I recently set up a new TV/home theater system for my dad. The TV had 3 HDMI devices plugged into it. I plugged a 4th HDMI cable into the TV, and as I reached for the opposite end of the cable with the intent of plugging it into another HDMI device, I received a shock from the metal shield on the HDMI connector. This was not a brief static electricity shock--it was a sustained, AC shock that went up my hand into my wrist.

I investigated the outlet powering the TV and associated equipment (all plugged into the same surge strip), and I found that 1.) hot and neutral were reversed, and 2.) although the outlet had an earth ground prong, the house actually has no wiring for an earth ground (it's an old building). I replaced the outlet with a new GFCI outlet, correcting the hot/neutral reverse. After making this change, I found that the HDMI cable no longer shocks me.

I have found several posts around the internet discussing similar situations, and suggesting that a hot/neutral reverse is a common culprit. However, I am struggling to understand why exactly this wiring issue would lead to the HDMI cable shocking me. Most of the devices plugged into the power strip at the time of the shock (TV, cable box, AV receiver, etc.) were 2-prong, double-insulated devices. My understanding is that these devices do not "care" whether hot and neutral is reversed, and electrically isolate their shield/chassis from both hot and neutral. There was one device with a 3-prong plug (a computer) plugged into the surge strip (but not the TV) at the time of the shock, but I'm not sure whether that's relevant.

So, could someone please help explain: how does a hot/neutral reverse lead to mains voltage leaking into the HDMI shield of a double-insulated device?

Many thanks!

  • \$\begingroup\$ Sounds like your normal Y-capacitor leakage, but can you confirm with a multimeter measurement of your open circuit voltage and your short circuit current? \$\endgroup\$ – winny Oct 30 '20 at 7:39
  • \$\begingroup\$ I had thought Y-capacitor leakage as well, but why would a reversed hot and neutral affect the amount of Y-cap leakage? Before replacing the outlet/correcting the wiring, I got a shock; afterwards, I don’t. Unfortunately, I wasn’t able to take measurements before replacing the outlet. However, after replacing the outlet, I did measure the AC current between the HDMI shield and the coax cable shield (seemed closest thing to earth ground...) through a 50K resistor. I did this as a safety check before touching the HDMI again. Leakage current was minimal (0.25 mA) \$\endgroup\$ – bform Oct 30 '20 at 8:09
  • \$\begingroup\$ You need to include @ followed by a username or he/she won't be notified. I agree, you flipping the AC polarity should not matter. 250 uA is the legal limit, so if you are at it during normal operaion, I would say that's high. Are you in 220/230/240 V part of the world? \$\endgroup\$ – winny Oct 30 '20 at 8:46
  • \$\begingroup\$ @winny thanks, I’m in the US, so 120V \$\endgroup\$ – bform Oct 30 '20 at 8:55
  • \$\begingroup\$ @winny the 250 uA you mention is per product, right? That makes it seem more reasonable given that I’m seeing the leakage contributions of a bunch of devices (receiver, cable box, Blu-ray player, TV, etc.) all summed together, no? \$\endgroup\$ – bform Oct 30 '20 at 9:23

Devices with grounded plugs need the ground connection for their mains input EMI filter Y capacitors to work as intended. Not providing ground means the grounded parts of the device, including HDMI connector, will float capacitively at half the mains AC voltage. This not only can hurt when touched but can also damage equpment if you connect two devices together and the capacitors get discharged by connecting the HDMI cable.

Devices with ungrounded plugs can also hurt, if their power supply has EMI filter Y capacitor between mains side and isolated side. Depending on the power supply implementation, the leakage current could be different depending on which way Live and Neutral are. In some countries you can connect the mains plug to wall in any orientation so usually the design does not care which way Live and Neutral are, but it can also assume which way Live and Neutral are connected, and leakage can be different if it is the wrong way.

I just tested with a two-prong 4K TV with a metal frame, and I can feel AC mains vibrations when sliding my hand on the metal. Multimeter says there is half of mains AC voltage from USB chassis to mains earth, and 130 microamps AC when shorting the USB chassis to ground. Reading does not change if I change the mains plug orientation to swap Live and Neutral.

Lesson is to do like most consumer equipment manuals say : disconnect devices from mains before making other connections between them.


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