Close....
In the distribution system, the neutral wire is indeed
neutral, and is normally earthed at the sub station
transformer (note, not the same as your local earthing
wire!!!). Because the three phases are divided to have
approximately equal use, there is no significant net
current flow in the wire.
Within your single - phase home, however, it has only
one phase of return current flowing in it, which means
that there IS a net current flow, and due to the
resistance of the wire, a potential, whose size
depends on the current flowing and how far it is to
the substation. So don't treat it as naturally safe -
that's what the local earth wire is!
Julian
PS this is a neat way to save copper, since for a
three - phase mains distribution system, you only need
wires for the three lives. The common neutral (which
should be wire 4) is formed by the earthing at the
substation and at the generating plant.
PPS you really don't want to get one of those 415V
cross - phase shocks.
--- Chris Barker <ftog@xxxxxxxxxxxxxxxx> wrote: >
> Thanks Piers. I must admit that AC electricity has
> always had me
> somewhat puzzled; I tamper with ring main wiring
> with great caution!
>
> Chris
>
> On 15 Apr 2004, at 11:46, Piers Hemy wrote:
>
> >
> > Quite right, Chris, and (as far as UK supply is
> concerned) quite wrong
> > Daniel, in the sense that you would be ill-advised
> to mix them up!
> > But I
> > will be happy for Tim Hughes to tell me otherwise
> (again!).
> >
> > Thw neutral wire *is* neutral, with zero current
> flowing, when the
> > entire
> > system is running properly - the entire system is
> a three-phase AC
> > system,
> > able to provide 415V three-phase, and 24v from any
> one phase. For
> > domestic
> > supply, single pahse is the norm, and assuming all
> three phases are
> > balanced, there will be no current flowing on the
> neutral (which is
> > common
> > to all three pahses).
> >
> > It was a question that bugged me at school too
> (thinking of James
> > Royall's
> > original question) but I got the answer in
> Chemistry rather than
> > Physics
> > class. Put that down to the approachability of
> the teacher.
> >
> > Piers
> >
> >
> <|_:-)_|>
>
> C M I Barker
> Cambridgeshire, Great Britain.
>
> +44 (0)7092 251126
> ftog at threeshoes.co.uk
> http://www.threeshoes.co.uk
> http://homepage.mac.com/zuiko
> ... a nascent photo library.
>
>
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