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I have had an extension built to a 60's bungalow.
I have extended the lighting circuit to the new extension but have a problem.
The metal switch boxes appear to have mains voltage on them.
When I connected an earth wire from the ring main earth to a metal switch box the main fuse blows.
Can anyone suggest a cure to this problem?

Thanks

Don't like to state the obvious,
but check you conductors are terminated correctly at ceiling rose on both the new light fitting and the one you have taken supply from.
There are diagrams to help on the project pages.
KB

Why doesn't the lighting circuit earth cause a trip as soon as circuit
re-energised? a clue maybe?
As KB says you must have connected the earth to a live terminal at source of extension,
How did you discover the mistake & did it hurt?
If you had been able to do the required insulation tests this would have shown up before circuit was livened up, strangely this is required of registered leckies but DIYers can 'chance it', no wonder so many die each year!

[quote="sparx"]Why doesn't the lighting circuit earth cause a trip as soon as circuit
re-energised? a clue maybe?
As KB says you must have connected the earth to a live terminal at source of extension,
How did you discover the mistake & did it hurt?
If you had been able to do the required insulation tests this would have shown up before circuit was livened up, strangely this is required of registered leckies but DIYers can 'chance it', no wonder so many die each year![/quote]

I felt a tngling when I touched the screws connecting the light switches.
I thought the lack of an earth was causing the earth to be live so I connected a temporary earth wire from the ring main earth to the metal lightswitch case. It was at that point that the main fuse tripped.
I have insulated the screws until I find the problem.
I take on board your comments about testing the circuit before it is put in service.

As you suggest I will go back to the original connection point and check the connection and then work through the lighting circuit until I find the problem.

Thanks for your advice.

[quote="sparx"]Why doesn't the lighting circuit earth cause a trip as soon as circuit
re-energised? a clue maybe?
As KB says you must have connected the earth to a live terminal at source of extension,
How did you discover the mistake & did it hurt?
If you had been able to do the required insulation tests this would have shown up before circuit was livened up, strangely this is required of registered leckies but DIYers can 'chance it', no wonder so many die each year![/quote]


Measured beween earth and neutral and I'm readng approximately 90 volts a.c.
Is this the sort of voltage expected or does it indicate a fault?

Hi agn,
certainly not correct, since earth & neutral are connected together at sub station and possible other points including at your intake point if a pme/tnc-s supply.
There should be no more than a few milli-volts between N
& E.
You appear to have 2 problems which may be due to 1 fault, first your neutral cannot be connected properly or it would not have volts appearing on it! and as said elsewhere your earth is not connected either.
I suspect the neutral has got free and is touching earth somewhere after the light so putting the lamp in line ie in series with your test meter.
If possible remove lamp(bulb) and retest N-E, if volts drop then look for neutral fault/connection such as in ceiling rose.

regards SPARX

[quote="sparx"]Hi agn,
certainly not correct, since earth & neutral are connected together at sub station and possible other points including at your intake point if a pme/tnc-s supply.
There should be no more than a few milli-volts between N
& E.
You appear to have 2 problems which may be due to 1 fault, first your neutral cannot be connected properly or it would not have volts appearing on it! and as said elsewhere your earth is not connected either.
I suspect the neutral has got free and is touching earth somewhere after the light so putting the lamp in line ie in series with your test meter.
If possible remove lamp(bulb) and retest N-E, if volts drop then look for neutral fault/connection such as in ceiling rose.

regards SPARX[/quote]

It seems that it is not a good idea to connect old lighting ccts to new.

I disconnected the new lighting cct from the existing lighting cct and connected directly to the main fusebox and that has solved the problem.

When the meter is moved and updated does that mean the existing lighting cct will need to be rewired?

Thanks