electric shock from chassis

 

Hello Group:

I have an older computer which was moved to another room. It seemed to
run fine, but one day the monitor (LCD) was off, so I looked behind it
and the power cord wasn't pushed all the way in. I pushed it in and
the monitor came on. Everything seemed fine, but then I tried to move
the computer to pick up something which had fallen behind it. When I
touched the back of the chassis, I got a tooth-rattling shock. I had
sneakers on, so I wasn't grounded, but I did put both hands on it. I
pulled the AC power cord and looked inside the computer for some sign
of insulation failure or whatever might have electrified the chassis.
But I couldn't find anything by visual inspection.

Could it be the PSU?

I am also going to have an electrician check the wall outlet it was
connected to.

Any suggestions? The obvious suggestion is to take it to a computer
repair place, which I'll probably do, but I thougt I'd ask for advice
about what might be going on, for my own curiosity.

Many thanks.

Jack

 



A vital question here is whether your wall outlet is properly grounded.
Computer PSUs leak current to the chassis via their built-in spike
suppression circuits. This leakage current is not high enough to pose a
health hazard to a healthy person with a dry skin. It is supposed to drain
out to ground via the power cord, but if the wall socket is not grounded, it
makes the chassis partially live and can give an unpleasant shock,
especially if your hands are damp.

 



More likely, you picked up a static charge, which discharged when you
touched the pc. If your pc's outlet isn't grounded, and a fault in the
machine made the chassis live, touching it wouldn't cause you to get a shock
unless you were otherwise grounded, which you say you weren't.

 

RBM wrote:

Assuming the OP is in the northern hemisphere where it's currently
summer...not much of a chance it was static discharge...
any way, not worth the risk!


YES definitely have an electrician check the outlet...if the power
supply was shorted, it should have tripped the breaker.

Also: Good chance the power supply is defective as even if the machine
was not grounded properly one should not have gotten shocked.


Standard 115vac CAN be lethal!

Not worth taking a chance!

 

JClark wrote:

The power supply has filtering at the AC input. The purpose of the
filtering, is to prevent switching noise from inside the supply,
from traveling out onto the line cord. Not all supplies are well
designed this way, and my old Antec used to emit enough signal
I could see it on a TV set.

The following device, is a line filter. This embodies a chunk of
circuitry your ATX power supply has. The example filters are
one and two stage filters. These would typically be included inside
equipment, to prevent noise from going back to AC.

http://media.digikey.com/PDF/Data%20Sheets/Tyco%20Electronics%20Corcom%20PDFs/N%20Series%20RFI%20Filters.pdf

In that Corcom N series filter datasheet, there is a leakage spec
stated. That means, they know the nature of the filter design,
allows some 60Hz current to flow from line into safety ground.
The premise of that filter, is it is used in a "three prong" circuit.

The ATX supply would share something in common with those one
or two stage filters. There may be a small amount of leakage
current, flowing into safety ground.

When is this an issue ? If your wall outlet has no safety ground,
then the chassis of the computer will be electrified.

The old house I used to live in, had two prong outlets. So there
was no safety ground provided.

If the shock was tooth rattling, it could be the leakage is
higher than normal.

But the thing is, if safety ground is present, there should be
continuity from the third prong at the wall outlet, to
the metal chassis of the computer. Any leakage current, up to
the amps level, should flow back into the safety ground.

So the first thing to check is safety ground. Does the
outlet have three prongs ? If you insert an outlet tester,
is the pattern on the outlet tester consistent with a properly
wired outlet ? And if you use a multimeter, and measure from
the third prong of the line cord, to some shiny metal on
the computer chassis, does that read zero ohms, indicating a good
connection ? Somewhere along the way, it looks like you lost
your connection to safety ground.

http://upload.wikimedia.org/wikipedia/commons/thumb/7/78/Receptacle_tester.jpg/800px-Receptacle_tester.jpg

    Paul