Running a 220v computer in a 110v environment

From: Philip.Belben_at_pgen.com <(Philip.Belben_at_pgen.com)>
Date: Wed Oct 13 11:47:12 1999

Peter Pachla quoting [I forget whom]

> > A fuse will never protect _you_ nor the device. It is only ment
> >to protect the house/the installation....
>
> This I have to take issue with (though I'm probably quibbling over
> semantics?).
>
> The purpose of the fuse in the PLUG is to protect the power cable between the
> plug and the device from carrying an unsafe current in the event of a problem
> with that CABLE.
>
> The device itself should contain a second fuse which is there to protect the
> PSU in the event of a failure or overload.
>
> At least that's what we were taught when I did an electronics course a few
> years back.


Right. I think there are some misconceptions around here that need clearing up.
I will put in a disclaimer that I am not an electronic engineer but an
electrical engineer - my field ends roughly where the power cord enters the
computer! - but many of the principles carry across between the fields.

Misconception 1: A fuse or breaker in the live (hot) wire means that a person
touching live metalwork will not get a fatal shock. THIS IS TOTALLY FALSE -
which is I think what our previous contributor was getting at.

A fuse or breaker is intended to interrupt the circuit when there is an
excessive current flowing. Since it can take as little as a milliamp to kill
someone, and very few people survive more than 30mA, a fuse is unlikely to help.

The fuse - as Tony and Peter have both pointed out - performs what might be
called damage limitation on the device. One fault causes excessive current; the
fuse prevents the excessive current causing further faults, either upstream or
dounstream.

Misconception 2: Earthed (grounded) metalwork is safe. It is usually safe,
PROVIDED fuses and things are in place, AND THE EQUIPMENT HAS BEEN PROPERLY
DESIGNED.

Anyone who has designed electronics will have encountered earth path resistance.
If there is a large current flowing in the earth path, the potential of
nominally grounded points can rise. (In digital circuits this is usually a
transient phenomenon, where a switching surge puts glitches on signals that
should be constant at ground potential. This has the disadvantage that
inductance is also significant...)

When designing electrical equipment, it is important to keep the resistance of
the earth path low enough that in the event of a short circuit, the potential of
(say) the chassis will not rise too high.

So how high is too high? The answer depends on the length of time for which the
potential will be present - the longer it is there, the more likely it is to
kill the person writhing on the floor unable to release their finger muscles...

The length of time the high potential is present depends - of course - on the
fuse or breaker. Fortunately there is negative feedback - the lower the
current, the longer the fuse takes to blow; but the lower will be the potential
rise, so the lower will be the current flowing in the person touching it. In
this sense the fuse protects you - in the event of a short circuit, it cuts off
the current before a high voltage has been present on the "earthed" chassis long
enough to kill you.

But remember, THE CURRENT FLOWING IN _YOU_ IS DEPENDENT ONLY ON VOLTAGE (and on
how you make contact - but not on the circuit to which you're connected) - your
resistance is so high (kilohms) that your effect on short circuit currents or
fuses is NEGLIGIBLE. (OK, perhaps someone will come up with an exception - but
it is best to assume that this always applies. I know of no exceptions on
domestic mains...)

If you are running electrical equipment and a fuse blows, YOU MUST DISCONNECT IT
before you go poking around inside. NEVER rely on the open-circuited fuse to
protect you (which may also be what our previous contributor was getting at).

If you are running British equipment from a US 120-0-120 supply, and there is a
short to ground near the hot end, the fuse will blow in the normal way. This
will interrupt most of the current, but will leave the other side of the supply
connected through most of the device to ground. An earth leakage trip (ELCB,
RCD, GFCI) [1] is required to pick up the remainder of the fault current and
disconnect altogether.

If there is a short to ground near the neutral end, only the ELCB (and possibly
the main breaker or fuse at the consumer unit) will see it. This is the
disadvantage of running British (or European) equipment on US 220V mains.

The fact that parts of the innards will be live (hot) when a single pole switch
is opened or a fuse blows is IRRELEVANT, because YOU SHOULD BE UNPLUGGING BEFORE
YOU GO POKING AROUND INSIDE ANYWAY.

If you have to run stuff with the cover off, be aware of the dangers, and take
precautions to minimise the risk. But this too goes for all equipment anyway.

I hope this clears up some misconceptions.

Philip.

[1] Earth leakage trips:

ELCB = Earth Leakage Circuit Breaker (UK)
GFCI = Ground Fault Circuit Interrupter (US)

These are generally Residual Current Devices (RCD) - detecting the difference
between hot and neutral currents. Some older trip switches were not RCDs, but
measured ground potential rise instead, but these are not recommended for new
designs - or for old ones for that matter! They should still work for any fault
that makes a chassis potentially (pun intended) dangerous to touch.






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Received on Wed Oct 13 1999 - 11:47:12 BST