Lars wrote:
My electrical theory is fair but not great. Can someone please
advise me.

I have an old Anglepoise lamp with painted metal arms and painted
metal lampshade (Anglepoise model 90). It has a two-core mains
lead. I am in the UK so this is all at 230-240 volts.

Today I touched the outside of the lampshade and got a sort of vey
mild tingle feeling which felt "odd". When I used a mains tester
screwdriver on the exposed metal (at the joint of the lampshade
and support arm) then it glowed as if the metal of the Anglepoise
lamp was live.

I unplugged the lamp and tested the resistence between the live
pin on the mains plug and some exposed metal on the lamp. I got
no resistence reading at all (i.e. it must have been a very high
resistence). I then tested the neutral pin in the same way and
got the same high resistence result.

So the lamp seems ok. But something seems to be wrong!

QUESTION: Is my lamp safe to use and could I get a shock from it
in its present condition?

QUESTION: If my lamp is unsafe then is there a repair I can do?


There are several potential causes. IMHO, the lampholder
would be the prime suspect. Over many years of being baked
by the proximity of the lamp below it, the surface can
degrade and allow a minute leakage current to flow. This can
be made worse by dust/dirt getting into small cracks.

A normal multimeter type tester is not a lot of use for
testing for leakage - that is why electrical safety testing
is done using special testers energised at 250 or 500 v.

If it was a three wire cable, this very small leakage
current would have a return path. The leakage current would
have to increase to a point where the RCD/ELCB tripped, if
fitted, or the plugtop fuse blows. That could take many
years. No tingle though.

With a two wire cable, the leakage current will be able to
increase with time until it trips the RCD/ELCD via the
person touching the exposed metalwork. The RCD/ELCB should
mean that this will probably not be fatal. Without one
fitted, the current will rise to a point where it is high
enough and flows for long enough to easily be fatal - it is
then Russian Roulette as to whether it kills or not.

So, basically, you can ignore this and hope that the leakage
grows only slowly. If you have an RCD/ELCB, probably no one
will die if your hopes prove optimistic. If you don't have
one, then you are playing for pretty large stakes..

So, your options a

Test the lamp using a proper insulation tester. You may find
that the lab tech in the electrical engineering department
of a local university will do this for you for the cost of a
beer. The lamp may actually be fine as some people are able
to detect leakage currents way lower than anything demanded
by the rules.

Fit an RCD/ELCB at the supply or replace the consumer unit
with a split box - if not already fitted.

Replace the cable with a three wire one and connect the
earth wire to the metalwork. That should stop the tingle. If
the insulation iof the lampholder does deteriorate further
then all that should happen is the RCD/ELCB will trip.

Replace the lampholder in the assembly with a new one. I
have managed to fit a standard ceiling lampholder to an
Anglepoise in the past. You may be able to get hold of the
genuine article.

--

HTH

Sue