Compaq//safety issues

From: Max Eskin <>
Date: Thu Jul 23 11:56:18 1998

LARTed with a tesla coil, right? So how do you propose using a pc in
a building without three-prong sockets?
>I do, and if I catch anyone removing safety earth connections in my
>workshop, they will be LARTed. I am serious. I've worked enough with
>mains to know how dangerous it can be.

I doubt 240 vs. 110 is much difference as far as heart muscle is
>(a) If I want to commit suicide I will do :-). But I don't want to set
>booby traps in my workshop. Perhaps I'm more careful becausewe've got
>mains here.

So, why have I never felt anything touching a PC case? I've done it
many times, I've touched insides many times, and I've never felt so
much as a tingle. Would I be able to use a VOM to find this voltage?

>Right, an explanation. The 'Earth' wire goes to the metal PC case, and
>also to the thrid pin of the plug. 'Neutral' is connected to earth at
>main distribution board (US), last distribution transformer (UK), and
>effectively earth. And live is 110V (or 240V) above that.
>Now, the 2 capacitors on the left form a capacitive potential divider.
>if the earth wire is not bonded to ground, then the midpoint of that is
>going to be at half mains voltage - 55V (or 120V) wrt neutral, which is
>efectively local earth. That means the case of the PC - the external
>case - is at that voltage wrt local earth. Now the capacitor values
>_should_ limit the available current, but I'd not bet on it. Also, the
>capacitors should be a special safety type which are unable to fail
>shorted (I forget the type - it's either class X or class Y), but I've
>seen plenty of cheap-n-nasty PSUs with just 600V capacitors in this
>position. If I spot them like that, they get replaced, BTW.
>And that's what I'm going on about. Not that the keyboard was live, but
>that the metal case of the PC had a significant voltage wrt local
>Normally you'd not notice it - the front panel is probably plastic, so
>are the floppy drives, etc. But reach round the back to plug in the
>keyboard cable, touch the back of the case, and zap!
>> like mine, it's very difficult to maintain wiring and even talk about
>> sometime...I'm very lax with safety precautions. There is no way the
>I am not fanatical about safety - I do plenty of things that can be
>considered dangerous. I use machine tools, I change CRTs, I work on
>30kV supplies, I work on mains, I repair SMPSUs, I even work on live
>equipment if necessary. And yes I defeat safety interlocks if there's a
>good reason for so doing, and I know what the consequences are. But
>no way you'll catch me removing an earth connection without a _very_
>> boy could have electrocuted himself with any modern keyboard I know
>> of, since it's mostly impossible to get to the components inside them
>> w/o taking the thing apart. An XT keyboard, OTOH, is metal which
>The backplate of an XT keyboard is connected to the body of the DIN
>If that's live for any reason, then there's something seriously wrong -
>see above.
>> could kill someone if the keyboard is damaged or plugged into the
>There is no voltage on a keyboard connector that can kill - at least
>if the rest of the PC is correctly wired. I'd touch any component in a
>working XT keyboard and I'd live to tell the tale.
>> wrong place. More likely is that he touched a bad power cable with
>> hand. If the PC got moved and the keyboard got unplugged, it could
>> undo any cables spliced together w/scotch tape (another of my bad
>Argh!!! I prefer to use proper cable joined with proper connectors -
>insulation and strain reliefs.

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Received on Thu Jul 23 1998 - 11:56:18 BST

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