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.