Compaq//safety issues - test-drb@ccmp.vtda.org

23 Jul 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  the 
...
 mains to know how dangerous it can be.

I doubt 240 vs. 110 is much difference as far as heart muscle is
concerned
...
 (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  240V
...
 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  the 
...
 main distribution board (US), last distribution
transformer (UK), and  is 
...
 effectively earth. And live is 110V (or 240V) above
that.

Now, the 2 capacitors on the left form a capacitive potential divider.  So
...
 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  metal
...
 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  earth. 
...
 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  there's
...
 no way you'll catch me removing an earth connection
without a _very_  good 
...
 reason.

  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  plug. 
...
 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  not 
...
 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 -  with 
...
 insulation and strain reliefs. 

-tony

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