On London Underground the DC traction current circuit to and from the
trains is via an insulated third live rail and an insulated fourth
live rail, and the earthed running rails are not used for traction
current. Would it be possible to use an isolation transformer to
prevent the electrocution of a person who touched one of the live
rails on this type of railway, or possibly on an AC version of it?
My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together. I would be very grateful to anyone who can explain
further.
Dominic

On 20/09/10 20:19, Dominic wrote:
On London Underground the DC traction current circuit to and from the
trains is via an insulated third live rail and an insulated fourth
live rail, and the earthed running rails are not used for traction
current. Would it be possible to use an isolation transformer to
prevent the electrocution of a person who touched one of the live
rails on this type of railway, or possibly on an AC version of it?
My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together. I would be very grateful to anyone who can explain
further.

Transformers don't work for DC, and aiui the 4th (return) rail exists to
reduce the corrosion effects of stray currents in the rather moist
environment of the tunnels.

To prevent the live rail - running rail shock risk, isolate the traction
supplies from the running rails. However, I suspect that not having
either side of the traction supply tied to earth brings back those stray
current corrosion issues, and / or it may have other issues too, like
affecting track circuits.

Rgds

Denis McMahon

In message
,
Dominic writes

On London Underground the DC traction current circuit to and from the
trains is via an insulated third live rail and an insulated fourth
live rail, and the earthed running rails are not used for traction
current. Would it be possible to use an isolation transformer to
prevent the electrocution of a person who touched one of the live
rails on this type of railway, or possibly on an AC version of it?
My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together. I would be very grateful to anyone who can explain
further.

No.

Transformers won't work with DC so the whole system would first have to
be converted to AC to even have a chance of working. I suppose it would
prevent the risk of electrocution as no electricity would get anywhere
useful.

Then you have to decide what you are isolating from what? Isolating
transformers don't stop electrocution; they just reduce the risks in
certain circumstances such as faults to earth (which, of course, LUL
nominally doesn't have as it is a 'floating' system).

The main issue would be that you still need the potential difference
between the 3rd and 4th rails to drive a train. If your electrocutee
touches both of these rails there will be no difference than if it was a
train there and it will hurt - a lot!.

In short, if it had a hope of working it would have doubtlessly already
been done. The easiest way is to just keep people away from the nasty
electricity as in any similar situation involving high risks of death.

--
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 Sep 20, 8:37*pm, Denis McMahon
wrote:
On 20/09/10 20:19, Dominic wrote:

On London Underground the DC traction current circuit to and from the
trains is via an insulated third live rail and an insulated fourth
live rail, and the earthed running rails are not used for traction
current. Would it be possible to use an isolation transformer to
prevent the electrocution of a person who touched one of the live
rails on this type of railway, or possibly on an AC version of it?
My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together. I would be very grateful to anyone who can explain
further.

Transformers don't work for DC, and aiui the 4th (return) rail exists to
reduce the corrosion effects of stray currents in the rather moist
environment of the tunnels.

Correct. In particular, the use of fourth rails was the result of the
use of cast-iron segments to line the tunnels, as these would have
been very vulnerable to corrosion.


To prevent the live rail - running rail shock risk, isolate the traction
supplies from the running rails. However, I suspect that not having
either side of the traction supply tied to earth brings back those stray
current corrosion issues, and / or it may have other issues too, like
affecting track circuits.

Rgds

Denis McMahon

Not directly earthing the traction return system is the best means to
prevent stray current corrosion. On a third-rail line, the running
rails are mounted on insulated fastenings for this reason, and the
negative busbar at a DC substation is insulated from earth. No third
rail system can ever be immune to stray current corrosion, espacially
at an AC/DC interface as the running rails at such points must be
earthed, but it can be managed to a level that is ALARP (As Low As
Reasonably Practicable). The drawbacks of a fourth rail system are (a)
additional complexity for the pway and pick-up arrangements and (b)
only one rail for the return circuit (on third rail systems, both
running rails are used for traction return, thereby reducing the
circuit resistance and allowing a small reduction in the number of
substations).

The Scarborough RT system in Toronto uses an ingenious fourth rail
system where two shrouded rails are used, one above the other; imagine
the DLR with two conductor rails one above the other and you get the
idea.

"Dominic" wrote in message
...
On London Underground the DC traction current circuit to and from the
trains is via an insulated third live rail and an insulated fourth
live rail, and the earthed running rails are not used for traction
current. Would it be possible to use an isolation transformer to
prevent the electrocution of a person who touched one of the live
rails on this type of railway, or possibly on an AC version of it?
My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together. I would be very grateful to anyone who can explain
further.
Dominic

You can't use a transformer on a DC system, that is totally impossible.
Transformers ONLY work on AC.

On Mon, 20 Sep 2010 13:03:29 -0700 (PDT), The Gardener
wrote:

On Sep 20, 8:37*pm, Denis McMahon
wrote:
On 20/09/10 20:19, Dominic wrote:

On London Underground the DC traction current circuit to and from the
trains is via an insulated third live rail and an insulated fourth
live rail, and the earthed running rails are not used for traction
current. Would it be possible to use an isolation transformer to
prevent the electrocution of a person who touched one of the live
rails on this type of railway, or possibly on an AC version of it?
My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together. I would be very grateful to anyone who can explain
further.

Transformers don't work for DC, and aiui the 4th (return) rail exists to
reduce the corrosion effects of stray currents in the rather moist
environment of the tunnels.

The LU system is not completely isolated from earth. The rails are
loosely tied to earth at the substation (IIRC ~400ohms +ve to earth,
~200ohms negative to earth) to maintain the conductor rails at about
+420v and -210v; should an earth fault occur on either rail then the
equipment at the substation detects this providing an alarm (and
tripping the supply ?).

Correct. In particular, the use of fourth rails was the result of the
use of cast-iron segments to line the tunnels, as these would have
been very vulnerable to corrosion.


To prevent the live rail - running rail shock risk, isolate the traction
supplies from the running rails. However, I suspect that not having
either side of the traction supply tied to earth brings back those stray
current corrosion issues, and / or it may have other issues too, like
affecting track circuits.

Rgds

Denis McMahon

Not directly earthing the traction return system is the best means to
prevent stray current corrosion.

There is still some form of earthing at the substation otherwise this
would produce a floating supply (generally deprecated in the
electrical world) which in very dry conditions could allow hazardous
voltages to exist on the traction return rail. The return path is
designed to tie it as close as possible to the substation earth by
providing a low resistance but also avoiding the opportunity for a
deliberate or accidental earthing of the return path at any point away
from the substation.

http://www.wsatkins.co.uk/Images/The...tcm12-2262.pdf
[http://tinyurl.com/34skod2]
shows arrangements for DC and AC supplies for overhead
electrification. IIRC there is a Railway Group Standard showing DC
traction supply arrangements (which I can't find ATM) which indicates
earthing of one side of the supply at the substation BUT at NO OTHER
point outwith the substation (as you indicate/imply above).

http://www.rgsonline.co.uk/Railway_G...20Iss%201a.pdf
[http://tinyurl.com/24cubfk] (a withdrawn document)
Deals with traction bonding (but not the supply origin), including :-
"5.1 The design of the return circuits shall be such that
there are no deliberate points of contact with the general mass
of the earth."

On a third-rail line, the running
rails are mounted on insulated fastenings for this reason, and the
negative busbar at a DC substation is insulated from earth. No third
rail system can ever be immune to stray current corrosion, espacially
at an AC/DC interface as the running rails at such points must be
earthed, but it can be managed to a level that is ALARP (As Low As
Reasonably Practicable). The drawbacks of a fourth rail system are (a)
additional complexity for the pway and pick-up arrangements and (b)
only one rail for the return circuit (on third rail systems, both
running rails are used for traction return,

No they aren't. _One_ of the running rails is normally used for
traction return as can often be discerned by the difference between
the traction bonds on one running rail and the much smaller signalling
bonds on the other side where there is a break (other than an
insulated break) in a running rail. The other running rail is
generally used for track circuits.

thereby reducing the
circuit resistance and allowing a small reduction in the number of
substations).

and giving a permanent "track occupied" indication on the track
circuit.

The Scarborough RT system in Toronto uses an ingenious fourth rail
system where two shrouded rails are used, one above the other; imagine
the DLR with two conductor rails one above the other and you get the
idea.

On Mon, 20 Sep 2010 12:19:00 -0700 (PDT) someone who may be Dominic
wrote this:-

My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together.

In addition to what others have said, if it worked such a system
would provide a false sense of security.

The existing system can cope with one earth fault, my memory is that
staff are alerted to it so that they can rectify it. The problem is
what happens when a second earth fault occurs, if that is in the
other pole then both are shorted together via the ground or a train
depending on the circumstances.

It is much the same with earth free supplies, such as that for mains
operated shavers. They protect against the first earth fault, but
fail in a much worse way if a second earth fault happens. If the
supply was earthed in the conventional way then there are problems,
but the supply should be disconnected automatically when the first
earth fault happens. Shaver leads are short and curled to guard
against the problem and reduce it to as low as reasonably practical.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000...#pt3-pb3-l1g54

On 20 Sep., 22:23, "Ray Shafranski" wrote:


You can't use a transformer on a DC system, that is totally impossible.
Transformers ONLY work on AC.

I supose the OP meant something along the lines of a transformer with
a rectifier bridge and filter stuck on the DC side.

On Sep 20, 11:38*pm, Charles Ellson
wrote:
On Mon, 20 Sep 2010 13:03:29 -0700 (PDT), The Gardener



wrote:
On Sep 20, 8:37*pm, Denis McMahon
wrote:
On 20/09/10 20:19, Dominic wrote:

On London Underground the DC traction current circuit to and from the
trains is via an insulated third live rail and an insulated fourth
live rail, and the earthed running rails are not used for traction
current. Would it be possible to use an isolation transformer to
prevent the electrocution of a person who touched one of the live
rails on this type of railway, or possibly on an AC version of it?
My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together. I would be very grateful to anyone who can explain
further.

Transformers don't work for DC, and aiui the 4th (return) rail exists to
reduce the corrosion effects of stray currents in the rather moist
environment of the tunnels.

The LU system is not completely isolated from earth. The rails are
loosely tied to earth at the substation (IIRC ~400ohms +ve to earth,
~200ohms negative to earth) to maintain the conductor rails at about
+420v and -210v; should an earth fault occur on either rail then the
equipment at the substation detects this providing an alarm (and
tripping the supply ?).



Correct. In particular, the use of fourth rails was the result of the
use of cast-iron segments to line the tunnels, as these would have
been very vulnerable to corrosion.

To prevent the live rail - running rail shock risk, isolate the traction
supplies from the running rails. However, I suspect that not having
either side of the traction supply tied to earth brings back those stray
current corrosion issues, and / or it may have other issues too, like
affecting track circuits.

Rgds

Denis McMahon

Not directly earthing the traction return system is the best means to
prevent stray current corrosion.

There is still some form of earthing at the substation otherwise this
would produce a floating supply (generally deprecated in the
electrical world) which in very dry conditions could allow hazardous
voltages to exist on the traction return rail. The return path is
designed to tie it as close as possible to the substation earth by
providing a low resistance but also avoiding the opportunity for a
deliberate or accidental earthing of the return path at any point away
from the substation.

http://www.wsatkins.co.uk/Images/The...a%20at%20Inter...
[http://tinyurl.com/34skod2]
shows arrangements for DC and AC supplies for overhead
electrification. IIRC there is a Railway Group Standard showing DC
traction supply arrangements (which I can't find ATM) which indicates
earthing of one side of the supply at the substation BUT at NO OTHER
point outwith the substation (as you indicate/imply above).

I know which standard you mean - like you I can't find it at the
moment!


http://www.rgsonline.co.uk/Railway_G...ing%20Stock/Ot...
[http://tinyurl.com/24cubfk] (a withdrawn document)
Deals with traction bonding (but not the supply origin), including :-
"5.1 The design of the return circuits shall be such that
there are no deliberate points of contact with the general mass
of the earth."

On a third-rail line, the running
rails are mounted on insulated fastenings for this reason, and the
negative busbar at a DC substation is insulated from earth. No third
rail system can ever be immune to stray current corrosion, espacially
at an AC/DC interface as the running rails at such points must be
earthed, but it can be managed to a level that is ALARP (As Low As
Reasonably Practicable). The drawbacks of a fourth rail system are (a)
additional complexity for the pway and pick-up arrangements and (b)
only one rail for the return circuit (on third rail systems, both
running rails are used for traction return,

No they aren't. _One_ of the running rails is normally used for
traction return as can often be discerned by the difference between
the traction bonds on one running rail and the much smaller signalling
bonds on the other side where there is a break (other than an
insulated break) in a running rail. The other running rail is
generally used for track circuits.

On the Southern, both running rails are used for traction return.
Track circuits are AC as a result; historically 50 Hz but modern track
circuits (known as TI for Traction Immune) use higher frequencies (I
believe in the range 1.1-1.3 kHz) to avoid the risk of harmonics in
the return current giving a false clear indication. This was a
particular problem with Networkers, which is why they are still
prohibited from large areas of the Southern. Single rail track
circuits are, however, used where there are switches and crossings.
Impedance bonds are used to separate track circuits. ICBW but I also
understand that the Tyne and Wear Metro uses both rails for traction
return.

AIUI, only the Euston - Watford DC line uses only one running rail for
traction return and this is why the redundant fourth rail remains
north of Harrow and Wealdstone; it is bonded to the return rail to
reduce the return circuit resistance.

The use of one rail for traction return is, of course, standard
practice on AC lines.

On 20 Sep, 20:19, Dominic wrote:
On London Underground the DC traction current circuit to and from the
trains is via an insulated third live rail and an insulated fourth
live rail, and the earthed running rails are not used for traction
current. Would it be possible to use an isolation transformer to
prevent the electrocution of a person who touched one of the live
rails on this type of railway, or possibly on an AC version of it?
My understanding, which I am sure will be corrected, is that an
isolation transformer could prevent the electrocution of a person who
connected one live rail to earth, but would not prevent the
electrocution of a person who connected the third live rail and fourth
live rail together. I would be very grateful to anyone who can explain
further.
Dominic

Isolation of the system, with all conductors insulated from Earth,
won't prevent electric shocks, because there is always some leakage
through the insulation, it only takes a few milliamps to get a shock,
and less than an amp to kill. 'Electrical Seperation', as the IEE
Regulations call it, is only effective on small installations.