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Hi,

My PII motherboard died a week ago - just 4 days after installing - cause
unknown. Of course the vendor replaced it. I was checking whether my SMPS
was giving correct voltage before I plugged in the new motherboard. The
voltage outputs were all right but I found that the pin giving -5 V DC was
giving the same voltage even after switching off. I was wondering if that is
normal.

The SMPS is AT type. Also there is leakage current in the case enough to
give a good shock. The leakage occurs only when load is connected (ie
motherboard).


Bharat
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My first concern is that no one has picked up on the hazard of case that
gives "a good shock".  The power supply should be GROUNDED.

I presume that this is not just a static spark going to ground, as he
refers to a "leakage", which implies that it was continuous as long as
he was touching it.  It sounds like there is a fault which has connected
the case to LINE potential.  As long as he touches just the case (line),
and nothing grounded, he will only feel a "tingle" due to the high
resistance of most flooring, etc.  Should he also touch a ground (such as
a radiator, water tap, etc.) he could suffer a FATAL shock.

If he checks the potential between the case (system on) and a good ground
such as a water tap (careful!) with a voltmeter, he may be surprised.

The fault could be in the power supply, but it could also be in wiring to
the socket.  A few years back, a woman who lived down the street from my
aunt was killed while using an electric lawnmower.  The "ground" connection
was not grounded, instead it was connected to the line. She was OK as long
as she held on to the rubber grip, and stood on her rubber-soled shoes,
but afterwards, she switched it off and knelt on the damp ground while
holding the metal frame.  That's how they found her.

If you "must" test for line voltage using yourself as a voltmeter (I
definitely DO NOT recommend this), it is supposedly safer to use the
back of the knuckles to make contact.  The theory is that a severe shock
will cause the muscles to contract, pulling your fingers AWAY from it.
(Rather than closing on the handle like that poor woman.) Of course, this
may not matter if you wind up on the floor with no pulse.

Incidentally, if the case is "live", connecting a modem (telephone line
has a ground) could kill the motherboard and the modem.

Boyd Ramsay

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