Floating grounds and Computer equipment.

Thread Starter


Joined Oct 13, 2018
Recently ive noticed when I plug my external powered USB Harddrive into my PC it causes my screen to flicker off as the USB shield touches the PC.
Another monitor in my house causes game systems to reset when the HDMI is plugged into them.
I've seen it spark so I thought id check what potential there was between the 2.
I measured from the HDMI shield to the game system shield and it shows close to 100vac ( I live in Australia with 240vac mains)
Ive also measured from my external USB HDD sheild to my PC case, same 100vac. Its very low current, if i touch the wire while measuring it goes down to around 12v which is what the USB HDD is powered by.
So I started to check my 12v power supplies around the house to see what potential they had to the house earth, all of my 12v adapters show the 100vac, but a usb 5v charger does not.

I cautiously removed the ground pin from an adapter and tested my system without any house grounding, sure enough the screen wont flicker when the HDD is plugged in, and there is no more 100vac between the 2.

I initially thought I had an earth leak in the house but tested with a fresh earth stake and all readings were good.

So I guess my question is, why do the 5v adapters not have this issue but majority of my 12v ones do?
Why does my TV not have 100vac from hdmi shield to console but my ACER computer monitor does?
(TV is direct connection to 240vac, where the ACER monitor uses a power brick and 19VDC, no earth prong.)

Must I add capacitance to the circuit to bring the 100vac down to a suitable level? I feel this is damaging my equipment.


Joined Jul 18, 2013
The problem is with separately powered HD's compared with one powered by the port is they are not earth grounded, while the PC P.S. common is referenced to earth ground.
Usually the separately powered is via a two pin power socket, hence no earth.

Thread Starter


Joined Oct 13, 2018
I was not asking why it happens, although your answer is kind of close you have forgotten to mention the cause which would be the PF capacitor on the 12v supply bleeding active back to neutral. Thanks anyway.

I was asking how to fix this or stop it happening as im sure not everyone in the world puts up with 100vac sparks flying out of their HDD or thousand dollar computer monitors flickering on and off when peripherals are plugged in.