On Sun, 30 Nov 2008 19:11:56 +0000 someone who may be Tom Anderson
wrote this:-

Ah, so although there was no potential between the two power rails,
there was a potential between them and the ground?

Sounds like it. However, I wouldn't like to go much further without
knowing the complicated reasons why it occurred. There are a few
ways this could happen.

Really, you need to connect the two rails to each other and also to the
ground. I was thinking the running rails would make a good ground
substitute here.

It should be enough to connect the conductor rails to each other.
However, there are failure modes where one pole of the circuit fails
(broken cable and non-interlinked fuses/circuit breaker are the
obvious ones) and if this happens unexpected voltages can be
present. Although AC circuits can suffer these problems they tend to
be minimised by various means. Two, three and five wire DC circuits
can suffer a number of maladies which those who work on them would
be wise to be aware of.

There is a lot to be said for treating all equipment as energised at
all times except when working on it under a safe system of work. I
recall one fairly well known evacuation of a tube train, probably
2-3 decades ago, when the conductor rails remained energised for a
short distance, but were discharged for most of the way along the
tunnel on either side of this short section. The fire brigade,
police, ambulance and passengers walked alongside these energised
conductor rails, fortunately without incident.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54