"Dr J R Stockton" wrote in message
nvalid...

Now join the three
neutrals into one; the three return currents, being of different phases,
add to nothing, with balanced loads; therefore the neutral, carrying
only off-balance currents, can be comparatively thin. See sig.

Or not... Subject to the type of load it is recommended that you up-rate the
neutral's

Capacitive loads, those with a poor crest factor and high THD will
(unchecked) burn your neutral bars/links out in no time at all.

This has been a big problem in I.T installs of ye olden days using current
kit and high density computing.

Then we get into the problems with kVAr...

On Fri, 16 Apr 2010 01:34:32 +0100, "Q" ..@.. wrote:


"Dr J R Stockton" wrote in message
. invalid...

Now join the three
neutrals into one; the three return currents, being of different phases,
add to nothing, with balanced loads; therefore the neutral, carrying
only off-balance currents, can be comparatively thin. See sig.

Or not... Subject to the type of load it is recommended that you up-rate the
neutral's

Doesn't the cheat for that one involve the use of star-delta
transformers at each end of the circuit (i.e. no neutral at all on the
"trunk" part of the circuit) and controlling the out-of-balance
currents appropriately ?

Capacitive loads, those with a poor crest factor and high THD will
(unchecked) burn your neutral bars/links out in no time at all.

This has been a big problem in I.T installs of ye olden days using current
kit and high density computing.

Then we get into the problems with kVAr...

I have, somewhere in my possession, a table of typical signalling
equipment loads, which I will try to find and summarise on this group
next week. As far as LED signals are concerned, I believe that they
have a higher demand than bulb signals, owing to the large number of
individual LEDs as opposed to a single bulb.

Hope this is of further use.

Very interesting that .. be nice to see the table you refer to in due
course..
--
Tony Sayer

In uk.transport.london message
, Fri, 16 Apr 2010 01:34:32, Q posted:

"Dr J R Stockton" wrote in message
. invalid...

Now join the three
neutrals into one; the three return currents, being of different phases,
add to nothing, with balanced loads; therefore the neutral, carrying
only off-balance currents, can be comparatively thin. See sig.

Or not... Subject to the type of load it is recommended that you up-rate the
neutral's

Capacitive loads, those with a poor crest factor and high THD will
(unchecked) burn your neutral bars/links out in no time at all.

Symmetrical linear loads, such as pure capacitative or inductive,
maintain the balance of neutral currents, by symmetry.

Third (and its multiples) harmonic distortion currents will add in the
neutral rather than cancelling. But it takes a lot of THD to give the
same current in the neutral as is in each of the lives.

In the quoted part, "comparatively" is a comparison between the joined
neutral and the three unjoined ones; and "thin" really means cross-
section rather than diameter.

--
(c) John Stockton, near London.
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
Correct = 4-line sig. separator as above, a line precisely "-- " (RFC5536/7)
Do not Mail News to me. Before a reply, quote with "" or " " (RFC5536/7)

On Apr 12, 2:57*pm, "Peter Corser"
wrote:

The Central Line is already 750V - it was converted with the last upgrade..


Was it ?

Not according to what I see , and what I have checked on, today, with
the DC traction SCADA indicating line wide values fluctuating around
620 +/- 10 V ... that suggests its 630 V

--
Nick