Touching a PSU can kill you?

TomMe · Post by **TomMe** » Thu Jun 22, 2006 3:01 am

When yesterday my uncle and I were fixing a problem with an undetected HDD inside a PC, I put it temporarily on the PSU and my uncle warned me I should never do such a thing since touching a PSU is very dangerous and can kill you (it was unplugged).

Since he has a background in electronics as a hobbyist I wouldn't want to ignore his advice. But if he's right then touching my case may kill me since it's all metal, I would have to be surprised I'm still living.

He warned me about the electrolytes, and I know that the inside can be very dangerous..but surely there must be some safety in place concerning the PSU casing?

Or in other words, is it safe to touch the PSU on the outside (even with the PC running)?

BillyBuerger · Post by **BillyBuerger** » Thu Jun 22, 2006 3:21 am

Touching the outside of a PSU is not dangerous. If you have a fairly open fan grill or something, it's possible you could accidentally stick something inside which could be dangerous even if the PSU is unplugged. PSUs have capacitors inside that hold a charge even when unplugged. Usually, pressing the power button on the PC after unplugging the PSU will discharge any stored electricity. Although you still should be careful any time you open one.

I think your uncle was just being cautious. Maybe a little over cautious.

burebista · Post by **burebista** » Thu Jun 22, 2006 3:54 am

TomMe wrote:Or in other words, is it safe to touch the PSU on the outside (even with the PC running)?

Yes.

BillyBuerger wrote:Usually, pressing the power button on the PC after unplugging the PSU will discharge any stored electricity.

Not quite. IIRC, PSU, when system is power off, sends a 720mA at 5V (pin 9 from ATX connector). In practice this is the current for WOL. With this current when you press power button MB "tell" PSU to go on full power (more on less). This current is from separate circuit than the circuit which power other components.
So if the PSU is unplugged, even if you press Power button, you only discharge capacitors that are on stand-by rail. But you don't want to discharge those capacitors.

My barbarian method is to unplug the power cord from computer while it's in POST (if you have some concerns about HDD, disconnect it before this maneuver). Never fail until now.

TomMe · Post by **TomMe** » Thu Jun 22, 2006 4:26 am

Thanks guys! This confirms what I suspected.

nick705 · Post by **nick705** » Thu Jun 22, 2006 4:52 am

TomMe wrote:Thanks guys! This confirms what I suspected.

There's some useful info in this sticky....

I agree your uncle was probably just being extra cautious (understandably so given the... ahem... "potential" consequences). I suppose if there were a fault with the PSU's earthing (grounding), even the outer casing might in some circumstances give you a nasty jolt.

TomMe · Post by **TomMe** » Thu Jun 22, 2006 4:55 am

I read the threads about draining a PSU, rubber gloves etc but couldn't find anything in them about the casing itself so I thought I'd ask the question myself.

Lawrence Lee · Post by **Lawrence Lee** » Thu Jun 22, 2006 12:37 pm

Yes, this is true. I got zapped twice while fiddling inside a power supply and i died both times.

jaganath · Post by **jaganath** » Thu Jun 22, 2006 1:15 pm

Amourek wrote:Yes, this is true. I got zapped twice while fiddling inside a power supply and i died both times.

I always said this forum was full of zombies!

On a more serious note, you can get a serious or even fatal shock if you touch the live parts inside a PSU while it is connected to the mains. Generally the outside is safe to touch even when the PS is on, because the live PCB is grounded.

wolfboy · Post by **wolfboy** » Thu Jun 22, 2006 3:33 pm

Kind of Off-Topic, but is it normal if the PSU is really hot if you touch the top of your PC case in normal room temperature?

wwenze · Post by **wwenze** » Thu Jun 22, 2006 9:32 pm

Amourek wrote:Yes, this is true. I got zapped twice while fiddling inside a power supply and i died both times.

Ha, I only died once.

Didn't die when touching the heat sink but died touching the smoothing caps when the system was on.

TomMe · Post by **TomMe** » Thu Jun 22, 2006 11:22 pm

wolfboy wrote:Kind of Off-Topic, but is it normal if the PSU is really hot if you touch the top of your PC case in normal room temperature?

Could be that the PSU is venting hot air into the inside of your case, as it was with me. I solved it by lowering the pressure inside the case (venting hot air out the back with an additional fan) allowing cool air to be drawn in from the front.

wolfboy · Post by **wolfboy** » Fri Jun 23, 2006 1:28 am

TomMe wrote:
wolfboy wrote:Kind of Off-Topic, but is it normal if the PSU is really hot if you touch the top of your PC case in normal room temperature?
Could be that the PSU is venting hot air into the inside of your case, as it was with me. I solved it by lowering the pressure inside the case (venting hot air out the back with an additional fan) allowing cool air to be drawn in from the front.

Yeah, it must be this hot weather.

BrianE · Post by **BrianE** » Fri Jun 23, 2006 10:36 pm

nick705 wrote: I agree your uncle was probably just being extra cautious (understandably so given the... ahem... "potential" consequences). I suppose if there were a fault with the PSU's earthing (grounding), even the outer casing might in some circumstances give you a nasty jolt.

That reminds me... it's probably not a bad idea for all of us to check to make sure their house wiring doesn't have a bad ground. (I wonder if that's what some UPS devices check for?) The point of the third electrode is to make sure that any voltage that may reach the outer metal casing can be bled off to ground instead of zapping you. There's still the possibility that the PSU is grounded fine, but your home wiring isn't.

darthan · Post by **darthan** » Fri Jun 23, 2006 11:17 pm

BrianE, it would take a shockingly bad ground in your house wiring to make the ground fail. After all, if the third wire simply touches the frame in the wall of your house it is a better ground than the current going from the PSU to you to the house frame. But don't play with plumbing and electronics at once.

Shadowknight · Post by **Shadowknight** » Fri Jun 23, 2006 11:55 pm

Amourek wrote:Yes, this is true. I got zapped twice while fiddling inside a power supply and i died both times.

So, I guess that explains why SPCRians are obsessed with contemplating every possible meaning of the phrase "as quiet as the grave".

wwenze · Post by **wwenze** » Sat Jun 24, 2006 2:36 am

A big-time misconception is that the ground wire is used to discharge live current on the case to the ground; what it can do is discharge static, even a minute current through the ground will switch the house's electrical circuit off (the more important part in ensuring safety).

But I can only say this for areas that have this protection, other areas... heh heh. And if your wires are not even fused, congrats.

If there is an undetected (and unstopped) short circuit from live to ground, your casing would be glowing and a fire will start. Well, it won't be undetected for long.

jaganath · Post by **jaganath** » Sat Jun 24, 2006 2:48 am

So, I guess that explains why SPCRians are obsessed with contemplating every possible meaning of the phrase "as quiet as the grave".

All those worms and beetles, I bet they are noisy b*st**ds, maybe I will have to put some Acoustipack in my coffin.........

BrianE · Post by **BrianE** » Sat Jun 24, 2006 11:36 am

darthan wrote:BrianE, it would take a shockingly bad ground in your house wiring to make the ground fail. After all, if the third wire simply touches the frame in the wall of your house it is a better ground than the current going from the PSU to you to the house frame. But don't play with plumbing and electronics at once.

Oh... I don't know very much about home construction/wiring. What is the ground wire connected to again? I always thought it was connected to the hydro ground outside the house or a ground rod somewhere....

Lawrence Lee · Post by **Lawrence Lee** » Sat Jun 24, 2006 1:13 pm

Okay so maybe I didn't die, but I did get a bit of a shock both times. Of course I've been lightly electrocuted at least a dozen times over the years. I'm a bit of a clutz and I ain't that smart either.

Oleg Artamonov · Post by **Oleg Artamonov** » Sat Jun 24, 2006 3:21 pm

wwenze wrote:A big-time misconception is that the ground wire is used to discharge live current on the case to the ground; what it can do is discharge static

Wrong. Ground wire is used for
a) supress common-mode noise currents; ground wire is connected to power wires (L and N) with two Y2-class ceramic capacitors (see here, pages 3-4); current flows through capacitors is about 1 mA;
b) protect users from electric shock if PSU fails in the way that high voltage will be connected to the PSUs' case.

even a minute current through the ground will switch the house's electrical circuit off (the more important part in ensuring safety)

Wrong again. There is no protection against current through the ground wire.

wwenze · Post by **wwenze** » Sat Jun 24, 2006 4:39 pm

Whoops, I misinterpreted the concept of ground fault circuit interruption, thx for the correction.

Touching a PSU can kill you?

Touching a PSU can kill you?

Re: Touching a PSU can kill you?