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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: sketch
  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.

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.
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  • 4
    \$\begingroup\$ Something is very wrong, I can tell you that much for sure. Probably with the monitor, possibly with the lamp. \$\endgroup\$
    – Hearth
    Jan 1 at 1:55
  • \$\begingroup\$ Measure DC and AC voltage with a multimeter from the lamp base to the metal chassis of the monitor. \$\endgroup\$
    – tobalt
    Jan 1 at 7:20
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    \$\begingroup\$ Most likely lamp wiring is damaged and lamp metal parts have become live with mains. You don't tell much details like if the power strip, or any other thing have earthed mains plugs and connected to earthed socked, but lamp looks ungrounded and so this is possible. Lamp is now dangerous to touch so unplug it and then examine where wiring is broken and touches the metal, most likely burn marks at that point too. \$\endgroup\$
    – Justme
    Jan 1 at 8:37
  • 1
    \$\begingroup\$ Good info, thanks, so most likely other equipment were properly grounded and because of they were the lamp fault became apparent and no one got killed. Either you only touched the lamp and nothing else when it was live, or the lamp exterior is only live when the mains cord is at a certain orientation and angle. \$\endgroup\$
    – Justme
    Jan 1 at 10:09
  • 3
    \$\begingroup\$ Please stop using every part of this setup immediately until the problem has been found. This is a serious hazard. \$\endgroup\$
    – winny
    Jan 1 at 10:29

3 Answers 3

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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.

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  • \$\begingroup\$ Thanks for sharing your thoughts. To clarify, yes, the HDMI was plugged into the monitor when the incident happened. I have a few follow-up questions. You say that the outer part of the HDMI plug should be earthed, but is it possible that the plug can have some nonzero voltage due to being connected to a powered-on monitor? Also, you said that the lamp fuse may still be working because the neutral might have been connected to the metal of the base rather than the live, but ideally (if the lamp were in perfect condition), neither should be connected to the metal of the base, right? \$\endgroup\$
    – dfncrbl
    Jan 1 at 4:23
  • \$\begingroup\$ One more follow up question: If I wanted to disassemble the lamp to inspect its wiring, with the knowledge that there may be a loose wire, how can I do this safely? \$\endgroup\$
    – dfncrbl
    Jan 1 at 4:43
  • \$\begingroup\$ @dfncrbl After it's unplugged, it is usually considered safe. If it has any electronics, it can have capacitors still holding charge, and they might actually be dangerous. Most of those DO have a resistor connected so they drain seconds after it's unplugged, but to be extra safe you could wait after you unplug it, up to 10-15 mins. I suggest making a tea in the meantime. EDIT: Also use a voltage tester if you can get access to one! \$\endgroup\$
    – Sasszem
    Jan 1 at 5:22
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    \$\begingroup\$ Since there are no energy storage devices in most lamps, it should be perfectly safe to disassemble it so long as it's unplugged . True story, don't disassemble lamp by the light of that lamp like my old maths teacher claimed he did!! \$\endgroup\$ Jan 1 at 13:18
  • \$\begingroup\$ In general all parts of electronic equipment that are at ground potential, the mains earth, the 0V rail in the power supply, the metal work and chassis, cable and plug shields etc should be bonded together at one or more points. So I would not expect any potential difference between various parts. There are exceptions, some particularly sensitive circuits or those that operate at high voltages may be completely isolated, but in that case they would also be well insulated as well. \$\endgroup\$ Jan 1 at 13:25
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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.

enter image description here

(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.

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  • \$\begingroup\$ @bob_monsen thanks for your suggestion. I do live in the US. I currently suspect that the lamp case was hot, but I will also purchase an outlet tester to test the outlet and the outlets on the power strip that I was using. \$\endgroup\$
    – dfncrbl
    Jan 1 at 19:32
  • \$\begingroup\$ @dfncrbl outlet testers are very useful, and for less than $10, seem like a good idea for almost any toolbag. I bought one to use while replacing outlets in my house (for asthetics!) That was 20 years ago, and they are actually cheaper now! \$\endgroup\$
    – bob_monsen
    Jan 1 at 20:14
  • \$\begingroup\$ I purchased an outlet tester and tested all the outlets I was using, including the power strip ones, and verified that they all tested normal \$\endgroup\$
    – dfncrbl
    Jan 2 at 2:29
  • \$\begingroup\$ Also, to clarify, the lamp plug is polarized (they are NOT the same size) \$\endgroup\$
    – dfncrbl
    Jan 2 at 2:30
  • 1
    \$\begingroup\$ Well, I'm wrong then. However, you are now the proud owner of an outlet tester. Enjoy it for the next 40 years! \$\endgroup\$
    – bob_monsen
    Jan 3 at 4:18
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enter image description here

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.

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  • \$\begingroup\$ Thanks for your detailed advice. I want to clarify a few things about the continuity testing that you mentioned. First of all, no the lamp does not have a 3rd Earth prong. I can connect one of the multimeter probes to the "live" prong of the plug and the other probe to the lamp case, making sure to touch metal in the crater. If wiring is good (or transiently good), I should expect to see "OL". If the result is anything other than "OL", then current is flowing from the live prong to the lamp case, right? Is using the "live" prong vs. the "neutral" prong any different for this test? \$\endgroup\$
    – dfncrbl
    Jan 2 at 2:01
  • \$\begingroup\$ Also, can you tell me if the following would be an equivalent way of testing?: Suppose I plug in the lamp and take a voltage measurement. One probe is connected to ground and another probe is connected to the lamp case (again making sure to touch metal). If the wiring is good (or transiently good), I would see a 0V difference. If I see any nonzero voltage, it means that current is flowing from the mains to the lamp case. If these two tests are equivalent for what I am investigating, is it true that the continuity test is safer because I can avoid actually plugging in the lamp? \$\endgroup\$
    – dfncrbl
    Jan 2 at 2:08
  • \$\begingroup\$ Lastly, I see that you mention it is a good idea to check for continuity between the Earth prong and PC/Monitor chassis. I understand that this verifies if the chassis is actually connected to ground. Q: Is it also safe to perform a "no continuity" test between the live prong and chassis (similar to what you suggest for the lamp)? To be clear, I mean to connect one probe to the "live" prong of the PC power supply (whose switch is flipped ON), connect the probe to the PC chassis, and validate that the result is "OL". If I did this, would the continuity test actually turn on the PC power supply? \$\endgroup\$
    – dfncrbl
    Jan 2 at 2:13
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    \$\begingroup\$ Yes to most of the above, except: the continuity test won't turn on the PC because the multimeter uses about 2V at very low current to do its test in order to not damage electronic components being probed. So it's safe. It will detect a hard short but it won't detect a bad insulation that only leaks current at high voltage, but in this case you have a hard short so it's fine. \$\endgroup\$
    – bobflux
    Jan 2 at 7:50
  • \$\begingroup\$ There shouldn't be continuity between Neutral prong and chassis either, although I'm not sure about this for the US. You should also check if the socket is wired with neutral and live on the correct sides (check voltage between socket hole and earth). And you can also check voltage between chassis and Earth while the lamp is plugged (AC mode of course), but because the multimeter has a high impedance and pulls very little current, it won't make a difference between a very small leak and a hard short. \$\endgroup\$
    – bobflux
    Jan 2 at 7:57

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