Exterior (spike) lights

[Aloha]

Hello all, new to the forum, go easy on me! 

Wondered if if anybody has had a similar problem with garden spike lights, gu10, namely Knightsbridge.

A lady asked me to look at her garden lights which intermittently trip the Rcd...

From stripping it down to perform visual inspection for ingress of water, Here is what the install consists of.

4mm 3 core swa feeding a wise box in the shed, out of the wise box comes a 4 core 1.5mm (2 lives, N,E) 

lives feed Wiska boxes, only 4 of them. Out Of the wiskas are a few spike lights with aurora LED lamps. 

The second live feeds a water feature.

as soon as I turned the water feature on it tripped the Rcd. Disconnected the pump, performed insulation resistance on the pump (1m flex to it) and got 0.20 ohms at 500v... Initial reactions are, it's the pump. 

The lights came on fine, but I thought I would test them too just to make sure. Each fitting is a simple 2m flex, removed the lamps, and put 500v through them, live to earth and neutral to earth similar readings of 0.02 ohms! She tells me they are 2 years old. There is zero evidence of any damage to the cables, and it's all of the fittings that are showing a reading like this.. Lights performed fine, very strange, but tripped again after about an hours buildup...

Therefore i have 2 questions : 

1, why the hell would such a low reading not trip an Rcd straight away, and give itself an hour to do so,

2, could the flex really be that porous? It's obviously coming across to me that the flex is just sucking up water as I say, visually there is no damage but the 2m of flex is giving me a 0.02 ohms reading..

any help? 

kind regards

tom 

 

[Doc Hudson]

Welcome to the forum, are you saying 0.2ohms  and 0.02ohms.  Or do you mean  0.2Megohms  and 0.02Megohms.  e.g. 200,000ohms and 20,000ohms. neither of which would trip an RCD instantly. I would have thought these items were double insulated. My guess is bad joint in one of the boxes.

Doc H.

 

[Aloha]

Hi doc, thanks for the reply, what a schoolboy error on my part, must have been rush typing! Yes I meant mega ohms  

regards

 

[kerching]

My money is on slugs......they have been causing mine to trip. Until i threw them into my nieighbours garden and filled my joint boxes with gel

 

[Aloha]

Hi kerching, yeah I've had that before too!  it's just even in disconnecting the fittings and holding the whole thing in my hands, visually inspecting it with no apparent faults and no lamp fitted, I'm still getting a low reading... Strange!

Would like to think performing IR at 500v would cause a slug to dry up pretty fast

 

[Rob_the_rich]

I think I know why, if the four flexes to the lights are not wired in series, from one to the next, but are wired in a star pattern, then the fault to earth path resistances are in parallel. This gives the combined resistance as (1/20000)+(1/20000)+(1/20000)+(1/20000)=(1/0.0002)=5000ohms which at 240V gives a current of 0.048A or 48mA, which is very close to the tripping current of the RCD.

Does this make sense?