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?

Thank you for any info.
Lars

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PS: Picture of Anglepoise model 90:


The two questions I haven't noticed anyone asking you directly are these:

What are you standing or sitting on and/or touching when you feel that
tingle?

And, what is the "cold end" of the "screwdriver" mains tester connected
to when you see it lighting?

If you can reasonably feel that your body is well insulated from ground
when you feel that tingle, like you are standing on dry carpeting on a
wooden floor inside a house, then I'll put my chips in with the folks
who say you are probably just feeling a capacitivly coupled ac current
flowing between the hot lead in the lamp and the lamp's metal body which
is then charging and discharging your body's capacitance.

How about trying this? connect a wire to a known ground, get the
smallest wattage 240 volt bulb you can find, unplug the lamp and connect
that bulb beween the metal lamp body and the grounded wire. Then plug
the lamp back in.

Does the bulb light, with the lamp either off or on? If it doesn't, then
the current flow (in amps) is less than the wattage of the bulb divided
by 240.

If it does light, you DO have a dangerous condition of deteriorated
insulation or some other conductive path within the lamp, STOP and get
it fixed.

If the test bulb doesn't light, replace it with an ac ammeter set to a
range higher than that bulb would draw at 240 volts and then switch to
lower ranges until you can measure the "leakage" current. I'm betting
you'll find it will be less than a milliampre, and is likely
capacitively coupled.

While it's nice to hear that all metal bodied lamps ought to have a
direct connection from their metal parts to ground through a three wire
cord and appropriate plugs and recepticals, that is certainly the
exception rather than the rule for household use in the USA. I've got
all sorts of metal bodied lamps in my home, all of them purchased new,
some as recently as last year, and not one of them came equipped with a
three wire cord.

In fact, I've installed capacitive "touch switch" multi level dimmers in
four of those lamps, using their bodies as the sensing elements. I
couldn't do that if we had to ground the lamp bodies.

Let us know what you find,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"