Matthew Geier wrote:
On Sun, 06 Apr 2008 03:28:47 +0100, Charles Ellson wrote:

On Sun, 06 Apr 2008 02:33:53 +0100 (BST), "Dave Liquorice"
wrote:

On Sat, 05 Apr 2008 23:59:50 +0100, Bruce Fletcher (Stronsay, Orkney)
wrote:

Test if a wire is live by touching it to the BACK of the hand. The
muscle action is then to throw your hand away
Better still, test with a meter!
A meter can be misleading if it has a high impedance, as many modern
meters do. It'll measure voltages induced into the wire that are not
actually there if you apply even a small "load".

In that case it will still show _at least_ the possible voltage that
contact might be made with even if the available current is minimal.

This is why the serious 'sparkies' carry analogue meters and 'test
lamps'.
Digital Meters can read apparent high voltages that were induced by
induction from near by cables. Analogue meters tend not to do that - they
pull more power from the load being tested. (A tradesmen is unlikely to
be carrying around a super expensive high-impedance laboratory analogue
meter, were as high impedance digital meters are common).
I have a recollection that at least one test manufacture makes DVMs that
have a lower than normal impedance (for a DVM) for exactly this sort of
use, ensuring circuits are well and truly dead before beginning work on
them.

Of course test lamps are a 'real load' and tell the story. But the test
lamps used by an electrician when working on 'domestic' mains won't last
very long if you tried to use them to see if the 3rd (or 4th) rail was
live. A bit too much juice!
Some one who regularly works on railway power systems may actually have
in their kit a 'test lamp' for 750v supplies.


Four regular 40w lamps wired in parallel should do the trick. The problem comes
when one of them blows