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[Taxi_Driver]

What could be causing an intermittent Neutral to Earth fault that makes the main trip in the fuse board to go??
I've discovered its on the downstairs lighting circuit......sometimes its weeks, but for last few days its been everyday the main trip has gone.....sometimes the main breaker will turn back on fine, other times i have to turn off the downstairs lighting circuit to get the main trip to stay on.

The reason i reckon its an N/E fault as the trip will go even if all the downstairs lights are off.

I did wonder if it could be a low energy bulb doing it......theres 5 on the circuit 

[aaronjb]

What kind of breaker is it throwing - RCD/RCBO/etc or MCB?

[TheBoy]

Some are wired with switched live, some switched neutral. So could be Live.

Is it the earth leakage trip that fires, or a circuit specific, or the main 100A one?

[Taxi_Driver]

Some are wired with switched live, some switched neutral. So could be Live.

Is it the earth leakage trip that fires, or a circuit specific, or the main 100A one?

Its the main 80A one, which i assume has earth leakage builtin, as i dont have a seperate earth leakage breaker. The 5amp breaker for the lighting circuit doesnt trip.

Its says on it RCO circuit breaker. Max current 80Amps 30ma trip.

[tigers_gonads]

Any strip lights on the circuit ?

[tigers_gonads]

Has your consumer unit just got the one combined rcd / main switch or is it a split load unit ?
What makes you think its the lighting circuit ?

[Taxi_Driver]

Any strip lights on the circuit ?

Nope, only 5 low energy bulbs, which i did wonder about.. 

[tigers_gonads]

Is it tripping when you switch something on or does it trip when it feels like it ?

[tigers_gonads]

Sorry, just read all the post 

Doing about 10 different things at once at the moment 

I'd swap the energy saving bulbs for normal bulbs for starters.

Bear in mind that if you only have a combined rcd / main switch, you consumer unit will have a common busbar for the neutral so the fault could be on any circuit.
Kettles and fridge / freezer compresors are a favorite 

Also bear in mind that just switching the 13 amp socket off doesn't always totally isolate the appliance.
Most sockets just switch the live leaving the neutral and earth conected 

[TheBoy]

Kettles and fridge / freezer compresors are a favorite 

dishwashers

[Taxi_Driver]

Has your consumer unit just got the one combined rcd / main switch or is it a split load unit ?
What makes you think its the lighting circuit ?

Sometimes the main breaker wont turn back on.....turn off this circuit and it will...

[Taxi_Driver]

Sorry, just read all the post 

Doing about 10 different things at once at the moment 

I'd swap the energy saving bulbs for normal bulbs for starters.

Bear in mind that if you only have a combined rcd / main switch, you consumer unit will have a common busbar for the neutral so the fault could be on any circuit.
Kettles and fridge / freezer compresors are a favorite 

Also bear in mind that just switching the 13 amp socket off doesn't always totally isolate the appliance.
Most sockets just switch the live leaving the neutral and earth conected 

Done that now......still doing it.....seems to be getting worse....tripping about every 15mins now 

Think i'll have to get an electrician in.....

[Kevin Wood]

I reckon you've got some damp in the circuit somewhere, i.e. bathroom plumbing leaking into a downstairs ceiling rose?

However, as said, it might be that there are other faults on other circuits that are all contributing to the leakage and it just happens to drop below the 30mA trip if you isolate this circuit.

Only real way to narrow it down would be to disconnect the circuits one by one at the consumer unit and megger the live and neutral against earth to see if you can spot where the worst of the leakage is.

EDIT: By disconnect above I mean remove both live and neutral connections at both ends of the ring.

Low energy bulbs don't normally have any earth connection (the whole bayonet connector is usually plastic).

Any outside lights on that circuit (paranoia lights. etc)?

[tigers_gonads]

As Kevin said 

Also have you replaced any light fittings recently ?

What circuit is tripping ?
Up or down ?

If your handy with a screwdriver, and know the approx route of the cables. You could try and split the circuit up by picking a light fitting half way round.  TURN OFF THE CIRCUIT and remove the live and neutral wires out of the fitting the power it back up 
If it trips again, do the same and work your way back to the consumer unit.  If the circuit holds, TURN OFF THE CIRCUIT and disconect the wiring further up until you isolate the part of the circuit that is tripping 

[Martian]

Any strip lights on the circuit ?

Nope, only 5 low energy bulbs, which i did wonder about.. 

It's not uncommon for CFL's to cause nuisance tripping, especially if they are the cheap crap that you find on the supermarket shelves.

As it's tripping so often, fault finding should be relatively quick.
Start by removing ALL the CFL's from the lampholders on the circuit you believe to be at fault and wait. If the RCD still trips then I'd be thinking along the same lines as Kevin in that a bit of damp has found it's way in to a ceiling rose, which means that visual inspection is the diagnostic tool of choice.
If the RCD stays in after you have removed the CFL's, that means one of them has gone a bit leaky and the only way to diagnose the culprit will be to reinstate them one by one until the fault reappears.

There is also the possibility that the RCD itself has gone a bit weak (I've had that happen on a brand new CU that was less than 2 weeks old), in which case replacement is the only cure.

Please note that while replacement of the main switch/RCD is fairly straightforward, it is a job for a QUALIFIED sparks only.
I say that not so much because of the Part P 'dangle berries', but mainly because you have no safe way of isolating the meter tails.
If you were to make a mistake while working in that area of the CU, then it would more than likely be your last as there is 100A sitting on the end of those tails and the only fuse in that scenario will be you (and you would fail at around 125mA).