Weird Earth Continuity on Ring Main

[Steve3948]

Hey guys, I have a new install and I have racked my brains trying to decided how this can be, so if anyone has any ideas then please add your input.

New House.
Ring main at fuseboard all disconnected.

Insulation Resistance L-L 0.0Meg Ohms, N-N 0.0Meg Ohms, E-E 0.0Meg Ohms.

Continuity Resistance L-L 0.97Ohms, N-N 0.97Ohms, E-E 0.74Ohms.
As you can see the earth continuity is not what I would expect, and would expect around 1.6Ohms.

Now leaving the ring disconnected at the fuseboard and going to mid point in the ring and disconnecting all legs I find the following,

L-L Clear, N-N Clear, E-E 0.0Meg Ohms ??
How can this be ? with ring broken each end.
L-E on one leg 299Meg Ohms, L-E on other leg 54Meg Ohms.

It seems like the earth is bridged somewhere but it's not as L-L, N-N are fine. Although a different Meg Ohm reading appears on each side at this point.

Another odd thing is it's a TT system and with the L-L,N-N,E-E all disconnected at fuseboard, then megger between each one leg of ring earth and earth bar which only has spike connected I get 0.0Meg Ohms ?

If it was not for the weird earth continuity readings on Live and Neutral all seems ok, Rcd's hold no tripping etc.

It is a new build with walls freshly painted and typical damp from plaster/render, and considering you a have fixed lug on each outlet could this give the earth continuity lower than the L & N.

Has anyone else had this and found why.

Thanks for any help.

 

[RF Lighting]

Is there a central heating boiler on the circuit?

 

[Adam_151]

Parralel paths, probably through central heating boiler as RF suggests, but its not the only thing that can cause parrallel paths....

Immersion heaters, undersink water heaters, gas hobs, metallic containment, metallic structure to which boxes are fixed, etc, etc. Its not a big issue, but you should attempt to try and eliminate it for your initial verification testing though (disconnect CH systems, water heaters etc from spurs)

 

[Steve3948]

Thanks for the quick replies.
This is a clean ring nothing connected new house and empty.
We have around 6 FCU's as its feeding a kitchen and dining room, however as I said it's empty no kitchen installed yet.

I'm thinking parrellel earths but cannot see how, with nothing plugged in.

One thing that I have not checked is the FCU's, some with 2m leads at low level as requested by kitchen fitter for outlet plates, although fuse removed and they are switched off obviousely the earth is connected could even be touching damp floor screed.
There is also a wine cellar sunk in the ground its corrigated steel circular structure with concrete poured round it and comes complete pre-wired and fed from one of our FCU's, I bet this structure is earthed inside with a fly lead or somthing.

Will check this in the next couple of days.

 

[Spark123]

Why are you IR testing L-L, N-N, E-E?

 

[1john]

Why are you IR testing the individual legs of the RING FINAL CIRCUIT.

What area is that circuit covering?

What are the proper IR readings on the circuit?

Is there any chance you know someone competent that could check what you are doing?

 

[Steve3948]

Why are you IR testing the individual legs of the RING FINAL CIRCUIT.

What area is that circuit covering?

What are the proper IR readings on the circuit?

Is there any chance you know someone competent that could check what you are doing?

Well after 37 years in the trade i do know what I am doing, the problem is I don't know what one of my employees is doing and he is away on 2 weeks holiday. he did not connect the circuit because he said it had a strange reading on the earth continuity compared to the other ring circuits.

The circuit is feeding as posted above a kitchen and dining room, and I thought nothing was connected, so started with a straight continuity test end to end, L 0.9, N 0.9, E 0.7. IR >100Mohms, the R1+R2 was not consistent.

The 0.7 is what my electrician was not happy with, so as he is now away I have taken over.

I have found the reason for the lower than expected reading, there is pre-fab wine cellar in the kitchen sunk into the ground, it has a corrugated steel surround with steel bracing, inside it is an electric actuator for the floor door and a light, all fixed to the steel, it comes pre-wired with a tail to connect to a FCU, which my man has done, and removed fuse ready for the cellar company to commission. It was this that was giving our lower than expected earth continuity reading and this is the only item on the circuit. Disconnecting this gave the expected earth continuity reading.

The reason for IR on LL and NN was to see if a nail may have been through either so instead of dead short I would have a reading.

Anyway all good now and my man can come back knowing it's nothing he has done.

 

[aptsys]

The reason for IR on LL and NN was to see if a nail may have been through either so instead of dead short I would have a reading.

That doesn't make sense.

 

[JohnW2]

[The reason for IR on LL and NN was to see if a nail may have been through either so instead of dead short I would have a reading.

I'm not sure that I really understand that, particularly give that you already had the LL and NN continuity figures.

Kind Regards, John.

 

[Steve3948]

[The reason for IR on LL and NN was to see if a nail may have been through either so instead of dead short I would have a reading.

I'm not sure that I really understand that, particularly give that you already had the LL and NN continuity figures.

Kind Regards, John.

Well my thinking here was that maybe 0.9 Ohms was not the correct reading.
Not being aware of how the circuit was wired in terms of length etc.
So I started thinking maybe the earth is correct and its either L or No that is not, I've had a neutral nailed before into the wall that gave continuity but not zero meg ohms.

 

[JohnW2]

Well my thinking here was that maybe 0.9 Ohms was not the correct reading.
Not being aware of how the circuit was wired in terms of length etc.
So I started thinking maybe the earth is correct and its either L or No that is not, I've had a neutral nailed before into the wall that gave continuity but not zero meg ohms.

I'm afraid I still don't really understand, unless you're sugegsting that you thought you meter might be faulty when measuring low resistances (continuity). If the meter was working properly and you got a reading of 0.9Ω with low-voltage continuity testing, you would inevitably get a zero IR reading.

Kind Regards, John.

 

[Steve3948]

Well my thinking here was that maybe 0.9 Ohms was not the correct reading.
Not being aware of how the circuit was wired in terms of length etc.
So I started thinking maybe the earth is correct and its either L or No that is not, I've had a neutral nailed before into the wall that gave continuity but not zero meg ohms.

I'm afraid I still don't really understand, unless you're sugegsting that you thought you meter might be faulty when measuring low resistances (continuity). If the meter was working properly and you got a reading of 0.9Ω with low-voltage continuity testing, you would inevitably get a zero IR reading.

Kind Regards, John.

Well this was my thinking and my Megger tester seems to play up when batteries are getting low, starts giving very odd readings, but no low battery warning. Sticking 250 up it just to confirm, more habit than anything else.

 

[JohnW2]

Well this was my thinking and my Megger tester seems to play up when batteries are getting low, starts giving very odd readings, but no low battery warning. Sticking 250 up it just to confirm, more habit than anything else.

Fair enough - it's obviously up to you do decide what you do, and it certainly didn't do any harm. I suppose anything is possible, but I don't think that it would have occurred to me that doing an IR test would help if (very) low voltage testing was indicating a very low resistance, even if the batteries were on their way out. Indeed, if the batteries were on their last legs I would probably expect that the 'continuity' test results would,if anything, be high and that the IR testing might be iffy, since it relies on enough juice being available to power the inverter (or whatever) that produces the high test voltage.

Kind Regards, John.

 

[1john]

How on earth is an IR test going to tell you anything more than a dead short when you have 0.9 ohms continuity??

 

[EFLImpudence]

If the meter was sensitive enough and had enough digits the IR test would show -

0.0000009 MΩ