On Wed, 14 Apr 2010 10:28:42 +0100, tony sayer
wrote:

In article , Charles Ellson
scribeth thus
On Tue, 13 Apr 2010 22:00:22 +0100, tony sayer
wrote:

The WCML modernisation scheme uses (mostly!) 400 V three-phase, but
this has not been a great success owing to the higher currents (and
therefore greater number of sources of supply, known as Principal
Supply Points or PSPs) required.

Ummm.. Rather interesting this but this 400 volt 3 phase supply was
"difficult" in quite what way for signalling purposes?..

As implied above, it requires more transformation points and/or more
copper in the ground. If it replaces a single-phase supply, it also
(assuming 3P+N) doubles the number of wires to be played with (or
stolen) and enables the various "interesting" effects caused by breaks
in the neutral or one phase wire without other conductors being
interrupted.

OK what sort of current draw does a typical signal post, gantry frame,
need then?...

The relevant figure is probably something more like how much average
current is drawn overall by the various equipment in the general area
x miles away at the end of the cable, remembering that other than
signal lamps and track circuits ...

Suppose in that you could include points motors etc...

.... a lot of it is transient.

And that much difference in overall power supplied 650 V single phase
rather then I presume 415 or 440 or was it actually 400 three phase?.

It's years since I played with 3-phase calculations (and in 20 years I
only ever had one 3-phase device to deal with) but IIRC the
(theoretical) "trick" is that with a balanced load your neutral
conductor can be almost non-existent (as occasionally occurs on LU
when it gets stolen) thus significantly cutting down on the amount of
copper compared with a single-phase circuit delivering the same amount
of power over two wires. Someone who has covered the subject rather
more recently might be able to do a quick back-of-a-Niquitin-patch
calculation but assuming ideal conditions my quick calculation is that
the CSA reduction of 3-phase (sans neutral) over 1-phase for the same
total current delivered is around 50% although I have a nagging doubt
that root-2 comes into it somewhere and pushes the reduction down to
25-33%.

Take your point re more cable required seem the price of copper is on
the up again. Bit on the news of some scrota almost frying himself in a
sub station...