Monitor Plug Ground Prong	Free end of HDMI cable (while other end is plugged into monitor HDMI port)	YES
	Monitor Plug Hot Prong	Monitor cable plate where the various ports are located	NO
	Monitor Plug Hot Prong	Metal bottom of monitor stand	NO
	Monitor Plug Hot Prong	Free end of HDMI cable (while other end is plugged into monitor HDMI port)	NO
LAMP
	Lamp Plug Hot Prong	Lamp base (not inside crater)	NO	Probably due to paint insulation as suggested by @bobflux
	Lamp Plug Hot Prong	Lamp base inside crater	YES reading 12 ohms	FAULTY: This tells me that there is something connecting the metal lamp casing to the internal wiring!
	Lamp Plug Neutral Prong	Lamp base (not inside crater)	NO
	Lamp Plug Neutral Prong	Lamp base inside crater	NO	Not sure why, but possibly because the wire which is touching the metal case is the hot wire and the neutral wire is not touching. Pending opening up the lamp to investigate.
PC CASE AND POWER SUPPLY UNIT (PSU)
	PC PSU Plug Ground Prong	PC PSU case	NO	Not sure why, maybe due to paint insulation? Here is a pic of the PSU.
	PC PSU Plug Ground Prong	Inside of screws affixing PSU to PC Case	YES	I think this is because the screw has direct contact with the PSU metallic chassis. I think this is sufficient to show the PSU is properly grounded.
	PC PSU Plug Ground Prong	PC Case	NO	Not sure why, maybe due to paint insulation? Here is a pic of the PC case
	PC PSU Plug Ground Prong	Insides of various screws on PC Case	YES	I think this is for the same reason - screws have direct contact with the metal frame. I think this is sufficient to show the PC case is properly grounded
	PC PSU Plug Hot Prong	PC PSU Case	NO
	PC PSU Plug Hot Prong	Inside of screw affixing PSU to PC case	NO
	PC PSU Plug Hot Prong	PC Case	NO
	PC PSU Plug Hot Prong	Inside of various screws on PC Case	NO

Conclusion (work in progress)

The spark was a result of a short circuit for the mains electricity coming from my wall outlet. The short circuit was: [source (transformer/powerlines) -> the hot prong of my lamp -> metal casing of my lamp -> HDMI cord -> monitor chassis -> ground wire of monitor -> source]. This short circuit caused my circuit breaker in to trip and cut off all the power to outlets in my room. The fact that I had plugged in the HDMI cable to my new PC that I was building the previous day was irrelevant.

How do you know there was a connection from hot prong of lamp to metal casing of lamp?

Confirmed continuity between the lamp's hot prong and the metal in the base of the lamp (which got exposed due to the spark).

Why was there a connection from hot prong of lamp to metal casing of lamp?

Still under investigation. Need to open up the lamp case to inspect the wiring.

How do you know that the monitor was not sending mains to the lamp as opposed to lamp sending mains into the monitor?

Absence of continuity from monitor hot prong to HDMI cable end, and presence of continuity from monitor ground prong to HDMI cable end. Assuming my power strip and outlet grounds are wired correctly, if the monitor had a ground fault that caused its chassis and consequently the HDMI cord to contact the monitor's hot wire, that would have created a short circuit [source -> monitor hot prong -> monitor chassis/HDMI cord -> monitor ground prong -> source] as soon as the monitor was plugged in which would have tripped my circuit breaker. This did not happen.

Why did the lamp not cause the circuit breaker to trigger by itself?

The lamp has a two prong plug (no ground) and it was situated on a wood floor. When plugged in, the current in the lamp was essentially flowing as if it were a perfectly normal lamp. This is because even though a wire was in contact with the metal casing, no current went down that path because there would have been no path for the current to go back to the source (the only path available would be through the wood floor, which is an insulator). Thus, there was no short circuit when the lamp was plugged in, so there was no tripped circuit breaker.

Why did I not get shocked from handing the lamp?

I was never touching the lamp and something connected to ground at the same time. As described in the previous section there is no path from the lamp metal casing back to the source to complete circuit. When I touched the lamp, this did not change because there was still no path through the metal casing back to the source. Had I touched both the lamp and the HDMI cord at the same time, there would be a path [source -> lamp hot prong -> lamp metal casing -> me -> hdmi cord -> monitor ground prong -> source]

Why is the lamp still working after the incident?

The lamp does not have a fuse in its plug. Perhaps because the circuit breaker kicked in and shut off the power so fast, the wiring in the lamp is still working as normal.

Other possible outcomes

From worst to best

Scenario	Difference from reality	Outcome
Person touched lamp and something grounded at the same time	the person	Death / serious injury
Person touched lamp and something grounded at the same time, but lamp was in a GFCI outlet	the person, the GFCI outlet	Death / serious injury -- not sure how much GFCI would have reduced the impact
Person touched lamp and something grounded at the same time, but lamp had a fuse	the person, the fuse	Again not sure how much a fuse would reduce the impact
Person touched lamp and something grounded at the same time, but lamp was plugged into a surge protector	the person, the surge protector	Again not sure how much it would reduce the impact
Exactly the same as reality	N/A	sparks, fried monitor HDMI connectivity (?)
Lamp was grounded via 3-prong plug and plugged in	the 3-prong plug	As soon as I turned the lamp on the first time, circuit breaker trips immediately
Lamp was grounded via 3-prong plug and had a fuse	the 3-prong plug, the fuse	If the fuse melted before the circuit breaker tripped, there would be no chance of using the lamp again. But also maybe no indication of a high voltage short circuit.

Lessons learned

1. If outside of appliance is metal, make sure it has a 3-prong plug.

Answer 1

If everything had been working previously in some configuration with the HDMI cable plugged in at both ends then my best guess is that there is a fault in your lamp. Maybe an earth has become detached and the metal base is somehow live. I'm assuming that the HDMI cable was plugged in to something at its other end at the time of the incident.

I believe the outer part of the HDMI plug, shown damaged, is earthed as part of the shielding and all the metalwork of your PC equipment and peripherals should be at a similar earth potential. I don't think the breakers would have tripped if it had been a low voltage event, say one of the PC power supply outputs getting grounded. They are able to withstand that sort of maltreatment and just shut down or current limit. It sounds far more like a mains short of some kind, nothing to do with the actual PC.

My advice is not to use the lamp until you have had it inspected and tested by a competent electrician. It might be extremely dangerous. The lamp fuse might not have blown if the fault was to connect the neutral (rather than the live) to the metal of the base because the fuse is in the live side. Whilst the neutral is nominally at earth potential, in old or badly installed mains wiring it can be quite a way from earth, sufficient to trip an earth leakage breaker if connected to earth via the HDMI cable.

later

Your continuity measurements just about confirm our suspicions. The lamp is faulty, you have a 12 ohm short between live and the metal work and the only thing protecting you is the paint!

Scrap the lamp completely and buy one either with a proper 3 wire earth, or that is marked "double insulated" (plastic body and internals).

The other readings are pretty much what I would expect for a properly grounded system. Where you couldn't get earth continuity try to scrape a bit of the paint off, but in any case those places are not essential.

Answer 2

If you aren't in the US, ignore this answer. Also, be very careful with live outlets.

The problem could be a miswired plug. You can buy an outlet tester for a few bucks at a hardware store. They light up when plugged in to show you the status of your outlet. They'll tell you if you have a working ground, and if the outlet is wired properly.

(image from HERE: https://www.amazon.com/Receptacle-Tester-Klein-Tools-RT110/dp/B01AKX3AYE/ref=sr_1_7)

The lamp could be shorted somehow, but that seems really improbable to me. It could, however, be very old, and not have a polarized plug (where the plug prongs are both the same size). If so, it could be plugged in the wrong way, and in case of a fault, the case could be hot. If so, buy a new lamp. They are cheap.

The monitor plug could also be improperly wired, but I'm guessing the HDMI cable's shielding is connected directly to ground, which, if connected, is unlikely to be miswired. Also, if the ground isn't connected in the outlet for some reason, you won't get sparks. If it is, you'll get sparks and a blown breaker when you touch it to the hot lamp case, which is what you saw.

Answer 3

The crater shows enough energy was released to vaporize metal. This means the usual causes of small sparks and shocks, like line filter capacitors, are not involved here. This looks like what happens with a full short across mains, which is confirmed by the circuit breakers. It is usually quite loud.

The base of the lamp looks painted. Paint is insulating at low voltage, unless it is scratched off by a flying HDMI connector. If the lamp is at fault here, it is possible you handled it and touched it before, and felt nothing just because the paint acted as an insulator. Obviously, this is not something you want to keep doing. But this is important: if you do any measurement on the lamp, make sure the multimeter probe contacts metal and not paint. You can stick the multimeter probe in the crater.

Let's proceed according the usual way of electronics: sort by cost, so lamp first.

You unplugged the lamp, right? If you did, it is safe. If there is an intermittent short inside, it may or may not still be shorted, if you moved it. So, keep in mind, measurements with a multimeter will tell you if a strand of wire got loose and is making contact with the metal body, but it will not tell you if the loose wire is juuuust about to make contact, but isn't.

Set the multimeter in beep-continuity mode. If the lamp has a 3 prong plug with Earth, it should ring between Earth and metal body. If it has a 2 prong plug, it should not ring between any of the prongs and the metal body.

Assuming it didn't ring, you can disassemble it and check for loose wires. Make sure to check for loose strands in multi stranded wires coming off the screw terminals, or carbonized remains of what used to be a loose strand.

If the lamp has a long stem with a bulb on top and the wire running inside the stem, also check the insulation on the wire where it bends.

It is very likely the culprit is the lamp, because you said you assembled the PC, which means it's a desktop. This means a power supply which is earthed, 3 prong IEC plug, and it connects the chassis to earth with screws. So the PC earths the HDMI cable, and if the monitor was sending mains into the cable shield, your breakers would have blown before.

Doesn't hurt to check continuity between Earth prong and chassis in both monitor and PC.