Massive sparks from HDMI cable

[freak]

We had a user with a Dell Vostro laptop attempt to plug in an HDMI cable from a TV to a USB port.  He didn't force it.  He simply touched the metal shield of the HDMI cable to the metal case of the laptop not realizing that it was the wrong port.  The laptop was being powered from it's original charger (3 prong) that was plugged in to a grounded power strip which was plugged in to a grounded outlet.  The TV was plugged in with it's original 2 prong cord which was plugged in to a grounded outlet.  Instantly there was a massive flash of sparks about like you would expect if you touched mains hot to neutral.  We have it on video however the HR department won't release it.  I do have a still shot.  No one was hurt, shocked or burned in any way.  The cable shield and laptop case are visibly burned.  The laptop charger no longer functions.  It does not trip a breaker or heat up up when plugged in.   Just no output.  The laptop works ok and will take a charge through USB-C.  We plugged a exact replacement working power supply into the barrel connector socket on the laptop.  Instantly the charger light on on the barrel connector goes out and the charger whines.  We unplugged it right away and it still works as tested on another laptop.  I checked the outlets involved and the hot, neutral and ground all had readings like you would expect in the US.  The laptop and non-functioning charger is being sent out for repair.  The TV appears ok and we tested it with another laptop without incident.  I know it's difficult but anyone want to speculate on a cause?

[madires]

Is anything else connected to the TV? A set-top box, cable TV or satellite TV?

[freak]

No.  Nothing else.

[Jeroen3]

One of the power supplies has lost primary insulation in the transformer or y-capacitor. Possibly the TV's, because it has no earth prong.
It could also have been some other appliance attached.

[wobbly]

Does the HDMI cable run a long way through the building?

[BrokenYugo]

I would think it has to be a power supply fault in the TV, no other path in the setup described. Was the second laptop on a 3 prong charger at the time?

Definitely looks like a 120V short in the pic, the one time I saw it happen (iron bedframe rail hopped over a plug plugged into the wall, hard L-N short for a split second) the breaker didn't trip either.

[JustMeHere]

I'd get one of those socket testers and test your wall sockets for a wiring fault.  For example, a socket that has hot and neutral swapped.

[radiolistener]

This is a common issue when one device is properly grounded while another uses a floating ground that is connected to the mains through filter capacitors.

Check all power supply for this label:


If any equipment features this label on its power supply unit (PSU), it may lead to this issue. Although the mains current is limited for such devices, it is unlikely to cause fatal harm, but it could potentially damage some equipment.

To avoid this issue, ensure that all PSUs have a ground terminal on their mains sockets and that this terminal is properly connected to the wall outlet. This includes the PSU for your laptop, the PSU for your display, and any other devices you plan to connect to the laptop.

This issue occurs when one device has a ground terminal on its mains socket and another device does not have one, and you connect them together through USB, HDMI, or another type of connection.

Measuring the AC voltage between the ground (GND) pins on these devices will reveal approximately 80-100 V AC. While it is normal to experience some sparks, it typically should not result in overheating due to the limited current. However, if you observe large sparks accompanied by melted metal, it indicates a problem with one of your PSUs.

[wasedadoc]

I'd get one of those socket testers and test your wall sockets for a wiring fault.  For example, a socket that has hot and neutral swapped.

No, hot and neutral swapped would not cause the reported incident.

[wasedadoc]

This is a common issue when one device is properly grounded while another uses a floating ground that is connected to the mains through filter capacitors.

Check all power supply for this label:


If any equipment features this label on its power supply unit (PSU), it may lead to this issue. Although the mains current is limited for such devices, it is unlikely to cause fatal harm, but it could potentially damage some equipment.

To avoid this issue, ensure that all PSUs have a ground terminal on their mains sockets and that this terminal is properly connected to the wall outlet. This includes the PSU for your laptop, the PSU for your display, and any other devices you plan to connect to the laptop.

This issue occurs when one device has a ground terminal on its mains socket and another device does not have one, and you connect them together through USB, HDMI, or another type of connection.

Measuring the AC voltage between the ground (GND) pins on these devices will reveal approximately 80-100 V AC. While it is normal to experience some sparks, it typically should not result in overheating due to the limited current. However, if you observe large sparks accompanied by melted metal, it indicates a problem with one of your PSUs.

This is not a case of normal leakage through suppression capacitors. That would never be enough to cause massive sparks and burning of connector metalwork.

[amyk]

Beware that outlet testers only check the relative voltages locally, while the scenario you described involves separate outlets. In particular, on a split-phase system with miswired outlets that are locally consistent, outlet testers may say they're fine but you could get 240V between them.

This is not a case of normal leakage through suppression capacitors. That would never be enough to cause massive sparks and burning of connector metalwork.

However, a defective suppression capacitor could certainly cause this.

I recommend checking the voltage between the HDMI cable shield and outlet ground on the TV, and between the shield and the case of another laptop set up similarly.

[Infraviolet]

Anything with the double insulated (concentric squares) symbol should usually act as a floating voltage shouldn't it? (if it is a device which has any electrically functional metal parts exposed). Making a connection from a floating device to a grounded one should never cause a spark, becase at the instant of coming together the floating device simply shifts it's floating "level" such that the part on it which has come in to contact with the grounded device is now referenced to the grounded device's ground. That this situation could occur seems pretty perplexing.

[radiolistener]

Yes, devices marked with the double insulated (concentric squares) symbol are typically designed to be Class II appliances, meaning they don't require a connection to earth ground because they have additional layers of insulation, ensuring that exposed conductive parts can't become live. This should result in the device having no reference to earth ground, making it "floating" in terms of electrical potential.

However, on many devices, Y-capacitors (between live/neutral and ground) are used for noise suppression. These capacitors create a capacitive connection between the power lines and the device's casing or GND, which can generate high-frequency AC voltages on the device's casing relative to ground, especially if it is floating.

Y-capacitors are designed to reduce electromagnetic interference and are often used in power supply filters. However, their capacitance, although small, can cause leakage of a small AC voltage onto the casing or GND of the device.

When connecting to a grounded device, this potential can quickly "discharge" to zero, accompanied by sparks, especially if the charge has accumulated on the capacitors.

The voltage between ground and such a "floating" GND is typically around 80-100 Volts AC. However, the maximum current under normal conditions is limited by safety requirements. Therefore, while this can lead to sparks, damage to devices, and even mild electric shocks, the current is usually within a safe range and should not be lethal, provided that the power supply is functioning correctly.

[bdunham7]

The TV appears ok and we tested it with another laptop without incident.  I know it's difficult but anyone want to speculate on a cause?

I think the simplest explanation would be a failed Y-capacitor (or possibly some other component) in the TV power supply.  The component would have been leaking or shorted but is now blown open by the huge short circuit current going through it.  It's possible the fault was in the laptop power supply, but that seems unlikely and would require a more complex set of failures.  Both devices would need to be disassembled and examined to know for sure.

[CaptDon]

I did a similar post a couple months back. My Gateway ROG450 laptop power supply had an internal short placing the full 120vac on the barrel of the coaxial power plug. It was apparently failed for a long time but went un-noticed. One day I plugged the audio output of the laptop into my grounded guitar amplifier and ripped the laptop a new ass, complete with smoke and popping parts on the motherboard. Guitar amp survived, laptop did not!! The laptop power supply was a two prong only mains connector.

[Doctorandus_P]

I used to have a stereo set on the other side of the room and it was connected to my PC with a 10m long analog audio cable. My PC is grounded while the stereo is double isolated.

Every now and then I had to temporarily unplug the stereo temporarily, and I felt quite heavy "tingles" when holding the analog cable end with one hand and the stereo set with the other. I also saw small sparks flying between the plug and the socket while reconnecting. It never damaged the inputs or outputs but I did not feel comfortable with it. I solved it by rolling out an extension cord and powering the stereo set from the same wall outlet as my PC. Here in the Netherlands it's quite common to have 3-phase power in residential areas, and I guess my stereo set was powered form a different phase, but I never looked into the real cause. The extension cord solved it for me and then I mostly forgot about it. I also tried rotating the power plug 180 degrees, but that did not help.