Charles Ellson wrote:
On Tue, 21 Sep 2010 10:15:19 -0700 (PDT), The Gardener
wrote:

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.

Adding to the previous comments re isolation transformers (which is
what you would actually have if the electricity supplier's substation
supply side did not have one pole deliberately earthed), the existence
of an isolated/non-earthed supply or supplies (in the absence of
"double-insulation" measures or similar used in domestic equipment)
enables faults which involve the addition of normally separate
supplies to dangerous levels even if each individual supply is at a
safe voltage. On the railway this could result in e.g. a 630v 4-rail
supply being charged by a broken overhead line at 1500V, 25kV or more
if the 4-rail supply had no earth connection at all.

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.

Ah! Maybe it was a mistake to think the SR would do it the same way as
the LMS.

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.

http://www.rgsonline.co.uk/Railway_G...%20Iss%201.pdf
[http://tinyurl.com/3ypbo7n]
has a few pictures which might save a few thousand words.

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

And NLL (where still DC) and WLL ?

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.

IMU rather to prevent increasing it IYSWIM (see also "chicken and
egg") as it was originally designed as 4-rail with substations spaced
appropriately.

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

Although IIRC, the action of the autotransformers along the line ensures
that traction return current is only carried by the track in the section the
train is in. Beyond that section it is in the opposite phase of the
(centre-tapped) 50Kv supply,

Jim Hawkins