MIG wrote:
On Nov 30, 11:14 am, David Hansen
wrote:
On Sat, 29 Nov 2008 09:26:38 -0800 (PST) someone who may be MIG
wrote this:-

LU's centre rail is I believe at minus 200 volts.

That's interesting, because I was under the impression that there
was no danger from the middle track,

A dangerous impression. If there was no (electrical) danger from the
central conductor rail then it would not be mounted on insulators.

Insulators are a give-away that something is energised.

Piecing it all together, I suppose it adds up, in that if a neutral
rail was to be used, it might as well the the running rails or
something earthed. So the fact that there's a special rail on
insulators means it can't be neutral. Never really thought it through
though.

No, the fourth rail was used because allowing the current to return though
earthed running rails causes corrosion to any metal utility pipes in the
area, so the fourth rail and insulators are there to protect water mains.

On Sun, 30 Nov 2008 03:46:02 -0800 (PST) someone who may be MIG
wrote this:-

Piecing it all together, I suppose it adds up, in that if a neutral
rail was to be used, it might as well the the running rails or
something earthed. So the fact that there's a special rail on
insulators means it can't be neutral. Never really thought it through
though.

The reasons for adopting the four rail system remain sound. As well
as corrosion it allows a simpler arrangement for the traction and
other electrical systems, like signalling. With a three rail system
one needs gadgets like impedance bonds to keep the electrical
systems separate enough to avoid interference, but there is one less
rail to install and maintain.

Roughly speaking, in a small but complicated system the reduction in
the number of gadgets outweighs the extra rail, but in a less
complicated system over longer distances not having an extra rail is
the important factor. That assumes starting from scratch, but that
is not entirely accurate in a number of ways. Unprotected conductor
rails would not be allowed in a new system anyway and a protected
central conductor rail could probably not be devised, but protected
conductor rails could be fitted either side of the running rails.

None of that outweighs the fact that low voltage conductor rails,
especially unprotected ones, are not ideal and were things being
done from scratch a high voltage overhead system, with the larger
tunnels this implies, would be chosen. In fact an overhead system
wouldn't in fact involve any larger tunnels. As any new system would
be fitted with emergency walkways and so the tunnels would be larger
anyway.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Sun, 30 Nov 2008 13:57:11 +0000, David Hansen
wrote:

None of that outweighs the fact that low voltage conductor rails,
especially unprotected ones, are not ideal and were things being
done from scratch a high voltage overhead system, with the larger
tunnels this implies, would be chosen. In fact an overhead system
wouldn't in fact involve any larger tunnels. As any new system would
be fitted with emergency walkways and so the tunnels would be larger
anyway.

And would be better off larger so that more capacity can be provided
in a given platform length.

Neil

--
Neil Williams
Put my first name before the at to reply.

David Hansen writes:

A dangerous impression. If there was no (electrical) danger from the
central conductor rail then it would not be mounted on insulators.

Insulators are a give-away that something is energised.

Does that necessarily follow? If the reason for using a 4-rail system
rather than a 3-rail with return via the running rails were to avoid the
problems such as electrolysis and interaction with signalling, would it
not be possible to do it by having the centre insulated rail at a
nominal ground potential but only bonding it to ground at the
substations?

On Sun, 30 Nov 2008, Steve Fitzgerald wrote:

In message , Tom Anderson
writes

I recollect an incident a few years ago when the traction current had been
discharged (turned off) due to person under a train. For complicated
reasons, the centre rail had become live at +420v.

This must be some new meaning of the term 'turned off' of which i was not
previously aware! As you say, complicated reasons - but this sounds like
the kind of thing that really, really shouldn't happen. Crumbs.

It had been correctly discharged but been re-fed in error from
elsewhere. You will note that what should normally have been -210v had
now become +420v.

Aha.

Would it be sensible to electrically connect all four rails, with a set of
crocodile clips or something, when working on the track in situations like
this? Then, if there was a mistake which fed voltage to one or more rails,
it would short out, and circuit breakers located wherever the feed-in
was happening would break and cut it off. It would be something you could
do at the site which would absolutely guarantee that there was no
dangerous voltage there. The problem might be the effect it had on other
parts of the system, though.

tom

--
Scheme is simple and elegant *if you're a computer*.

In message
David Hansen wrote:

[snip]

Roughly speaking, in a small but complicated system the reduction in
the number of gadgets outweighs the extra rail, but in a less
complicated system over longer distances not having an extra rail is
the important factor. That assumes starting from scratch, but that
is not entirely accurate in a number of ways. Unprotected conductor
rails would not be allowed in a new system anyway and a protected
central conductor rail could probably not be devised,

You could do it providing you never turned the stock, so the Circle line
would be out for a start.


--
Graeme Wall
This address is not read, substitute trains for rail.
Transport Miscellany at http://www.greywall.demon.co.uk/rail/index.html

On 30 Nov, 15:43, Tom Anderson wrote:
Would it be sensible to electrically connect all four rails, with a set of
crocodile clips or something

Crocodile clips or something, and how to use them, in glossy brochure
form:
http://www.tubelines.com/whatwedo/tr...ing_device.pdf

U

On 30 Nov, 13:57, David Hansen
wrote:
Unprotected conductor
rails would not be allowed in a new system anyway and a protected
central conductor rail could probably not be devised, but protected
conductor rails could be fitted either side of the running rails.

Or you put the conductor rails above each other:
http://upload.wikimedia.org/wikipedi...amome-7280.jpg

U

In message
, Mr
Thant writes

Would it be sensible to electrically connect all four rails, with a set of
crocodile clips or something

Crocodile clips or something, and how to use them, in glossy brochure
form:
http://www.tubelines.com/whatwedo/tr...guidance/LU_sh
ort_circuiting_device.pdf

Which sadly in this case, the SCD was the cause of the problem.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)

On Sun, 30 Nov 2008 12:38:22 -0000, "John Rowland"
wrote:

MIG wrote:
On Nov 30, 11:14 am, David Hansen
wrote:
On Sat, 29 Nov 2008 09:26:38 -0800 (PST) someone who may be MIG
wrote this:-

LU's centre rail is I believe at minus 200 volts.

That's interesting, because I was under the impression that there
was no danger from the middle track,

A dangerous impression. If there was no (electrical) danger from the
central conductor rail then it would not be mounted on insulators.

Insulators are a give-away that something is energised.

Piecing it all together, I suppose it adds up, in that if a neutral
rail was to be used, it might as well the the running rails or
something earthed. So the fact that there's a special rail on
insulators means it can't be neutral. Never really thought it through
though.

No, the fourth rail was used because allowing the current to return though
earthed running rails causes corrosion to any metal utility pipes in the
area, so the fourth rail and insulators are there to protect water mains.

On a related note, the original LNWR/LMS electrification was 4th rail.
It was converted to three with the return and running rails bonded, in
1970.

Of course it made sense to use the existing 4 rail system because
apart from the LBSCR's overhead AC that was what the other London area
lines used. The LNWR electrification was planned in 1907 and opened in
1914, with the Bakerloo linking up a year later.

The LSWR 3 rail system was planned later with the first section
opening in 1915.