Ze Reading.

[hutch6447]

Working on a house that as an overhead supply TT but I think it as been converted to TNCS, done some testing and the Ze L-N is fine 0.23 but the L-PE is reading 0.49 now I know this is high for TNCS (0.35).

Sure it has been converted because its an overhead supply with Earthing Electrode but the main earth connection is 16mm and joins at the cut out, plus there is a sticker warning you of PME proctective multiple earthing so never really come across this before wondering if anyone has experienced this? Thanks Matt.

 

[telectrix]

poor earthing by DNO. get them out . they have an obligation to maintain the Zeat or below 0.35 ohms on a PME system.

 

[GLENNSPARK]

and would it not be PNB...not PME?...

 

[brman]

Are you saying it has both a earthing conductor to the cutout and one to an earth spike? Is that Ze measurement to the cutout earth only? (ie disconnected from bonding and other earth)

Assuming so I'd be tempted to call the DNO and say their PME needs sorting. If they suggest converting back to TT then you already have an earth spike to use :lol:

I should add this isn't something I have experienced, just what I would do if faced with it so others might have better ideas!

EDIT: Oops, Tel types faster than me....

 

[telectrix]

well, if it says PME on the tin, it ought to be PME in the tin.

 

[hutch6447]

Yeah did the Ze test with main earth conductor disconnected and it gives me a reading of 0.49 so it is higher than it should be.

 

[hutch6447]

No definitely PME. I am at a loss think I am going to sleep on it.

 

[GLENNSPARK]

we had something a while back....overhead cable with an earth link off the neutral...obviously to replace a TT setup....except there were 2 earth links....one went to the earth bar in the C/U....the other just went down the outside wall of the house and was cropped about 8" from the ground so if there had been a fault at star....could have been a voltage sitting on that .....rang the DNO about this one.....they have since been and sorted it...but really....

 

[Moses]

How far is the supply head from the CU and what are the tail sizes?

 

[sedgy34]

might be worth a call to the DNO

 

[hutch6447]

How far is the supply head from the CU and what are the tail sizes?

Yep 25mm meter tails, CU is 3 meters away from main fuse.

 

[hutch6447]

All is well DNO as put problem right Bravo.

 

[hutch6447]

DNO came out sent an engineer he did tests on the main incoming supply and it came out at 0.21ohms now because earth is linked to the neutral, when I disconnected the main earth in consumer unit I did the test with his machine and it came out as 0.23 ohms so it was taking in account the run to consumer unit. When I did the tests with my 1720 I put the red test lead on line in main switch and clipped the green lead on the disconnected main earth but had it in the neutral on tester and the reading came out as 0.23 ohms now when I put the green lead in the PE on tester the reading again came out high, the engineer said this is correct because the earth is running through the neutral.

This must be correct as I rang up to confirm and there Ze was 0.21ohms so I ended up just doing calculated readings (R1+R2) + Ze = Zs.

I think you have to switch a few leads round on your tester if you have a multi function going to ring Megger up anyway.

Spoke to loads of people that are getting high Ze readings on TNCS systems but I was getting one but the engineer did test in front of me and he tested earth through the neutral which makes sense considering they are linked and not separate.

Think people just need to call DNO out and have them do test infront of them especially if it is a converted TT to TNCS.

 

[Engineer54]

Not sure from your description of testing, but on a PME system testing between L-N and L-E will be almost identical or as near as dam it!! lol!!

 

[hutch6447]

Yeah I know but this engineer that came out from DNO tested on the live and neutral where the earth was connected to because system is TNCS and his reading came out at 0.21ohms did the same test at board with main earth disconnected and it came out at 0.22ohms.

Tested on mine(1720) and if testing between L-N the reading came out at 0.22ohms, that main earth connection will be running straight into neutral but if tested on L-PE the reading came out at 0.49ohms. Used my tester on TNS and all reading are fine it was fully calibrated by megger.

Just trying to get some input of people that have used the megger 1720 in this position because rang up DNO and they confirmed the Ze to supply of 0.21 and the Engineer that came out confirmed it also so just wondering where this reading of 0.49ohms comes from testing on L-PE on 1720.

With earth and neutral combined wondering if it has something to do with the positioning of leads on your tester(1720).

Thanks any info welcome.