Weird RCD tripping

[markythesparky]

Hi guys,

I've installed a new c/u for a swimming pool plant room (very small) consisting of circuits for a pump, electrics heater, pool and garden lighting and timer control for the heating/ filtration.

it has an RCD main switch. when commissioning everything worked fine although the pump had no water and was just fired up briefly to check operation. anyway, the pool guy comes back and connects up all the pipework and now when he switches the pump on the RCD trips. I went back and found the fault to be on the lighting circuit so I disconnected it.

Question:
Why would the RCD trip when the pump is started and has no fault but the lighting circuit does?

as always, thanks in advance.

 

[telectrix]

Re: Wierd RCD tripping

neutral failt on the light circuit.

 

[markythesparky]

Re: Wierd RCD tripping

it is a neutral-earth fault but why would the pump trip the RCD when its not the pump at fault?

neutral failt on the light circuit.

 

[Simonslimline]

Re: Wierd RCD tripping

Have you dissed the lighting circuit neutral as well?

 

[Richard Burns]

Re: Wierd RCD tripping

Little diagram for you

works the same even if they are on the same RCD.

 

[Octopus]

Re: Wierd RCD tripping

If you have an "up front" RCD, a fault on ANY of the circuits could trip the board - and yes turning on 1 circuit could affect another circuit - as you have seen!

 

[markythesparky]

Re: Wierd RCD tripping

just disconnected the lighting earth to find the fault. funny thing is that I could connect the earth for the lighting back once the pump was running! if I left the lighting earth connected and then started the pump it would trip every time. lighting circuit is totally dis'd now.

Have you dissed the lighting circuit neutral as well?

 

[Octopus]

Re: Wierd RCD tripping

just disconnected the lighting earth to find the fault. funny thing is that I could connect the earth for the lighting back once the pump was running! if I left the lighting earth connected and then started the pump it would trip every time. lighting circuit is totally dis'd now.

Really?

 

[markythesparky]

Re: Wierd RCD tripping

Yep. this was the strangest thing. I tried it about five times and every time the pump was running the lighting earth could be reconnected. but if I turned the pump off and then back on, it would trip again.

Really?

 

[Richard Burns]

Re: Wierd RCD tripping

Probably not the best way to fault find by possibly making the lighting earthing live.

It would be that the majority of the current will flow via the neutral so if at a 6A running current say 10mA was passing through the fault but at 20 or 30A start up current over 30mA would pass through the fault and trip the RCD.

 

[Deleted member 9648]

Re: Wierd RCD tripping

Never heard of insulation resistance testing?

 

[markythesparky]

Re: Wierd RCD tripping

you are correct but it was the easiest way and I was in my own so no danger of anyone else in the area. so even though the fault is on another circuit, some of the current flow from the pump neutral would go back down the earth fault as the neutrals are all connected at the same point. I get it now Richard. many thanks mate.

Probably not the best way to fault find by possibly making the lighting earthing live.

It would be that the majority of the current will flow via the neutral so if at a 6A running current say 10mA was passing through the fault but at 20 or 30A start up current over 30mA would pass through the fault and trip the RCD.

 

[Lucien Nunes]

it was the easiest way and I was in my own so no danger of anyone else in the area

If you must test live (and in this case it wasn't necessary) it's much better to dis L & N than the CPC. Quite apart from possible hazards of a floating CPC, if the circuit is picking up an earth anywhere along its length, dissing the CPC won't localise the fault and will give a false negative result. But dissing L & N will always find the offending circuit.