106Vac between earths - FIXED

[Willewonte]

Just gone to turn the lights off in my living room, and at the same time turn on the upstairs landing lights - you can touch both easily.

I felt a 'tingle' when doing the above, but only when touching the retaining screw of the living room lights (the only exposed metal on this switch, back box is earthed) and the front plate of the landing light (metal front plate and earthed back box)

Not a proper belt, more a tickle really, and nothing to cause any hurt.

The measurement was made with a Fluke 83 DMM. The voltage does disappear if I drop a low-impedance load across the two earths (incandescent bulb on a lead)

I'm more an electronics guy rather than electrical.

I was wondering what would cause this voltage (with no apparent current behind it) and if it's something I need to have sorted out?

 

[PrenticeBoyofDerry]

If you isolate the system do you still get a voltage reading?

 

[plugwash]

Yes it is something you should be concerned about. One of the two "earths" is obviously not properly connected to the house's earthing system and furthermore something is leaking enough current to give you a tingle. The former is definitely a problem, the latter is probablly a problem though some devices are leaky by their nature.

 

[Willewonte]

If you isolate the system do you still get a voltage reading?

When you say isolate, do you mean tripping the MCB for the lighting or disconnecting the wiring in the consumer unit?

 

[bernardgreen]

I was wondering what would cause this voltage (with no apparent current behind it) and if it's something I need to have sorted out?

One of the "earths" is not connected to the MET ( main earth terminal) This earth wire has capacitive coupling to the Live wire and also to the the Neutral wire in the twin and earth cables.

Live------||------earth-----||------Neutral

Hence the voltage on the earth wire in the result of a capacitve voltage divider between 230 and 0 volts. The current is limited by the impedance of the capacitive coupling.

 

[Lectrician]

As said, one earth is floating and has lost it's connection to the earth somewhere along it's route.

This earth is then picking up some voltage via capacitive coupling in the cable. This voltage will vanish if you put a load onto it (as you have found). A digital voltmeter has a very high impedance and can measure these voltages.

 

[ericmark]

Was the last re-wire before 1960? Although often wired with twin and earth pre-1960 lights were often not earthed. It is easy for some one who has not done a proper test to just connect wires which may not be connected at the other end and the inductance and capacitance in the cable can result in a voltage being put an any items connected to this aerial wire.

After 1960 we earthed lights so if after that day then something serious is wrong.

However testing for earth we would normally use an earth loop impedance meter and at the switch this is a problem as often no neutral to power the meter so again often not tested as it should be.

There is often no alternative but use a long wire and a low ohm meter. These meters have to draw at least 200 ma to test so they should highlight poor connections.

Clearly it does need testing but both the meters are not cheap and it is likely cheaper to get an electrician to test than hire the meters.

 

[knowall]

If the switches are so close to each other why don't you loop an earth from one box to the other ?

NA

 

[JohnW2]

If the switches are so close to each other why don't you loop an earth from one box to the other ?

That would obviously address the one immediate issue that has been identified by the OP, but would leave answered the question as to what is wrong with the earthing system (which might be more general, affecting other things, and maybe not solved by that added 'loop'). In other words, it would/could, IMO, be a type of 'papering over cracks'.

Kind Regards, John

 

[securespark]

Was the last re-wire before 1960?

IME, pre-1966.

The 14th introduced cpc's in lighting, AFAIR.

 

[Willewonte]

Hi Gents,

Thanks for the responses, they are very much appreciated.

As a bit of further information, I turned off the lighting circuit at the MCB and rechecked the voltage across the two light switches. This is now down to 1.3Vac. When I turn the lighting mcb back on, the voltage increases to 88Vac (with no lights on in the house) and then back up to 106Vac when the upstairs landing light is turned on. Turning the living room light on makes no difference to the voltage.

Although I understand that I really do need to get a an electrician in to sort this properly, I was thinking if the following would be useful information to have and pass on to the lucky tradesman?

If I remove the front of the CU and attach a flying lead to the main earth, and then use this lead and my DMM to create a list of light switches that show a voltage? My thinking is that those light switches that are properly earthed will show no voltage and ones after the fault will show a voltage.

Further info: House was built in the mid-70's, but has experienced 'interesting' wiring during that time. We bought the house a couple of years back, and during the renovations we are doing, we've found mains and lighting cables travelling diagonally across walls and choc-bloc mains connections behind plastered walls...

The lamp I placed across the two switches did not light, so no current? Perhaps I should measure it.

Also, when I switched the light circuit off, I noticed that the shaver socket in our en-suite also went off, so that's connected into the lighting circuit. Should the shaver socket be connected to this circuit, especially as it does not seem to be individually fused?

 

[ericmark]

Also, when I switched the light circuit off, I noticed that the shaver socket in our en-suite also went off, so that's connected into the lighting circuit. Should the shaver socket be connected to this circuit, especially as it does not seem to be individually fused?

Yes quite normal.

From what you have said I would expect to find the switches are simply not earthed and the capacitance in the cables is giving enough voltage for you to measure.

Since build date 1970's and MCB not fuse fitted there is clearly a fault one would hope you have not got same fault in sockets and likely using the earth pin of a three pin plug with suitable wandering lead to meter would be easy way to find point where earth is lost.

However in view of number of faults found I would not trust sockets either so really you need some better method of testing. Since you seem to have done so well so far you may wish to continue but the problem is cost of test gear.

The cheap option Found as low as £42 these will do a reasonable job testing earths not to standard that electricians use but much cheaper than the £750 I paid for a test set. However even £42 is not cheap and then you have to interpret the results but once you know sockets are OK you have reference point to test the rest.

What you have to consider is it worth buying tester or should you play safe and get an electrician in to do a full electrical installation condition report (EICR was called PIR)?

Only you know how good or bad you are and you have to decide if worth taking a chance with DIY or if time to bite the bullet and get an electrician in. I would until problem resolved put a spot of silicon sealant on the screws not a cure but at least reduces risk until solved.

I would start at ceiling roses but to be honest working up steps to test ceiling roses is not easy and you really do need to do a risk assessment and decide if really worth DIY.

 

[Willewonte]

Thanks for the suggestion regarding the mains sockets, it's something that needs serious consideration. I have one of those testers, and *think* I've tested the sockets in this house, but will dig it out and check the sockets to be on the safe side. Also have one of those plugs with an external earth connection (used for connecting an anti-static wristband) so will use that for some initial testing before getting an electrician in.


Although I'm more electronics biased, I worked as a maintenance engineer for several years - lots of install wiring and controls fault finding (Injection Presses with 250mm steel armored!) but no experience of house hold fault finding. I work away a lot now, so time at home is precious, better to get someone in who does this as a day job, rather than me learn something new when I could/should be with the family!

 

[Willewonte]

Looks like I've found the source of the problem!

I've been wall-papering the ceiling of our main bathroom this morning and during preparation I removed the ceiling light. When I removed it I found that two earth connections were connected to the light fitting, with a third tucked back up in the ceiling. I brought this third earth wire back down and connected it to the other two. After turning the power back on, I tested for the original fault at the light switches previously mentioned. No AC voltage measured!

I know that I've been lucky with this and I'll still be checking the mains sockets with the tester mentioned above, and then checking the other lights out.

Many Thanks to all those that posted and helped.

 

[securespark]

Excellent news!

As you say, it's worth going round with your test set-up checking to see if any other points are showing a voltage.