Tripping mcb problem

[Niscars]

I am having problems with an intermittant fault on one of my lighting circuits. One of the 6A mcb has garage lights connected to it (3rd mcb from the left in the picture). As I hope you can see, the mcb is on the non rcd side of the distribution box. The trouble is that often when the light is switched on or off, the rcd trips but not the mcb. Is this correct ?

To see if I can narrow the problem down, I am thinking of swapping the 2 6A mcb over. Can anyone can see an obvious reason for this fault.
Thanks.

 

[plugwash]

what is the trip point of the rcd?
and what is the power rating of the light in the garage?

if you had the neutral in the wrong neutral bar then i would expect the rcd to trip every time the light was turned on but if it was a high trip point rcd and a low power light i can see ow it could be borderline.

 

[Niscars]

It is a 80A, 30mA RCD and the lights are 2 x 40w strip lights.

 

[Damocles]

There are two reds coming out of that mcb, so i guess it is controlling more than the garage light. what else?

But otherwise, make sure that BOTH corresponding neutrals are on the non-RCD neutral bar.

 

[securespark]

looks like both sides are RCD, no?

 

[Damocles]

well spotted that man. Though mixing up the neutrals would still cause tripping. Given that both switches are RCD, which one is tripping?

 

[Niscars]

I have just confirmed that all neutrals go to the rcd side.
Noting Securespark's comment about 2 rcd, I thought that the switch on the left was just a main switch but on inspection it may be a type of rcd. The figures are difficult to read but I can make out the following markings: DOJ, 80A, 0.1A, 220 there is also a test button.
Of note, this is not the item which keeps tripping, that's the one on the right.
I was also not descriptive enough about the 6A mcb - It feeds one of the lighting circuits and the garage lights.

 

[ban-all-sheds]

well spotted that man. Though mixing up the neutrals would still cause tripping. Given that both switches are RCD, which one is tripping?

Either or both? Turn-and-turn-about?

If a neutral was cross connected, both RCDs should see an imbalance....

 

[plugwash]

looks like an install for a TT supply

the rcd on the left is in the input and is a 100ma device. and this is fed over to the second. 30ma rcd on the right.

 

[mapj1]

As a diagnostic, it might be worth unshipping one or both of the fluroescent fittings. Depending on the suppressor arangements inside they may have a significant earth leakage.
Note, on a TT installation it is normal for a lighting circuit to be protected only by the larger RCD, in this case the 100mA one (0.1A), not the 30mA one -that should feed sockets and portable appliences - it may simply be on the wrong side of the board for reasons of convenience.
regards M.

 

[Niscars]

The supply is either TN-C-S or TN-S, I can't see which without taking bits apart. Looking at the pictures on this forum, it's definately not TT.
I will try isolating one of the strip lights to see if that fixes it.

The thing that I am still not understanding at the moment is why a lighting circuit on the left hand side of the CU trips the RCD on the right of the split board knocking out the 13A sockets ?

PS. The strip lights remain on !

 

[Niscars]

I have been doing some more fault finding and have found a possible problem in the wiring from the CU to the lights.

The wires on the left come from the CU (top) & to a single pole switch (bottom), the wire on the right goes to the lights. The switched live goes to what I think is neutral on the strip light (just numbered 1 & 2 - not L & N). I swapped them over and the lights still work and after several tests have not tripped the RCD yet.
Does this make any sense to anyone ?

PS I have marked up the switched live properly too !

 

[andy]

in the pic, can i ask y there are 2 reds in the same terminal, yet there is a terminal to the side, designed for that other wire

 

[andy]

The thing that I am still not understanding at the moment is why a lighting circuit on the left hand side of the CU trips the RCD on the right of the split board knocking out the 13A sockets ?

PS. The strip lights remain on !

if it has nay contact with neutral on the right hand side then itll trip the right hand side RCD. the other RCD may not trip fast enough so the other RCD trips and the faults goes, leaveing the other RCD on

 

[securespark]

If it is def. not a TT supply, then the incomer could be 100mA and the split could be 30mA. I've seen this before a few times.

Makes sense to double check that every neutral conductor is in the correct bar.