Very strange fault

[GaryMo]

Small flat, TN-C-S supply, Ze 0.19ohms.
16th edition installation, split RCD board.

Non RCD
1 - Boiler (16A)
2 - Lights (6A)
3 - Security Alarm (6A)

RCD
1 - Oven (32A)
2 - Sockets (32A)
3 - Sockets (32A)

IR for all circuit (L&N - E) >299Mohms (no problem there).
30mA RCD trips within maximum time, trips at 21mA on ramp (no problem there).
Zs for lighting circuit 0.98ohms (no problem there).

Customer calls to say when he switches his outside light off which is fed from the lighting circuit (W2, not protected by RCD) the RCD trips.
He demonstrates this and yes, the RCD does trip with all MCB's still energised.
Split board wired correctly, no neutrals crossed over. The RCD only trips when the outside light is switched off.

I measured IR between circuits for separation and again, all read >299Mohms.

Any idea's?

 

[Spark123]

Switch arcing?

 

[RF Lighting]

Have you checked the tightness of the incomming connections, especially the neutral around the cutout / meter / main switch?

Also tightness of connections onto the busbar etc?

 

[securespark]

NE reversal or NE fault on outside light and/or wiring?

 

[GaryMo]

Have you checked the tightness of the incomming connections, especially the neutral around the cutout / meter / main switch?

Also tightness of connections onto the busbar etc?

Yes, everything tight, all busbar tags correctly gripped by MCB terminal.

 

[GaryMo]

NE reversal or NE fault on outside light and/or wiring?

Forgot to say, polarity fine.
Outside lights connected correctly, all wiring appears to be fine...

 

[GaryMo]

Switch arcing?

Nothing I noticed. Outside lights are 2 x 16W 2D fittings.
I wouldn't have thought an arcing switch fed from the no RCD side would cause the RCD to trip.

It's a real headscratcher!

 

[Spark123]

How is the switch wired?
What make of RCD - does it have a functional earth?

 

[GaryMo]

How is the switch wired?
What make of RCD - does it have a functional earth?

1.5mm t&e to switch (Live and Switch Wire), metal switch, earthed (obviously).

Wylex RCD and consumer unit, no functional earth.

 

[RF Lighting]

Has the RCD tripped since the lights were installed, or has it started of it's own accord?

The only think I can think of is something like a spike of back EMF as the magnetic feild in the chokes of the 2D lights collapses.

Having said that, it's getting a bit outside my knowledge base, so maybe someone else could say yay or nay?

 

[GaryMo]

Has the RCD tripped since the lights were installed, or has it started of it's own accord?

The only think I can think of is something like a spike of back EMF as the magnetic feild in the chokes of the 2D lights collapses.

Having said that, it's getting a bit outside my knowledge base, so maybe someone else could say yay or nay?

Started to trip recently. Also tripped when I had both 2D fittings disconnected

 

[RF Lighting]

So it tripped as you switched the switch off, even with the lights disconnected?

Was there any load at all on the switch at that point?

It does sound very odd indeed

 

[securespark]

Try changing the switch for a plastic one.

 

[GaryMo]

Yes though I had to operate the switch numerous times to trip the RCD with the lights disconnected.
At that point there was no load at all on the switch.

I'm just struggling to think how a circuit on the non RCD side of the board could trip the RCD.

 

[GaryMo]

Try changing the switch for a plastic one.

At this point, that's worth a try though god knows why!