On Tue, 30 Mar 1999 Bharat M Ale <[log in to unmask]> wrote:

<snip>
>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).

I note that you only experience the shock when the motherboard
is connected, which suggests your SMPS is grounded only through
the motherboard, an unusual situation which you can confirm with
an ohmmeter after first disconnecting the mains supply cable and
DC output cables. There _ought_ to be a solid, zero-Ohms connection
between the ground pin of the input mains power connector and chassis.
(Physically, this should be provided by a ground wire inside the SMPS
near the mains in socket, though it might be by an earthing tag
on the front panel power switch.)

If you don't have this connection I would guess that you also
don't have a 3-wire power connection: line, neutral and ground;
or that your ground connection is faulty.

In your SMPS there is an input filter containing 3 delta-connected
capacitors. (This filter is to stop noise leaking OUT OF your SMPS
into the power supply system, as well as stop interference and power
line spikes coming INTO your computer.)

One capacitor is X-connected between the two power lines, live and neutral:
the other two are Y-connected, one between each power line and ground.

Your computer chassis is also connected to the power supply ground
to provide safety earthing: a short from line to ground is supposed
to blow the protective fuse and isolate the equipment.

Hoewever, if there is no ground connection in your power supply wiring,
the two Y capacitors act as a capacitive potential divider and
force the chassis to float at half the power line voltage:
around 60V for US influenced countries and around 110V for
UK/European influenced countries.

Hence, when you touch the chassis and a true earth, a "tingle".

Luckily, the Y capacitors are small so the current is limited and
the shock is unlikely to be fatal, but the situation should be
corrected since it points to a lack of safety fusing.
It could also result in damaging voltages being applied
to peripheral interfaces.

If you cannot provide a proper low-impedance ground connection
you should consider using an ELCB (earth leakage circuit breaker),
also called a RCB (residual current breaker), for electrical safety
and also ensure that all conductive items are "grounded" together
so there is no risk of touching items at different voltages.
You could also obtain medical-grade power line filters, which
use much smaller Y capacitors, but this would require some
rebuilding of your equipment and will not on its own solve
the safety issue.


Len Warner <[log in to unmask]> http://easyweb.easynet.co.uk/~len/ ICQ:10120933

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