On Tue, 08 Apr 2008 20:08:55 +0100, Charles Ellson wrote:

but the usual rule about treating anything as
live unless it is obviously earthed seems to be the main defence.

As a friend who worked in the power industry once said - 'The only thing
I trust is a metre air gap'.
Any one working on a high power system will want the assurance some
idiot can't turn it back on and kill them, hence the highly visible short
to earth bars/wires. If some one tries to turn it back on, the short to
earth should be a sufficiently good 'fault' to trip it off again saving
your own skin from such stupidity.

An people do try to turn things back on again. I've seen at least one
report were a circuit was isolated for work, and a remote control centre
forgot about the work, saw the isolated circuit as a fault and tried to
remotely turn the power on again...
The earthing cables saved the guys out on site.

--
Matthew Geier