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.