[sdiy] My PSU...

Neil Johnson nej22 at hermes.cam.ac.uk
Sat Jul 27 20:33:27 CEST 2002

On Sat, 27 Jul 2002, Ingo Debus wrote:

> epeasant at telusplanet.net wrote:
>  > you fuse ONLY the "HOT" wire. In North America the ground
>  > is at the same potential as the neutral wire, but in Europe
>  > ground is midway between each line.
> No, that's wrong. We have a hot and a neutral wire too here, and the
> neutral is at earth potential, i.e. can be touched without danger... In
> other countries in Europe this is different, at least for the 3-prong
> plugs.

In the UK _all_ plugs are three-pin (although for double-insulated
equipment the earth pin is usually plastic).  We have:

- Earth
- Live
- Neutral

Now, Live is nominally 240V AC, 50Hz.  Earth is for protection in case of
fault, and is common throughout the house, and connects to the earth point
at the mains entry point.  The Neutral is also bonded to the SAME earth
point as the house earth.  You might think that Earth and Neutral are
redundant, that only one is necessary.  But they do have different

- the Neutral carries the main current, from the Live via the appliance. -
the Earth is for protection only, and only needs to carry sufficient
current to cause the fuse and/or circuit breaker to blow.  Thats why in
general the Earth wire is smaller than the Neutral, because it only needs
to carry the full current for a brief period.  An example: a distribution
board wired with 25mm2 cable only needs a 16mm2 Earth bond, because it
only needs to carry the full current for a fraction of a second before the
safety device (fuse, trip, whatever) blows.

We also have devices called Earth Leakage Circuit Breakers (ELCB) that
monitor the current flowing through the Earth wire.  If it goes above a
certain threshold (to allow for capacitive coupling, capacitor leakage,
etc) it trips.  This is different from Residual Current Circuit Breakers
(RCCB) that monitor the _difference_ in current flowing through the Live
and Neutral.  Typically, if the difference is greater than 30mA (because
it might be flowing through you to ground) it trips.  RCCBs tend to be
favoured these days, as the ELCB relies on the fault current flowing down
the Earth wire, which is not always the case :-(

And all our plugs have fuses fitted in them as standard.  That fuse is
there to protect the cable, not the appliance.  And we only have one fuse,
in the Live, in appliances.

I guess we take more care because we have more volts to play with/be
killed by.  Also, all mains sockets have a protective shutter over the
Live and Neutral holes, which is opened by the Earth pin (which is longer
than the other two pins).  Explains why double-insulated plugs have
plastic Earth pins, otherwise you would not be able to plug them in.

Hope that helps,

Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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