Due to some strange combination of circumstances last night, I found myself simultaneously touching one of the screws fixing a light switch to the metal wallbox with one hand and a copper heating pipe with the other (no, I can't quite remember what I was doing). Anyway, I got a very distinct tingling - there was definitely some current flowing through me, albeit not very much.
Now is this just a problem of earth bonding, ie the central heating system needing to be earthed properly, or is there also some fault in the lighting circiut - surely the screws of my light switches shouldn't be live, even if only a "little bit" live?
Probably lack of main bonding to the central heating. Your lighting circuit is probably correctly earthed. If it wasn't, the screws would have been open circuit, unless it was getting earthed through the building structure, which isn't particularly likely.
In fact, it has just occured to me that my CH system is probably only bonded via the supplementary bonding in the bathroom. The cylinder and boiler are in the loft and connected to the rest of the house via 15m of plastic pipework. The actual radiator circuits are mostly in copper.
Either as you say the light switch screws are live, or they could be earthed and the central heating could be "live" and or have some electrical leakage though it.
You could test this by running an earth wire from a known good earth to the switch as seeing if the "fault" disappears and if it does then there is some leakage there.
Most likely someone has left a break in the continuity of the earth wire in the lighting circuit somewhere probably thinking that lights don't need earthing anyway, which is partly true. The earth wire is more then likely connected to the metal backing box and is "floating" and thus inducing a small amount of current due to leakage effects.
The central heating is far less likely to be at fault IMHO....
My bet would be a fault on the lighting circuit earth. A fitting replaced a ceiling rose somewhere, and the earth not looped through. Happens all the time. If you've got a DVM, measure the screws on that switch to the screws on a socket. Should be no more than a couple of volts - if any. If the earth is actually disconnected somewhere on the lighting circuit, but the earth wire connected to the backing box, you'll get a substantial reading through capacitive coupling.
Another possiblility . . . The central heating *is* correctly earthed The switch back-box *isn't* earthed A live wire in the box is routed in line with the screw - such that the screw penetrated the insulation when screwed in - making it live
Very easy to check - first with an electrical screwdriver with a neon in the handle (with the power on) - and then by taking the screws out and looking inside (with the power to that circuit off).
On first reading, Dave, I thought you were suggesting that a DVM would give no more than a couplavolts reading in the presence of the fault (lack of earth continuity). Reading more carefully, I realise you meant that with a good earth, the voltage read will be minimal, whereas with the earth-wire floating, the reading will be high - which is indeed the case.
How high? Well, nudging the supply voltage, depending on the relative impedance of the DVM - typically at least 10Mohm - and the impedance of the "capacitor" formed by the insulation between the earth conductor and the live wire. A moment with our friend 1-over-two-pie-eff-cee suggests that at 50Hz, a 10Mohm impedance (which would equal the assumed impedance of the DVM, giving a reading of 120V or so) is what a 300pF capacitor has; since common cable has capacitances of the order of 30-90 pF/metre, that handwaves that 10m of mains cable would be enough to give a reading well over half the mains voltage - which corresponds to common experience. And the current from a 10Mohm impedance at mains voltage is
0.024mA - detectable as a tingle.
This little musing has, I think, prompted me to make up a shunt for my DVM, being a, let's see, 470kohm resistor wrapped in a couple of layers of heatshrink across a pair of red-n-black 4mm piggyback-capable plugs (yes, I'll use shrouded types) to deliberately "desensitise" the DVM. A
470kohm resistor (call it 480k) will pass a whole 0.5mA at mains voltage, dissipating 1/8th of one of your earth Watts, so won't glow in the dark; while being enough of an Extra Drain on a capacitively-coupled "this'll worry you unnecessarily" voltage to be usefully diagnostic between that and a give-you-a-belt low-resistance connection to Phase. No, I won't fit an in-line fuse, that's only more connections to work dangerously loose; if I ever connect this thing across a bigger voltage (even 415V will only triple the power dissipation, and I don't have
3-phase at home!) I'll have more to worry about than the disembowlment of a twopenny resistor...
And if you hear of a house in Bristol burning down, or if I end up contributing here via text-speech conversion having gotten bits of exploding resistor in both eyes, you'll know I've got my resistor-colour-code-reading badly wrong ;-)
Or possibly like we had when we moved in, heating system was at outside ground potential and electrics were at PME earth potential, in our case 42 volts away from outside.
Even touching a live with no other contact than your feet on carpet is quite a violent sensation. You wouldn't describe it as "tingling". More like being hit 50 times a second with a screwdriver handle.
Maybe I'm just more conductive??? I get a definite painful "buzz" sensation when touching a live even when wearing trainers on carpet with a wooden floor. I don't make a habit of it, you understand, but it happens occasionally...
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