UK Earth leakage issues.

[Edward67]

Hi all. Im looking for some help regards a massive earth leakage issue in a new i installation
Its a little more complicated than this but its hard to explain. New board 16 way tails splits due to old board still i operation for new few weeks. Large 4 bed house. Pme system. Earth verified bonding in place.
My main issue appears to becoming from a newly installed lighting circuit. It consists of downlights and standard pendant type lights and a bathroom fan and some wardrobe lighting.
I have tested the circuit R1 and R2 all fine 1.68. IR 999 so all fine there.
My first thought was damaged insulation causing but how can this be when IR is fine.
I have inspected whole circuit multiple times but nothing obvious.
Now here is what i dont understand. If i clamp at the meter i get 180 to 400 milliamps rapidly up and down where as my RCD is not tripping?
But if i test the rcd at a socket it tests fine.

So at this point I'm thinking WTF. Alot of workmen have been on site. Plasterers carpenters etc.

This is where things get stranger. I decide to go to first switch disconnect all seems fine test incoming feed 240v. I go to second switch and disconnect. So all i have is the feed from first switch. I test here but discover i am getting voltage reading between my incoming cpc and the light cpc even though they are completely separated.
Induced voltage? But why. We dont normally check voltage on the cpc so am i looking at something normal.
At this i put circuit back together go to the board. Out of interest i pull the circuit cpc (yes circuit was live so dont crucify me) and find if i link to main earth terminal i see 100v. Ok i put it back in then things are at same potential so it disappears.

So I asked an electrician with 20 years experience to come have a look. He is also stumped. I have not tested the full installation yet so maybe its fault affecting my circuit?

Thoughts helpful. If thos makes any sense at all.

 

[Edward67]

This isn't really a useful test but 100V is a perfectly reasonable result for induced voltage. One side of your meter sees earth, the other sees a floating length of CPC that isn't connected to any terminal of the supply. But there is capacitive leakage into it from nearby line conductors, and leakage out of it to nearby neutral conductors and earthy objects, and this forms a potential divider. The actual voltage, often between 1/4 and 1/2 of line voltage, doesn't signify very much other than that there are some live cables around the place.

If you had measured the current from the disconnected CPC to earth, you would probably have found that the leakage is tiny, a fraction of a milliamp.

Thanks for response. .
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Ok. I went and purchased a clamp meter today. For a little test I clamped my main live and neutral 10ma. Out of interest I clamped the main earth. Same reading. 10ma.
So I opened up my board and only cable in that I could easily clamp was my alarm cable. So I clamped live and neutral 130ma approximately. I then clamped the cpc on.its own. 1ma.
Could one of you kind people explain this?
Guessing its electronics in some form.

Also back onto my original large earth leakage issue. How common is it for tncs to be affected with earth leakage from sources external to the property:?
As I say it all points to that conclusion due to the rcds working fine within the installation but just find the issue disconcerting.
Also would you contact dno about it if it's happening or is it acceptable Because I got friend to clamp main earth on a 3 phase large commercial unit today and he was getting similar reading to the house I'm working on.

Sorry for all questions..

 

[Lucien Nunes]

The current you are measuring in the earthing conductor is not leakage, it's a fraction of the normal neutral current from any and all installations finding its way through parallel paths connected to the PEN. It's not a problem and it can't be eliminated.

 

[plugsandsparks]

Very common, see post 9

 

[Edward67]

Very common, see post 9

Thanks again. Look forward to clamping the offending tails again.

 

[Edward67]

Thankyou plugsandsparks. I went round today. Clamped my tails and got 5mA.
So lesson learned.