Lighting circuit - RCD trips when MCB is on

[edwardt]

Hi,

Our house had completely new wiring installed about 1.5 years ago. This morning (and I guess it happened when a light switch was turned on somewhere) one of the RCDs in the CU tripped.

After switching off all MCBs on this side of the RCD, resetting the RCD itself then switching each MCB back on, I've isolated the problem to one of our downstairs lighting rings.

On this ring, there are 15 regular CFL downlighters, 4 water resistant ones (bathroom), 1 bathroom mirror (also florescent, but with heating pad) and - unknown to me up to this point (qualified spark fitted the new system) - two lights that sit outside our front door (25w halogen). MCB is 6A.

As soon as I switch the MCB for this ring on, it trips the RCD (the MCB remains un-tripped) - it doesn't matter if all of the load is switched off (note that I can't switch the heating pad of the mirror off, and therefore even with all switches off there is a small load).

The obvious thing for me was the outdoor lights suffering from water ingress due to the rain last night - though we have disconnected the live from their indoor switch and still get the trip.

Does this sound like a dodgy MCB (note again that the MCB doesn't trip, only the RCD if that MCB is switched on - is that expected?) or is it still possible that the outside lights are causing a problem, even with the live disconnected from the switch?

So far we have disconnected all of the hallway lights (5 CFLs) and the bathroom (4 water resistant CFLs) as well as disconnecting the outdoor light's live. Still tripping.

Another note - if the circuit is left for a while then switched in again, the trip takes 10 seconds to happen - then after resetting, will trip instantly again (so magnetic trip vs thermal trip - does this give any clues?).

Final note - we've not had a single trip in this house for the entire 1.5 years. 3 weeks ago, a utility company came and replaced our old style main fuse bulkhead (metal type) with a plastic one (inc. digging up the driveway to connect). After that, the electricity company came and changed the meter. Since then we've had a single trip in the kitchen (which was fine after a reset), plus this new trip. Is it possible they changed the type of earthing during this process and something has gone weird?

Thanks in advance for any help!
Edward.

 

[PrenticeBoyofDerry]

Why can't you isolate the heating pad?
If this is the only load left, I would at least eliminate that.
Has any other work been done in the house recently, that could have effected the cables?

 

[edwardt]

Thanks for the quick reply - no, no other work has been done on the house at all (for at least a year).

The difficulty with isolating the heating pad is the extremely short cable on the back of the heavy mirror, which makes it very hard to remove and put back again - but I will try this tomorrow (too dark to try in there now).

Thanks,
Edward.

 

[PrenticeBoyofDerry]

I would at least eliminate that first before investigating further.
Isolate circuit first before removing, but still find it odd that there is no safe means of isolating that piece of equipment, without opening the whole circuit.

 

[Taylortwocities]

Edward

Please note that an RCD will trip if there is a fault on either the live or the neutral side of the circuit.

In many circuits - especially lighting - only the live side is switched leaving the neutral still connected to the RCD.
Also the MCB in the consumer unit will only isolate the live side of the circuit.

The best way that proper isolation (and fault location) can be carried out is with an insulation tester and a competent person with a screwdriver.

You may be able to sort it by trial and error but its a long, frustrating job.
Start with the obvious stuff - anywhere where water could meet electricity:-

Outside lights & sockets
Central heating pumps and valves
OUTSIDE CHRISTMAS LIGHTS

But remember, to isolate the circuit, you gotta disconnect live and neutral!

 

[EFLImpudence]

So far we have disconnected all of the hallway lights (5 CFLs) and the bathroom (4 water resistant CFLs) as well as disconnecting the outdoor light's live. Still tripping.

That will not remove the fault if it is between neutral and earth.

You would need to disconnect completely the cable at or/and to the outside lights.

 

[bernardgreen]

A Neutral to Earth fault on a circuit whose Live has been isolated by the MCD being turned off can still cause an RCD to trip.

Which means you can turn off the circuit with the fault, the RCD trips and from that you assume that the circuit that is OFF does not have the fault.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

The only reliable way to prove there is no Neutral to Earth fault on a circuit is to turn off the MCB and remove the Neutral from the neutral bar and measure resistance between the disconnected Neutral and its earth wire.