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