On Mon, 20 Sep 2010 12:19:00 -0700 (PDT) someone who may be Dominic
wrote this:-

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.

In addition to what others have said, if it worked such a system
would provide a false sense of security.

The existing system can cope with one earth fault, my memory is that
staff are alerted to it so that they can rectify it. The problem is
what happens when a second earth fault occurs, if that is in the
other pole then both are shorted together via the ground or a train
depending on the circumstances.

It is much the same with earth free supplies, such as that for mains
operated shavers. They protect against the first earth fault, but
fail in a much worse way if a second earth fault happens. If the
supply was earthed in the conventional way then there are problems,
but the supply should be disconnected automatically when the first
earth fault happens. Shaver leads are short and curled to guard
against the problem and reduce it to as low as reasonably practical.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000...#pt3-pb3-l1g54