N E fault on ring.

[Clark3169]

Hi guys,

I fitted a new CU last week. After IR testing I found a dead short 0 on N & E. advised the customer this would need to be sorted before installation as New CU will have a sexy little device called an RCD which will keep tripping when he turns lights on and off or plugs stuff into the ring main. When he asked me why? I told them that there would be an imbalance that the rcd will react to etc.. etc... But when asked about how the light switches might make the rcd trip as they weren't on the rcd I bundled through the explanation which seem to satisfy them but to be honest I would like to have the science of this explained to me just to make sure I have it correct in my head. Bit apprehensive about posting here again. I am pretty new to the industry and after a terminology slip up a week or so ago (been talking class two lights with a customer all day and referred to a plastic socket as class two, yeah I know! Nob ed!) I got a load of Micky taking etc... Nothing wrong with banter and I'm a big boy now ha ha but not very helpful when you need to get an answer. So please any help will be greatly appreciated as will any Micky taking as long as its in that order ha ha.

Thanks

 

[telectrix]

so why are the lights and other circuits not on RCD as per reg. 522.6.101?

 

[RISElectrical]

The lights should also be on the RCD, as for the dead short reading... Did you look into it? We're all loads unplugged from the ring that you tested etc?

 

[Limbo76]

The lights shouldn't trip the RCD if they're not fed by the RCD circuit (which they should be, as telectrix states above).

 

[Clark3169]

Hi guys sorry bad description on my part. The board is split load all circuits are on rcd what I should have said is not the rcd that cover the ring whee the fault is. I cleared the fault. My method was

1. Unplug everything and IR test again found same fault.
2. Broke ring in middle tested both ways.
3. Repeat step 2 until I found the fault.

It turned out to be a exposed N touch metal babackbox my Q is

what is the science that allows a light switch covered by a different RCD to trip the other rcd that covers the ring where the fault is.


ta guys

 

[telectrix]

what is the science that allows a light switch covered by a different RCD to trip the other rcd that covers the ring where the fault is.



usual cause of that is a borrowed neutral or a neutral cable inserted into the wrong N bar.

​

 

[Clark3169]

How do you know that the lighting cables weren't over 50mm in the wall or in protection. Your right of course they weren't ha ha

 

[Limbo76]

Experience :icon10:

 

[Clark3169]

;-) I know.

 

[baggy61]

Induction from lighting cables rcd'd cables running along side could cause tripping couldn't it ?

 

[telectrix]

not as i've ever seen. only incorrectly connected neutrals can cause this.

 

[oldtimer]

Also you would have saved yourself a lot of hassle by doing a global LN-E IR test before you started on the CU

 

[Clark3169]

I'm sure you'd struggle to leak 30ma through good insulation. Or even 25 + that it would take to trip the RCD.

 

[Clark3169]

Oldtimer that's exactly what I did and is how I discovered the fault.

 

[Clark3169]

I had another job. All neutrals were correctly connected, no borrowed neutrals with an N to E fault on the ring circuit. When switching lights on or off sometimes it tripped the rcd that covered the ring not the rcd that the lights. How would that happen does the sudden on or off of the lights cause a small surge or something in all other circuits that due to the N to E fault cause the other rcd covering the ring to trip? I ask this because after clearing that fault the rcd hasn't tripped since.