TT earth problems

richiek · Feb 11, 2010

Hi all, just heard about this forum and decided to give it a go.

I've only been a sparky for just over 2 years and last week a mate of mine who's a general handyman fitted a 12v door bell for a lady which is mains fed to a transformer. He wired it into a JB on the lighting circuit in the loft but called me as he noticed when he was touching the earth terminal with one of those harry tate screwdrivers that light up the light inside was flickering. I went round and put my fluke voltage tester across neutral and earth and sure enough there was a slight voltage there instead of the usual continuity. It was only very slight - prob less than 1v - but shouldn't be there nonetheless.

I carried out an earth loop test which read OL before eventually tripping the RCD as my Robin multi-tester doesn't have d-lock. I noticed she had a TT system and eventually located her earth rods concreted into her basement. They looked quite old and could be coroded so I guess this would be the cause of the earthing problems. Her entire consumer unit is RCD protected and I tested this and it trips in good time (19-29ms). Her next door neighbours have the same system which is also quite old but they don't have an RCD protected board, instead they have an old VOELCB which to be honest is a waste of space. They also have this problem of minute current in the earth but when I tested their earth loop I got a reading of 13.8 ohms which aint bad on a TT in my experience.

What I'm wondering is, would it be worth going through the hassle of replacing this lady's earth rods as she's well protected by her RCD and her existing ones are concreted in? They may even be connected to an earthing plate under the concrete. Would replacing them be guaranteed to cure this problem?

Any thoughts any of you more experienced guys?

bugsy · Feb 11, 2010

I would suggest spike test but this requires sp eqp. which you won't have , check , clean connections to rod ( could be v dry soil - I ve had tor water them before to get a decent reading - no joke)
process of elimination.

electric-ian · Feb 11, 2010

Hi Richiek,

Have you tested the loop at the incoming supply?

If there is no reading there, it's most likely a break in the earthing conductor or maybe the stake as you say.
Wants sorting urgently by the sound of it.

You could do a continuity check from the earth terminal in the cu to the stake to see if its the cable or the stake.

Ian

richiek · Feb 11, 2010

Hi guys, thanks for your input and advice.

I haven't got the equipment to do a spike test and watering the soil is out of the question considering it's covered in concrete lol.

I didn't do a loop test at the incoming supply but will do it when I go back there next week. I take it that will take the RCD out of the equation or will I need to remove the main earth from the board and clip it to the earth lead on my tester? I will also take a long lead with me and test continuity from the board to the spike to eliminate a broken CPC.

Des 56 · Feb 11, 2010

The neutral at the star point is a few volts above true earth

On a Tnc-s the earth connected directly to the incoming neutral will show no voltage,however, on a TT where the earth is connected to true earth,a few volts between neutral and earth can be expected

richiek · Feb 11, 2010

That's very interesting Dez. How does that come about if there is no direct link between the main earth and incoming neutral fom the pole or is the neutral earthed before it enters the property?

Also, my earth loop reading still shouldn't read OL but then again that could be caused by the RCD tripping before it has chance to complete the reading on a TT system.

electric-ian · Feb 11, 2010

richiek said:
I carried out an earth loop test which read OL before eventually tripping the RCD

From what you said, I assume that you got a reading - even if it was OL.

If you've just tested from a socket then maybe the cpc to the socket is faulty not the earthing conductor??

Sounds like you need a new tester. How can you do the required loop tests without bridging out the RCD? not really right, especially on a TT.
I suppose you could tie the occupants to a chair ( a plastic one of course ) while you do your testing....

Good luck with the fault finding..

Let us know how you get on.

gavinthom · Feb 12, 2010

and anyway why not try using a clamp metre

richiek said:
Hi all, just heard about this forum and decided to give it a go.

I've only been a sparky for just over 2 years and last week a mate of mine who's a general handyman fitted a 12v door bell for a lady which is mains fed to a transformer. He wired it into a JB on the lighting circuit in the loft but called me as he noticed when he was touching the earth terminal with one of those harry tate screwdrivers that light up the light inside was flickering. I went round and put my fluke voltage tester across neutral and earth and sure enough there was a slight voltage there instead of the usual continuity. It was only very slight - prob less than 1v - but shouldn't be there nonetheless.

I carried out an earth loop test which read OL before eventually tripping the RCD as my Robin multi-tester doesn't have d-lock. I noticed she had a TT system and eventually located her earth rods concreted into her basement. They looked quite old and could be coroded so I guess this would be the cause of the earthing problems. Her entire consumer unit is RCD protected and I tested this and it trips in good time (19-29ms). Her next door neighbours have the same system which is also quite old but they don't have an RCD protected board, instead they have an old VOELCB which to be honest is a waste of space. They also have this problem of minute current in the earth but when I tested their earth loop I got a reading of 13.8 ohms which aint bad on a TT in my experience.

What I'm wondering is, would it be worth going through the hassle of replacing this lady's earth rods as she's well protected by her RCD and her existing ones are concreted in? They may even be connected to an earthing plate under the concrete. Would replacing them be guaranteed to cure this problem?

Any thoughts any of you more experienced guys?

try cleaning up the connections and that might help try using a clamp meter that might dom the job