rcd activated - lighting circuit

[tinkering]

Tuesday night - switched on fluorescent light - took several seconds to come on (usually no delay).
Wednesday night - switched on fluorescent light - did not come on.
Thursday morning - all lights & some sockets not working.
Looked at consumer unit - one of the two RCDs had activated; all MCBs still on. Switched off all MCBs, switched on RCD, tried MCBs one at a time - downstairs lighting circuit caused RCD to activate ; it took several seconds to activate. Left that MCB off.
Dismantled fluorescent light - choke/coil? had burnt out.
Tried MCB again - RCD activated again.
The double switch for the fluorescent light also serves an outside light; there is no sign of water ingress. Have disconnected all wiring to this double switch.
Tried MCB again several times - if RCD is on when I try, now activates RCD instantly and cannot switch MCB fully on. If RCD is off when I try, I can put MCB fully on but cannot put RCD on.
Any suggestions for locating the problem please?

 

[AndyPRK]

try disconnecting the neutral from the light.

Replace light

 

[PrenticeBoyofDerry]

The first thing to do as the RCD is shared with other circuits, would be to take everything off load, so unplug appliance (not switch off) take the plug out of the socket, turn any fused connection and isolator switches off, have all lights in the off position.
Then see if the RCD still trips, if it does not then try switching lights on and see if the RCD stays stable, then plug in and re-energise each appliance at a time.

If the RCD does trip straight away, then as Andy has said disconnecting neutrals with the MCB off, could lead to which circuit is causing the fault.

It does sound like a lightning problem, but could be coincidental and issue lay elsewhere or is a combined build up of leakage.

 

[tinkering]

Thanks for your reply but sorry, I don't understand why you suggest unplugging appliances - the RCD activates only when the MCB for downstairs lights is switched on; if that MCB is off and all other MCBs on, there is no problem.
Have I misunderstood you or have you misunderstood me?

 

[bernardgreen]

The fault can be a neutral to earth fault anywhere in the installation. Even if the MCB for the faulty circuit is turned OFF the neutral is still connected. The fault could be in an appliance and even if the appliance is switched OFF the fault can still cause the RCD to trip when there is something else switched on.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

 

[EFLImpudence]

Thanks for your reply but sorry, I don't understand why you suggest unplugging appliances - the RCD activates only when the MCB for downstairs lights is switched on; if that MCB is off and all other MCBs on, there is no problem.
Have I misunderstood you or have you misunderstood me?

Yes.

It would appear that you have a Line(live) to earth fault if it disappears when you switch off the mcb.


Edit - Stupid error corrected.

 

[RF Lighting]

Would a line to neutral fault trip an rcd?

 

[EFLImpudence]

Oh I do beg your pardon, obviously meant earth - will edit.

 

[RF Lighting]

Just checking

 

[JohnW2]

It does sound like a lightning problem ....

I suppose anything's possible, but I don't think the OP mentioned thunder or suchlike

Kind Regards, John