earth leakage problem

Postby kenb » Thu May 15, 2008 7:24 pm

i have a problem with the lighting circuits giving a live reading of approx 50 volts. all the circuits have been checked by a sparky and he cannot find a fault with the wiring and the system does not trip either. he recommends having the lighting re-wired but i am not sure whether this will cur the problem. what do you think?
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Simply Build It

Postby kuzz » Thu May 15, 2008 9:36 pm

more likely to be induced voltages from lighting cables running next to other live cables, not a fault. not a re-wire. if there was leakage any decent electrician could identify, locate and rectify the problem with ease. don't get me wrong, faults can be time consuming, but identifying and locating is simple enough. rectifying is always going to be easier tan re-wiring. i think your electrician is a bit of a silly billy.
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Postby ericmark » Thu May 15, 2008 10:47 pm

“live reading of approx 50 voltsâ€

Postby kenb » Fri May 16, 2008 8:03 am

thanks for reply, my wife felt a shock from a light switch which when i put a neon screwdriver to the pins showed live. this was in the lounge downstairs where there isn't carpet. Upstairs didn't show on the tester because of the carpet etc.
I phoned a sparky, he came out to test the system with his equipment. He spent six hours checking the downstairs ceiling roses and the switch cables for good insulation, resistance, continuity etc. At first we thought it was only downstairs but when he checked upstairs which is on a seperate breaker they too showed the same fault.
His boss reckons it will take three days to remedy this by a re-wire of the ring on the lights both upstairs and downstairs and leave the switch cables if they test ok.
There are three lighting circuits wired to the CU. so could they all be disconnected from the CU and each one re-connected individually to narrow the fault down?
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Postby kenb » Fri May 16, 2008 10:19 am

Thanks for your replies. reading them made me think a little more.
So I disconnected all of the lighting circuits from the CU and tried each one individually until I had established that if I leave out the live from the downstairs circuit or trip the breaker my neon tester does not light.
Also with just upstairs L N & E connected and the E & N connected from the downstairs the tester does not light.
Does this mean that the fault could just be the downstairs circuit?
I know the neon screwdrivers are far from ideal, could I buy a multimeter to check things further?
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Postby kenb » Fri May 16, 2008 2:18 pm

If i didn't make it clear in previous notes the 50 volts reading was on the earth wires. I am following the cables around the house and disconnecting each cable to see if the fault remains at the previous rose/connection. Is this the correct method to trace the fault?
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Postby ericmark » Fri May 16, 2008 4:37 pm

You have an earth problem. But where it is not so easy. Earths can be got in two ways either from the supplier (PME = TN-C-S or TN-S) or from an earth stake (TT) both systems rely on an bonding system where water pipes, gas pipes, and in some cases even the reinforcement of the building are all connected together so that the whole building is the same voltage. With a TT system because an earth stake is not really that good also you have a special trip with disconnects the electrical supply should a fault occur. With PME or TN-C-S the supplier uses multiple earths all connected together with a common wire which is both earth and neutral and does not need special disconnection devices as enough current will run to blow a fuse or trip a miniature circuit breaker MCB but it does rely more on the bonding between all the services. In theory every 10 years all houses should have this all checked or when there is a change of occupant but with the exception of rented properties this is often missed. Over time earth wires get broken or removed and it is all too common to find houses without any earth connections or at least a lot missing. It is possible this has happened to your house. Also it has been known for the supply to the house to become faulty which is a really big problem causing allsorts of equipment in the house to blow. The device used to do most of the testing is an “Earth Loop Impedanceâ€

Postby kenb » Fri May 16, 2008 4:56 pm

Thanks ericmark, This is getting intersesting.
The guy who came had ''the works'' with him and he does all the tests and certs for his gaffer too. Anyway the water pipe is plastic and there is earth bonding in the airing cupboard between the central heating pipes etc and in the bathroom connecting the two taps on the basin but I can't see any more elsewhere in the house. Should there be?
The house is about 200 years old but was re-wired by the previous occupant within the last 14 years, so all the cables are pvc.
As for the type of earthing system, I haven't got a clue.
There was a meter last changed in '98 would the earthing have been checked then?

Do you consider a re-wire of the lighting circuits necessary or will I be wasting money?
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Postby ericmark » Fri May 16, 2008 6:45 pm

Until you have sorted out earths rewiring could be waist on money.
At the point where power enters the house there is normally an earth terminal and earth cables connecting to where gas, oil, water etc enters the house. Sometimes the earth bar inside the consumer unit is used. If the pipes inside the house are plastic except for short lengths where visible then they may not require earthing but if the pipes are copper even if main incomer is plastic they need earthing.
The supply authority have to tell you what earthing system you have it is up to them not you and has to be same as other houses in area.
If it comes in on overhead wires most likely TT and there will be an earth rod and an earth leakage trip if it is TN-C-S normally from an underground cable either way you can normally see the green and yellow wire somewhere need the meter.
If the electrician had the works I can't understand why he would leave without making safe. Something wrong but really if an electrician on site can't work it out we have little chance remote from the job.

Postby kenb » Fri May 16, 2008 7:47 pm

thanks ericmark for your reply. another sparky says the voltage on the earth could be from a neutral which may have been scuffed or under pressure (don't know what he meant by pressure) and that needs to be found and replaced.
Since your reply on earthing I have found the earth cable in from the CU and it does go down to the gas pipe under our boiler, so my info wasn't quite correct. The cold water main does enter in plastic then to copper above that. Should I link the earth from the gas pipe to the copper cold water and also place an earth link on my two kitchen taps to bond them?
Also, do all the pipes in the airing cupboard need linking including the shower and should i also earth from the pipes to the earth in the electric shower pull switch?
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Postby ericmark » Sat May 17, 2008 12:38 am

Linking gas to water yes but have power off before linking.
There can be no harm in bonding but remember it is likely it will then highlight the fault and trips or fuses may trip or blow.
I really do think you have a potential dangerous situation and I would like some one skilled to cure it so they can take the necessary precautions I know you had an electrician there but I don't feel any skilled man should walk away from what you have without making safe I would hope you could find a good tradesman who would find the fault and fit all required earths.

Postby kenb » Sat May 17, 2008 6:23 am

Thanks ericmark, I have asked another electrician to come and have a look AND rectify the problem, whatever it may be.
What is the difference between an RCD and an ELCB ?
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Postby ericmark » Sat May 17, 2008 11:09 am

An RCD is a type of ELCB
There are a number of names for basically the same thing. They are all designed to measure the leakage of electricity and when it reaches a predetermined current evel to switch of the power in a predetermined time.
To protect people it is 30ma at 40ms and for fire 300ma delayed I think 300ms but not sure on that. Where an earth rod is used 100ma is normal but under the new regulations I would expect 30ma on every circuit.
There is a combined RCD and MCB called a RCBO and they will have five ratings B, C, D shows how much overload is required to switch off with no delay and 16A etc shows amps that can be used for extended time the 30ma is the leakage amps lack of S will mean it will trip in 40ms and 4500 in square says it can switch 4500amp with a short circuit without damage.

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