Live-ish light switches

Firstly, I'd like to thank everyone who writes here. I've been doing my first (and last) renovation over the last three years and the advice here has been invaluable even though I've hardly had to post myself.
I wonder if anyone can work out what's going on here:
I've just replaced plastic light switches with ones that have metal faceplates, and I *just* can feel electricity when I brush my fingers against them lightly. I'm certain it's not static -- I can feel the 50hz cycle.
It only happens on one lighting circuit; on other circuits with the same new switches you can't feel anything.
Does this mean that the earth on that circuit isn't connected? Or that it is connected and there's electricity flowing continuously to earth due to a fault, a little of which diverts through me when I touch the switches? Should I ignore it or try to fix? (The case of my new laptop feels the same sometimes and the user's manual says it's normal.)
Any advice appreciated as always.
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Max Christian wrote:

Likeliest cause is that the earth connector isn't connected to earth somewhere further back than the switch, and that the 50Hz you can detect is capacitively coupled into the free-floating earth conductor which is correctly connected to the switch faceplate. You can check this out if you have a multimeter - you should find that the resistance between the faceplate and something-else-which-has-a-good-earth (e.g.: socket screw, socket earth pin, outer case of a plugged-in non-double-insulated metal appliance, etc.) is Huge, while the resistance from the faceplates of the other, non-tingling faceplates to the same point is Negligible (probably down at 0 ohms unless your meter has an accurate "low ohms" range). Turn off the power to the lighting circuit while you do this - less because of the risk of shock (you've shown the available current at the suspect faceplate is not currently (ha) dangerous), but becuase it'll louse up your resistance measurement.
But it *is* worth tracking down where the earth connection's missing, since without an effective earth at your metal faceplate you (and those who share the house) are at risk in the event of a "hard" fault where the live directly comes into contact with the faceplate; at that point what you want to happen is for a brief flash behind the faceplate and the MCB/fuse for the lighting circuit to trip, EEBADS-style; *not* for the faceplate to stay live, now with plenty of heart-stopping current available, to give a nasty (or worse) belt to the victim.
Your previous installation - with plastic faceplates - masked the fault I'm guessing you have; tens of years ago it wasn't even a requirement for lighting circuits to have an earth-continuity conductor. Until you fix the underlying fault (which you should do ASAP), replacing the metal-faced switch with the previous plastic one is a Good Move - but is no substitute for the proper fix, mind you...
(While your laptop might give you a similar weak tingle, we Presume it's a double-insulated/Class-II design where any such current is restricted; while the tingle you say you're getting suggests the absence of an effective earth in an arrangement where such an earth *is* a requirement.)
HTH - Stefek
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Hmm, I'd wondered about posting a similar question, I've also replaced some plastic face plates with metal ones and wondered what to do with the earth cable - as it's currently connected to the original *plastic* back plate. I can't feel any electricity, so I've left it as is now, should I move the earth to the face plate - or will it make no difference as it's only bolted to plastic anyway?
--
Mike Buckley
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writes

The face plate should be earthed, the connector in the box is there to let you have an easy way to earth the face plate with a short bit of wire.
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the
If it is a metal box, I thought it was there to earth the metal box. I'm sure most of us don't turn off the ring to loosen face plates for decorating. Mmm, stripping knifes touching the metal box.
-- Mike W
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VisionSet wrote:

We've had this out on the group a few times; the consensus, which I share, is that it's (marginally) preferable to run the incoming cable(s) to the earth terminal on the Accessory - if it has one - e.g. the socket, fused-connection-unit, whatever - so that there's one fewer connection to come adrift on the way to the "important" place, which is whatever the socket/FCU/whatever feeds. A tail to the backbox is then a better-than-relying-on-the-mounting-screws way to earth the backbox, and earths the mounting screws for those rare designs where there's an earthing terminal on the accessory which doesn't itself provide continuity with the mounting screw surrounds.
For light-switches, things can be different - plastick plateswitches don't have an earthing terminal, so parking the earth wires in the earth terminal (usually) provided in the backbox is a sensible way to keep it/them secure, and - if there are multiple cables, as there will be in some wiring setups where all the joining's done at the switch positions rather than in junction boxes and/or loop-in roses - does a "real" electrical-continuity job. Metal-faced switches generally have an earthing terminal - it's not simple to design them as double-insulated/Class-II, as the mounting screws will earth the faceplate when used with a metal backbox.
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<snip>
Ok - what would you do if there was no earth cable in the wiring and you wanted to fit a metal face plate to a plastic backing box? The plastic box does actually have an earth terminal in it - would you use a short piece of wire from the backing plate to the plastic box (why?)?
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Mike Buckley
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You run a seperate earth conductor or you don't use the metal faced switch plate, simple. Or of course you ignore all safety aspects, in which case you likely wont be asking advice anyway.
Dave
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How could I run a seperate earth conductor?
--
Mike Buckley
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Well as I don't know your layout I cant say exactly, but you should be able to pick up the earth conductor somewhere and join a single to it. The actual physical running is another matter and could involve a lot of extra work, ie lifting floorboards, chasing out etc. You might if you were lucky be able to feed a single conductor down the existing channeling. Very much a suck it and see operation. Done it a few times and it can be a pain in the a***. Costs the customer a lot of extra money!. One reason never to cut off earth conductors, people do because they think they wont be needed at a certain point, then again people are stupid.
Dave
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Great advice here thanks - there is a nearby socket I can use for this, although it's going to be a major pain in the arse for the other downstairs rooms. I guess it means a bit of plastering :-(
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If you're going to have to re-plaster, do the job properly and make sure there's an earth throughout the lighting circuit in the normal way. If the earth's been cut or left off, gawd knows what other bodges you'll find. If it never was present, the wiring is old.
--
*I don't know what your problem is, but I'll bet it's hard to pronounce

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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I suspect this house has never been re-wired since it was built - late 1930s. The wiring is just 2 core in a grey round sheath, all the back boxes are nasty grey plastic. Possibly upstairs has been re-wired with 3 core, but there's no obvious sign of re-plastering anywhere except the hall, everywhere else is the original lathe and plaster.
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Mike Buckley
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With lath and plaster you can do the majority of the runs without needing to re-plaster. Just the bits where you have to cross the noggins.
--
*If at first you don't succeed, destroy all evidence that you tried *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Excuse my french - but what's a noggin? I'm going to run a single 1.0mm earth wire to a socket down the edge of a door frame, all sheathed by graan/yellow. Bit of a pain in the arse, but nothing compared to the grief from the girlf of having a plastic socket in a house full of stainless ones....
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Mike Buckley
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A lath and plaster wall is hollow. It's constructed out of vertical studs. But at perhaps two places there will be horizontal 'noggins' to add strength and prevent twisting. These also stop you just dropping a cable from top to bottom. But you can channel out over the noggin and bring the cable round over it, then make good.
--
*It is easier to get older than it is to get wiser.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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writes> >> Great advice here thanks - there is a nearby socket I can use for this,

Borrowing an earth connection from the socket circuit is against regs, as its possible someone could one day disconnect the socket cct, but leave the lights powered up. However, it will clearly greatly increase the safety of your dodgy setup.

Uhoh. If its really that old, or even 40s or 50s, dangers are bound to be lurking. The number 2 rule with really old wiring is dont move it, not even slightly. A quarter inch of movement is enough to cause a short or fire if the rubber's perished - and it normally is.
The number 1 rule? Disconnect it and rewire. 1930s wiring is anything but safe: in fact 30s wiring in the condition it will be in today would have been condemned even back in the 30s. I dont know when lighting had to have earths, but 2 core lighting is very old, and probably well past its reasonably safe by date.
I _really_ wouldnt want metal light switches on a dodgy setup like that. Even when earthed, the level of shock protection is liable to be real poor, and the odds of earthed things going live quite high. Why? High earth impedances, wire fuses, no RCD, no crossbonding, no earthing on parts of the install, heavily corroded wiring, high probability of shorts, high leakage... no thank you. get it fixed.
Regards, NT
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Good point Dave
Dave
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Actually - none of the downstairs light switches have a cable for earth (upstairs do). I guess I'm going to have to run a cable from the switch/faceplate to a suitable earthing point - for instance central heating copper pipe?
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Mike Buckley
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NO NO, NOT A SUITABLE POINT. You need to take it to a earth point in a socket, junction box etc, but do make sure that the connection is part of the earth circuit.
Dave
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