This is not strictly a home improvement question but I'm trying to learn more about household electricity. I couldn't find a better place online to pose this question.

Lets say I'm bonded to a pc power supply which in turn is earthed through a connection to a grounded outlet. If the pc power supply is powered off, am I still at risk of electrocution and if so, why?

closed as off-topic by Tester101 Sep 15 '14 at 11:53

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  • You mean like while working inside the PC? If so, possibly. Capacitors can hold a charge, so if you touch the wrong thing the capacitor could discharge through you. – Tester101 Sep 12 '14 at 21:49
  • Yeah, I'll be assembling a PC. The capacitors wouldn't be a threat unless I was tinkering inside the power supply, right? What I was considering doing is plugging in the powered off power supply to a grounded outlet and then connecting an anti-static wristband to one of the power supply mounting screws to dissipate any and all static electricity build-up from myself. But, I've read that this method is dangerous and I can't figure out why it's considered dangerous by some. My electrical knowledge is very low. Thanks for answering – Sam Sep 12 '14 at 22:14
  • Why not just unplug the power supply while you're working? – Tester101 Sep 12 '14 at 22:16
  • Because plugging in the power supply is how I'm earthing myself I think. I'm not even sure such precautions are vital though. The computer building community seems divided over how much of a threat static electricity discharge is and this is my first build so I'm paranoid. I appreciate your help. – Sam Sep 12 '14 at 22:29
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    The anti-static strap puts you at the same electrical potential as the case, so static is no concern as long as the strap is on. – Tester101 Sep 12 '14 at 22:31

In working on computers, or for that matter of fact most electrical equipment, avoiding static damage is achieved by establishing a ground plane.

It's not whether you're connected to earth potential, but rather the fact that everything you're working on is at equal potential.

You can get by easily by having an antistatic mat on a desktop that you've laid all the components on, and then grounding yourself to that mat with your personal grounding wrist strap. This establishes a ground plane of equal potential as everything on the mat will bleed charge to equal potential. No potential differential, no sparks...

The same can be done with an unplugged computer simply by attaching yourself to the metal case with a grounding wrist strap and laying the plastic component packaging (antistatic conductive plastic) on bare metal in the case before opening. Once again, you're attempting zero potential between you, the computer case and the component, not absolute earth potential.

As to worries about computer power, the maximum voltage you'll encounter is 24V if you manage to bridge the powered output between +12V and -12V. The line current is pretty effectively contained inside the metal box the PSU resides in except for ancient AT style PSUs with the hot switch that switched full line power. Those could be dangerous.

I've also had situations where there were bad grounds in the power wiring that allowed for hot chassis, so if it's unplugged, you will find this is the safest mode of operation.

If you feel it's absolutely necessary to work on an earth potential chassis, there are green-cords available. They have only the ground lug attached. Using the ground plane methods described, I've never really needed such a thing and I've worked in some industrial situations where there's been some pretty heavy inductive magnetic fields available that have powered up the outer shields of ungrounded coax ethernet cabling. Computer system completely disconnected from all external devices with grounding strap worked the best.

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    Additional detail: The antistatic wrist-strap has a current-limiting resistor in it, so you can't get electrocuted through it. (In fact if you're feeling cheap or need one in a hurry you can make your own -- wrap bare wire (or if you want something more reusable, a loop of beaded chain) around your wrist, run an insulated wire connecting that to one side of a 1 megohm resistor, run a wire from the other side of that resistor to an appropriate clip to attach to the ground point. That's really all it takes to let the static bleed off at a harmless, controlled rate.) – keshlam Sep 13 '14 at 13:26

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