Abnormal Voltage Detection From Light Switch When Checked With Voltage Detector Pen


Postby robertarton » Mon Nov 11, 2013 12:55 pm

I wonder if anyone can explain this for me. I've just replaced a light switch in the house and noticed that the new one gave a very very slight funny feeling in the finger tips when run across the metal front panel. I immediately removed power from that lighting circuit to investigate it was wired correctly, as indeed it was. I replaced the switch with an all plastic one as a precaution until I can explain what was happening.

This may or may not be related to the problem, but is my query for this forum. I have a Megger non contact voltage detector pen. While checking the light switch during the replacement above, I noticed that I get a positive detection reading on the pen from around 15cm away from the socket. In fact the electric field is so strong that I am able to follow where the cable runs up inside the wall. No other light switch or plug socket i have tested so far gives such a strong reading. Generally I have to have the point touching the switch/socket to get a live reading.

From experience, can anyone explain what is going on here?

Thanks,

Rob
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Postby ericmark » Mon Nov 11, 2013 10:57 pm

By any chance was it a two way switch? Years ago it was common to wire the strappers between two way switches with twin and earth and use the line from an adjacent switch. This would in some combinations produce a large induction loop.

Also over the years it was common to split up and down stairs lighting as the ceiling roses were only rated at 5A so to get extra lights needed to feed. With lack of RCD protection it often resulted in borrowed neutrals where the switch used to take the line supply was from another fuse or trip.

As we now use RCD protection on lighting this practice has been causing a problem as it will trip the RCD. The problem is once one realises what is going on you can't have up and down stairs off different protection devices even though if they both come from same RCD coming from different MCB is still against the rules so only option is to come from same MCB.

Once this is done then the total current needs to be 5A max and so arrays of silly 50W spot lights become a problem the only option is to use LED lamps. However SELV types often have power supplies with min and well as max so it also means swapping power supply.

This is all because only twin and earth rather than triple and earth cable was used. Also often radio interference including using wireless LAN and even wireless door bells the knock on is huge. The best is even with all these problems although not recommended there is no rule stopping it's use.

Also before 1960 we did not need to earth lighting circuits and loops like this can induce current into any used wires in the cable. It is not unknown for people finding an earth wire disconnected to just connect without testing so what they have really done is connect an aerial wire to the switch box which will likely have an induced voltage so making the whole switch live in respect to earth even if the current is very low.

Electricians have meter called loop impedance meters which are used to measure the earth loop impedance but most DIY people don't even know they exist. Using these meters it is very easy to test an earth but without it is far more entailed. So as electricians we will spend 30 seconds to test but as DIY most will not spend the 1/2 hour needed to test without one of these meters.

The voltage detector pen we test by rubbing on our jumper and the reed switches can be very sensitive. As with the neon screwdriver they do have their place but they are not test equipment and one should use test equipment which requires at least two positive connections using ones body as a reference point to measure against is not really how it should be done.
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Postby proptech » Wed Nov 13, 2013 11:44 am

To just add to ericmark's excellent explanation.
Metal faced switches must be earthed. If the installation is not, or there is some doubt, please keep the plastic version.
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Postby ericmark » Thu Nov 14, 2013 8:00 am

[quote="proptech"]To just add to ericmark's excellent explanation.
Metal faced switches must be earthed. If the installation is not, or there is some doubt, please keep the plastic version.[/quote]
Warning when using plastic switches using screw covers is also required as the screws holding switch to wall can become live.

The electrical safety council do a series of best practice guides and these include what to do with lights without earths. I would not say I agree with all they say to me earths have been required on lights since 1960 and one is to have a PIR or EICR as now called every 10 years so house owner should have had 6 reports at 10 year intervals telling them they were no earths plus extra if occupants has changed. The regulations do say you can still use electrical installations which complied with the previous edition of BS7671 there have been three editions of BS7671 which are BS7671:1992, BS7671:2001 and BS7671:2008 before 1992 it was not British Standard the IEE which has become the IET has published seventeen editions but only last two were in conjunction with British Standard organisation.

Clearly the first edition is no longer acceptable and one does have to draw a line somewhere and going back one edition is clearly acceptable as to going back to 1992 then maybe that can be accepted going back two editions to start of BS7671 but pre 14th Edition is really going too far if we drive at 100 MPH in a 1960's car we will still get prosecuted if caught we can't say the rules when produced allowed one to drive at 100 MPH but we are given dispensation with seat belts.

Things have moved on and one major thing is water pipes and gas pipes are now often plastic so our houses have to modify their earthing system which includes earths provided by supplier of electric, earth rods, and RCD protection with a great push to double insulated equipment we must upgrade our houses to match we can't live in a bubble.

With this in mind I don't agree with the continued use of non earthed lighting circuits at low voltage. At extra low voltage OK but not at low voltage (230v). Would be interesting to hear others views on this.
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Postby proptech » Thu Nov 14, 2013 11:12 am

ericmark
I do wonder how many properties there are without earthed lighting circuits. When you see the vast numbers of metal faced switches in various designs flying off the shelves in places like B&Q, they seem to be 'must haves' . Then there's the light fittings, does the average DIY customer check the requirements before buying ?
it wouldn't appear so !
As for screw caps, I've only come across one or two makes of accessories that provide these. MK, as far as I know, don't.
in my opinion any unearthed lighting circuit should be replaced, and let's face it, in most cases it's not such a big job providing there is some degree of access. It just has to be a case of safety first.
Regards, John
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Postby ericmark » Sat Nov 16, 2013 3:25 am

The lack of testing by DIY people means rings become radials and earths are lost and even when earth wires are connected to a back box it does not mean they are connected at the other end.
For an electrician with a loop impedance tester checking earth is easy but for the DIY guy I really don't know how he would test if the earth is there. I can't think of a simple safe test that would work with all cases without the loop impedance tester.
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Postby proptech » Sat Nov 16, 2013 8:21 pm

The only other option would be with a R2 test wander lead on an isolated circuit. This, of course assumes that a decent continuity tester is available. Perhaps there's a market there for someone to produce a DIY equivalent, but I hate to think what rubbish that would be. One thing for certain, in DIY hands a loop tester on a lighting circuit would in no way be safe.
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Postby ericmark » Mon Nov 18, 2013 11:05 am

I have in my test bag some items which have been banned for years. For example a converter BA22d to 5A two pin which allows me to plug in to test. I think we all over the years make our own test leads to make it easy but today with screw and bayonet at 27mm, 22mm, 14mm there are so many different fittings being used it is a problem testing with a loop impedance tester and yes it is easy to get a shock while testing.

The socket tester with loop has resulted in DIY being able to test sockets but lights are another thing often one has to use steps to reach them. However "robertarton" has not returned to say how he got on so no point in going further. He did it seems return on the 14th to have a look but clearly not read last two posts.
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