Light fault 'tripping' RCD on the ring circuits.

[Feenster]

I have an offgrid set up with no mains connection at all.

Recently, an interesting thing happened. The RCD on the distribution panel 'tripped' when it was wet and windy. After much trial and error I found the problem to be an outdoor light fitting that water was getting into.
However, the RCD is only on the ring (socket) circuits and shouldn't have anything to do with the lights and yet it 'tripped'. I have 7 socket circuits (each a ring circuit), each with its own MCB, protected by one RCD. This is the RCD that 'trips'. I have no RCD on the radial circuits for the lights.
(The RCD is a Hager 63A 2 pole RCD 30mA (type A)).
Is this unusual?

 

[Fleeting]

Are you sure the light isn't tagged from the power circuit?

 

[Feenster]

I am sure.

 

[Geoff1946]

What are your earthing arrangements ? The outside light fault, which is presumably to ground, is apparently (somehow), sending a current through one pole of your RCD.

 

[Murdoch]

Having 7 socket circuits to a single RCD isn’t a great set up to start with

Extreme Earth leakage from “other” circuits can trip RCDs protecting other circuits.

 

[C15JON]

Neutral to earth low resistance will cause imbalance and trip RCD because the neutrals and CPCs are common to all your circuits presumably.

 

[kerching]

Are you sure the outside lights are on their own mcb and NOT spurred from the socket circuit?

 

[Mike P]

I agree with C15JON They all share neutrals with a common RCD, so low/short insulation N-E will give same result!!

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[Murdoch]

I agree with C15JON They all share neutrals with a common RCD, so low/short insulation N-E will give same result!!

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But the lights are not on the RCD

 

[Feenster]

What are your earthing arrangements

I have an earth rod hammered into the ground.

 

[Feenster]

Having 7 socket circuits to a single RCD isn’t a great set up to start with

I agree. When the days get longer and I get a chance I plan to replace the MCBs and RCD with RCBO's.

 

[Fleeting]

I agree. When the days get longer and I get a chance I plan to replace the MCBs and RCD with RCBO's.

Northern or Southern Ireland?

 

[Geoff1946]

I have an earth rod hammered into the ground.

Connected at a generator? Where is your neutral connected to earth? Are there other possible "earths", such as plumbing ?
My thinking is that part of the fault current from your non-rcd lighting is somehow going through one side (probably neutral) of your power circuit rcd.
This suggests to me that there is possibly neutral to earth leakage somewhere in your power circuits, not sufficient to trip the rcd normally, but giving fault current in your earth network an alternative path. Does that make sense?
Sheer guesswork, but you haven't told us much about your system and nothing about what is connected to it.

 

[C15JON]

That’s irrelevant. We’re not talking about the live, just the earth and neutral. Any earth-neutral connection will be sensed by the RCD, regardless of how the live connection of any particular circuit is wired.

 

[Murdoch]

That’s irrelevant. We’re not talking about the live, just the earth and neutral. Any earth-neutral connection will be sensed by the RCD, regardless of how the live connection of any particular circuit is wired.

You 100% sure about that in a board where only the socket circuits are protected by the RCD?

 

[C15JON]

Yes.
The fact that some sockets are protected by RCD means the live feeds of those protected circuits are cut through the RCD, correct?
The neutrals and earths are all connected to the same busbars - this applies for every circuit, correct?
So a neutral to earth fault on any circuit will be detected by the RCD.

 

[Murdoch]

Yes.
The fact that some sockets are protected by RCD means the live feeds of those protected circuits are cut through the RCD, correct?
The neutrals and earths are all connected to the same busbars - this applies for every circuit, correct?
So a neutral to earth fault on any circuit will be detected by the RCD.

Er, not correct

 

[C15JON]

Er, not correct

Okay - please educate me why this is?

 

[Murdoch]

Okay - please educate me why this is?

RCD's detect the in balance between L & N - so in this case the sockets are protected by the RCD. The live and neutral for the lighting circuits in the situation outlined do not use the RCD

 

[C15JON]

With respect, you are wrong. RCDs trip quite happily on neutral to earth leakage. You can prove this quite simply in the home!