"Max Demian" wrote in message
o.uk...
On 01/04/2021 12:29, Andrew wrote:
On 31/03/2021 17:39, Steve Walker wrote:
On 31/03/2021 15:36, Adam Funk wrote:

Better testing devices are now available (but I'm told my grandfather
used to use that kind of technique to check whether wiring was on or
off).

So did my grandfather. Unfortunately for other people, he seemed to be
electricity proof and what he could touch without a problem would give
others a hell of a belt.

Ex IT colleague went to work for London underground. He got the chance
to go into the tunnels one night and to test that the power was off they
had an ancient bit of wood with a number of bayonet-fitting bulbs
fitted along it, connecting the power rail to the running rail.

Actually the Underground has two power rails (centre and side). The
running rails are somewhere in between in potential.

Although presumably testing either the positive or the negative rail wrt
ground would indicate that the line was still electrified. I wonder if the
strip of bulbs would work with either rail (they are different voltages wrt
ground), without blowing the bulbs (in one case) or failing to light
sufficiently to see (in the other case).

I know that the four-rail system is used to try to lessen electrolytic
corrosion in the tunnel linings and underground pipes, but I've always
wondered why the two rails are not symmetrical about 0V - ie +315 V and -315
V rather than +420 V and - 210 V.