Hello people I was wondering if you could help me understand how a tncs ze value be 12 ohms. All breakers off main conductor disconnected and two lead test with megger 1553 between line and earth tester set on high as no trip not needed. Reading is 12 ohms,so to check this again I left earth disconnected at board and took reading with earth probe on the neutral/earth block. The weird thing is all zs readings complied with regs, there is no earth on the water so surely the readings cant just be complying due to the gas equiptional bonding? Any thoughts would be appreciated. Thank u in advance
ze 12 ohms
- Thread starter swift2
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Are you sure you tested the main earthing conductor and the live side of supply?
Run us by exactly what you did, when you performed the test.
Run us by exactly what you did, when you performed the test.
Those results obviously don't make sense - neither the Ze value (for a TN-C-S system), nor having Ze (very much) greater than Zs.
I'm not familiar with the Megger instrument, but are you sure that you can do two-lead loop impedance tests with it? With Fluke's one has to connect the neutral as well.
How did you measure Zs? If you did that with a plug adapter (hence 3-lead), that might explain why your Zs figures were correct but your ('two lead') attempt at a Ze measurement was not.
Kind Regards, John.
Could easily be if the gas service is metal, as the adjoining properties should have main bonding in place, so you could well be measuring Ze via their main earth connections.
Is the supply actually TNCS - or is it where someone has tried to convert it themselves? Is there any continuity between the incoming neutral and the earth connection?
12 ohms sounds more like a TNS where the underground connection to the cable sheath has rusted away.
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Sounds as if the N-E link is not in place (not uncommon)
JohnW2, we only use 2 lead instruments as we have to test at the cut-out or on the system on cable cores etc.
JohnW2, we only use 2 lead instruments as we have to test at the cut-out or on the system on cable cores etc.
That makes sense, in your line of work. However, as I said, that's not true of all MFTs.
(I've just looked at the Megger MFT1553 manual - and it does, indeed, do 2-lead tests - so much for that theory!)
Kind Regards, John.
Indeed. As I've reported before, my incoming water pipe has a 'Ze' of about 0.25Ω, presumably thanks to a neighbour's TN system - which makes the difference between Zs (with bonding connected) and Ze (without bonding) on my (TT) system far more dramatic than even the OP's!
Kind Regards, John
As per Westie, I have seen this a few times and always report it. You get reasonable Zs readings because of all the bonding is in place but when you check for Ze on what is supposed to be (or what you think is) a TN-C-S you get relatively high impedance which I have always assumed to be some parallel earth paths due to the neutral-earth-spike links further down the line. The DNO come out fit the link in the cut-out as a matter of emergency assuming the supply is listed as TN-C-S.
If it is, as I read it, a one-piece block it could be a more serious problem.
I think I must be missing something here. If the N-E link in the cutout is absent then, once one disconnects the earthing conductor from the installation (i.e. MET), it's just a floating bit of wire, so would one then not expect to get a measured (L-'E') Ze of 'infinity', not just a few ohms?
Kind Regards, John.
I forgot to write this and it's probably a question for Westie and the other DNO guys (or girls)
I have seen several TT installations with what looks like a TN-S or TN-C-S cut-out box. But usually nothing connected to the MET block that sticks out of the top. When I check the resistance between the MET and Neutral I most often see no connection at all (infinity) even on the insulation setting at 500V. Also nothing at all (as expected) if I try to get a Ze reading between phase and the MET so I assume this is just a modern cut-out fitted to a TT installation with a redundant MET?
But just once in a while I see anywhere from 10 to 100 ohms Ze on these un-used blocks and I have always assumed that it is the remnants of a partially rotted away old earth sheath. The interesting this is that on a recent one (about a year ago) the earth rod was quite new making me wonder if an old TN-S had been converted to TT because the outer sheath of the cable had rotten away? If that is true then of course whoever "fixed" the problem problem by installing the rod should really have called the DNO and reported that their earth was AWOL!!!
I have seen several TT installations with what looks like a TN-S or TN-C-S cut-out box. But usually nothing connected to the MET block that sticks out of the top. When I check the resistance between the MET and Neutral I most often see no connection at all (infinity) even on the insulation setting at 500V. Also nothing at all (as expected) if I try to get a Ze reading between phase and the MET so I assume this is just a modern cut-out fitted to a TT installation with a redundant MET?
But just once in a while I see anywhere from 10 to 100 ohms Ze on these un-used blocks and I have always assumed that it is the remnants of a partially rotted away old earth sheath. The interesting this is that on a recent one (about a year ago) the earth rod was quite new making me wonder if an old TN-S had been converted to TT because the outer sheath of the cable had rotten away? If that is true then of course whoever "fixed" the problem problem by installing the rod should really have called the DNO and reported that their earth was AWOL!!!
Yes but if it is not really TN-C-S but a rotted TN-S ?
Sure - but that's totally moving the goalposts in comparison with what you wrote (above)
If (having moved th goalposts!) it was 'partially rotted TN-S', then I agree that one could get a high, but measurable, Ze. However, if it were completely rotted, that would presumably be the same as TN-C-S without the local N-E link (i.e. an 'infinite' Ze)?
Kind Regards, John.
Sure - but that's totally moving the goalposts in comparison with what you wrote (above)
If (having moved th goalposts!) it was 'partially rotted TN-S', then I agree that one could get a high, but measurable, Ze. However, if it were completely rotted, that would presumably be the same as TN-C-S without the local N-E link (i.e. an 'infinite' Ze)?
You can still get a Ze across the broken (rotted) ends of the sheath via the small mass of earth but of course it becomes a high Ze.
I can sometimes get a Ze reading off a damp wall
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