Neutral - Earth problem?

choppertastic · 17 Mar 2007

Hi,

I have a weird problem with a lighting circuit.

If I put a meter across L-N I get 240v. If I go across L-E I get 75v and if I go N-E I get around 75v.

I have checked on a 13A ringmain and I get the following. L-N 240v, L-E 240v, N-E 0v which looks good to me.

I have also checked on the Conumer unit and i get the same readings as on the 13A Ring main.

I am stumped as to where the problem exists and any help would be much appreciated.

Cheers,
James

pdcelec · 17 Mar 2007

try an insulation resistance test and a R1+R2 and report the results

ColJack · 17 Mar 2007

what's on the lighting circuit?
sometimes you get feedback off the fittings..
are you reading at the light itself and do you have dimmers fitted? is it dimmed?

Spark123 · 17 Mar 2007

I'd vote for an open circuit earth connection, an R1+R2 check should reveal this.

ColJack · 17 Mar 2007

earth floating up to 165v.. ?

can you meter that earth to one from another circuit?

choppertastic · 18 Mar 2007

There are no dimmer switches fitted. There is 1 2-way switch. All the lamps are low energy (dont think that would make any difference).

If I take the celing rose off and meter across the terminals when the light is on I get the figures I mentioned earlier. This is the same if I meter behind the switch.

didthathurt · 18 Mar 2007

I know it sounds a dull question, but are you sure exactly what you are measuring? (Especially if they are not marked properly) it can be very easy to confuse different conductors. If you "break" the neutral of a radial circuit, it is possible to measure 240V between the ends you have just created despite the fact they are both "Neutrals". In non-PME systems you can measure a lesser PD between a neutral and an earth.

I ask because the results you quote are a tad contradictory. If the Live is at a potential of 240V above that of the neutral and 75V above that of the earth a certain logic says the potential difference between the neutral and earth must be around the 165V mark - which you don't have.

Spark123 · 18 Mar 2007

Unless the CPC is electrically floating

As ColJack mentioned before, with the electricity turned off can you connect a multimeter set to the lowest ohms range between the earth on a socket and the earth on the light switch/ceiling rose and tell us what it reads?

devonspark · 18 Mar 2007

I agree with SPARK123 an open circuit cpc withn a neutral - earth fault upstream.R1/r2 would prove open earth , or try measuring at each point from cu onwards , this would give an idea where fault lies as readings will differ before and after fault

Spark123 · 18 Mar 2007

Not sure about a N-E fault upstream, I was thinking along the lines of capacitive coupling of an open circuit CPC to the phase conductor. Using a high impedance DVM can produce some strange results!!

RF Lighting · 18 Mar 2007

O/C CPC connection is almost certain, as Spark123 has said 3 times

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Neutral - Earth problem?

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