In message , Tom
Anderson writes
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.
Don't tube trains still carry Short Circuiting Devices (SCDs) anymore?
It used to be that when traction current had been discharged a driver
would clip the device over the outside (live) rail, look away incase of
arcing, then smartly smack the other end down onto the centre negative
rail. It would then be left there in situ for safety until it was
required to restore traction current.
--
Clive

wrote:
In article ,
(Paul Scott) wrote:

wrote in message
On hybrid sections (e.g. East Putney to Wimbledon) the 4th rail is
at +630v.
That would be the third (positive) rail. Otherwise the 'third rail'
stock wouldn't work too well...

Yes, I meant the outside rail. I was counting across with the centre as
third.


If you count in such a way that the outside ("live") rail is the fourth,
then the centre is the second. If you do it the other way so that the
centre is third, then the live rail is first!

Peter Beale

On Sun, 30 Nov 2008 20:03:37 -0000 someone who may be "Peter Masson"
wrote this:-

The GNR sought running powers over the Canonbury Curve and
into Broad Street, but the LNWR prevented the North London granting these.
Accordingly the GNR invited the NLR to run trains from Broad Street out to
its suburban stations.

I have seen photographs of LMS locomotives operating trains amongst
LNER trains. The coaches were presumably LMS coaches too, possibly
former NLR coaches. IIRC this lasted until the early 1930s, when the
LNER took over at least the locomotives. I could be wrong about the
date though and it may have lasted into the 1950s.


--
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

In message , Clive
writes

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.
Don't tube trains still carry Short Circuiting Devices (SCDs) anymore?
It used to be that when traction current had been discharged a driver
would clip the device over the outside (live) rail, look away incase of
arcing, then smartly smack the other end down onto the centre negative
rail. It would then be left there in situ for safety until it was
required to restore traction current.

Yes, one at each end of the train with the emergency equipment.
--
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 19:11:56 +0000 someone who may be Tom Anderson
wrote this:-

Ah, so although there was no potential between the two power rails,
there was a potential between them and the ground?

Sounds like it. However, I wouldn't like to go much further without
knowing the complicated reasons why it occurred. There are a few
ways this could happen.

Really, you need to connect the two rails to each other and also to the
ground. I was thinking the running rails would make a good ground
substitute here.

It should be enough to connect the conductor rails to each other.
However, there are failure modes where one pole of the circuit fails
(broken cable and non-interlinked fuses/circuit breaker are the
obvious ones) and if this happens unexpected voltages can be
present. Although AC circuits can suffer these problems they tend to
be minimised by various means. Two, three and five wire DC circuits
can suffer a number of maladies which those who work on them would
be wise to be aware of.

There is a lot to be said for treating all equipment as energised at
all times except when working on it under a safe system of work. I
recall one fairly well known evacuation of a tube train, probably
2-3 decades ago, when the conductor rails remained energised for a
short distance, but were discharged for most of the way along the
tunnel on either side of this short section. The fire brigade,
police, ambulance and passengers walked alongside these energised
conductor rails, fortunately without incident.



--
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

In article , (Peter
Beale) wrote:

wrote:
In article ,
(Paul Scott) wrote:

wrote in message
On hybrid sections (e.g. East Putney to Wimbledon) the 4th rail is
at +630v.
That would be the third (positive) rail. Otherwise the 'third rail'
stock wouldn't work too well...

Yes, I meant the outside rail. I was counting across with the
centre as third.

If you count in such a way that the outside ("live") rail is the
fourth, then the centre is the second. If you do it the other way
so that the centre is third, then the live rail is first!

I was assuming, whatever else, that the running rails were first and
second!

Bear in mind that the first third rail systems were the City and South
London and Central London Railway's and theirs were between the running
rails.

--
Colin Rosenstiel

On Sun, 30 Nov 2008 16:00:14 +0000, Graeme Wall
wrote:

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.

You provide appropriate changeover switches as on the original Great
Northern and City line system which IIRC used two outer conductor
rails until later changed to conventional four-rail working.

On Sun, 30 Nov 2008 12:34:42 -0500, Christopher A. Lee
wrote:

On Sun, 30 Nov 2008 15:08:16 +0000, Graham Murray
wrote:

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?

There will always be leaks due to build up of dirt.

And it would assist the occurence of faults where the return path
breaks and the conductor on the load side achieves a good connection
via the wrong path (think RCD).

On Sun, 30 Nov 2008 19:11:56 +0000, Tom Anderson
wrote:

On Sun, 30 Nov 2008, David Hansen wrote:

On Sun, 30 Nov 2008 16:50:43 +0000 someone who may be Steve
Fitzgerald ] wrote this:-

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.

I guessed that it was one of a series of problems which caused the
voltage from a still energised positive rail to be transferred to the
negative rail. Presumably there was some sort of broken connection or
open circuit breaker/fuse in the feed to the negative rail.

Ah, so although there was no potential between the two power rails,
there was a potential between them and the ground?

Really, you need to connect the two rails to each other and also to the
ground. I was thinking the running rails would make a good ground
substitute here.

Only on one running rail of a 3-rail system where there is intentional
permanent bonding suitable for traction currents (and even then it
isn't necessarily at the same PD as the local earth). On a LU 4-rail
system the running rails are not "earthed" WRT traction supplies and
have no certain/permanent low-resistance path to earth even if they
did when a 3-rail SCD (positive-to-running rail) was first applied.
Think of the earth wire in a domestic system - this normally carries
no current but still has a minimum current rating and maximum
permitted impedance to the supply earth as it is an intentional path,
albeit with a limited expectation of actual use.

"Tom Anderson" wrote in message
.li...
On Sun, 30 Nov 2008, David Hansen wrote:

On Sun, 30 Nov 2008 16:50:43 +0000 someone who may be Steve
Fitzgerald ] wrote this:-

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.

I guessed that it was one of a series of problems which caused the
voltage from a still energised positive rail to be transferred to the
negative rail. Presumably there was some sort of broken connection or
open circuit breaker/fuse in the feed to the negative rail.

Ah, so although there was no potential between the two power rails, there
was a potential between them and the ground?

Really, you need to connect the two rails to each other and also to the
ground. I was thinking the running rails would make a good ground
substitute here.

Unfortunately, in the incident Steve is referring to, the "ground
substitute" turned out to be a para-medic... (She survived).
--
Cheers, Steve.
Change jealous to sad to reply.