Yesterday, I plugged a metal USB into the computer case. When I tried to unplug the USB when the computer is powered on, it shocked me. I thought it's electricity from a malfunctioning part, so I tried to check on the computer. Device Manager said 'The device is working properly.', I can also find the USB on Explorer and can use it, create folders, save files, use it normally. That case is made of plastic, and I realized that when I tried to insert a plastic memory card it didn't shock me at all. I plugged that USB into a different computer and it didn't shock me at all. What is the cause of this? Now I'm scared of metal USB!
Asked
Active
Viewed 1.2k times
2 Answers
6
One key piece of information you've left out is whether the shock will happen repeatably (i.e: touch twice within a few seconds).
One Time Shocks
If the shock happens once, but then not when touching again within a few seconds, then it is likely down to static electricity that has built up on you, discharging via the computer's low impedance path to ground.
This is a common issue when wearing synthetic clothing (e.g: tracksuit, socks on carpet, etc...) and touching anything that is properly grounded (e.g: pipework, cars, desktop PC, etc...).
You might hear a snap, and may even see a spark. It's a sharp pain that dissipates quickly.
Repeatable Shocks - due to Class-Y Capacitors
This can be caused by the power supply's Class-Y capacitors - you may effectively have a "floating ground" within the system. This "ground" can actually have a significant potential compared to the local mains ground (I've seen ~50% of mains, or ~110v AC in the UK), and in many cases it's possible to sink / source a few tens of milliamps between the device "ground" and the local mains ground.
You may experience this when connecting a computer to a monitor, between the computer's chassis and the cable's shielding, and can also be the cause of ground loops and other interference.
This doesn't feel nice at all, but isn't usually too concerning when it comes to your personal health - pacemakers and other medical electronics make this situation more concerning of course.
You won't likely see or hear anything, and the sensation will persist while you are touching the two items.
Repeatable Shocks - due to No Earth (!)
A repeatable shock can also be caused due to a device being improperly earthed.
In this situation, you will typically need another fault to present concurrently (i.e: a live wire touching the chassis) for any significant shock to appear.
You will certainly know about it if you've experienced it - I suspect you haven't.
NOTE: This is a scary scenario and can lead to death.
Double insulated devices (e.g: laptops, monitors, etc...) should never present this issue. Look for the box-in-a-box icon on the adapter:
Avoid "testing" the hypothesis by using your left and right hands, as this will pass the current directly up one arm, across your heart, and down your other arm.
If you must, then using two fingers of the same hand is a safer option - though it may sting more :-)
If you're in any doubt or would like to be sure, then I suggest getting a "Socket Tester" that will indicate whether the wall outlet is correctly wired and safe to use - there are far too many places that have poor wiring, and I really consider these to be an important household toolbox item.
As a side note, Device Manager is not a reliable source of whether "The device is working properly", especially when it comes to this sort of issue.
Attie
- 20,734
-1
This is called electrostatic discharge (ESD) and we all know that very well from school. I don't believe your computer doesn't have grounding. Instead you should thinking about other slippers maybe :)
alecxs
- 396
- 1
- 4
- 15