Problematic Ze........

[GBDamo]

I'm at a bit of a loss as to how to proceed.

Had a Ze measured at 0.55 Ohms on a TNCS, contacted DNO who gave a asserted value of 0.52 Ohms. DNO came out and confirmed the result then checked over the earth path to the transformer, worked some magic and came back and measured at 0.44 ohms, they also de-rated the main fuse to 80 Amps from 100 Amps.

All good then? well no, tested today and it's back to 0.55 Ohms. So really wondering how to proceed with EIC.

1, record the result the DNO tested - 0.44 Ohms
2, record my measured result but make a note on the EIC that the DNO is aware of the high Ze.

This is not a test instrument issue as my, another independent spark and the DNO test instrument all tested to within 0.01 Ohms of each other.

This is my second visit, 300 mile round trip, to try to resolve an issue with the DNOs equipment, where do my responsibilities with this end?

The DNO have been contacted again, not sure of what they'll do but if they decide to de-rate the main fuse again we're going to have some serious load issues.

Any thoughts??

 

[Pete999]

I'm at a bit of a loss as to how to proceed.

Had a Ze measured at 0.55 Ohms on a TNCS, contacted DNO who gave a asserted value of 0.52 Ohms. DNO came out and confirmed the result then checked over the earth path to the transformer, worked some magic and came back and measured at 0.44 ohms, they also de-rated the main fuse to 80 Amps from 100 Amps.

All good then? well no, tested today and it's back to 0.55 Ohms. regards to the So really wondering how to proceed with EIC.

1, record the result the DNO tested - 0.44 Ohms
2, record my measured result but make a note on the EIC that the DNO is aware of the high Ze.

This is not a test instrument issue as my, another independent spark and the DNO test instrument all tested to within 0.01 Ohms of each other.

This is my second visit, 300 mile round trip, to try to resolve an issue with the DNOs equipment, where do my responsibilities with this end?

The DNO have been contacted again, not sure of what they'll do but if they decide to de-rate the main fuse again we're going to have some serious load issues.

Any thoughts??

Do you mean this was a visit for an EICR ?

 

[Pete999]

Do you mean this was a visit for an EICR ?

 

[GBDamo]

No, still trying to to issue an Electrical Installation Certificate. Ripped out old heating system, installed new. When tested found high Ze, informed DNO who have been out and resolved, gone back to re-do test only to find it is still high.

 

[Ian1981]

DNO normally stipulate 0.35ohms max for a ze
reading for TN-CS unless they stipulate higher which can be the case for rural areas.
You sure it's not TN-S?

 

[Pete999]

No, still trying to to issue an Electrical Installation Certificate. Ripped out old heating system, installed new. When tested found high Ze, informed DNO who have been out and resolved, gone back to re-do test only to find it is still high.

Would have thought Ze should have been sorted at the onset of the works.

 

[GBDamo]

Would have thought Ze should have been sorted at the onset of the works.

Not possible with a two day job but it is something were looking to find a way to work with.
Just thinking out loud, but maybe paying a local spark to do Ze and sort with DNO if required.

 

[westward10]

0.35 is a typical value for a 100A fuse at the head. I suspect the reason they reduced the fuse to 80A is to allow for a higher Ze of 0.50 but I can't recall the exact figure.

 

[GBDamo]

DNO normally stipulate 0.35ohms max for a ze
reading for TN-CS unless they stipulate higher which can be the case for rural areas.
You sure it's not TN-S?

No, the Ze is ultimately what the DNO tell you it is. Not less than the 0.35 TNCS, 0.8 TNS, but they will, on request, give the exact figure for the area you are and this may well be higher than the above numbers.

 

[westward10]

What was the line/neutral reading.

 

[Andy-1960]

How close to the transformer were you when carrying out the measurement?

 

[GBDamo]

What was the line/neutral reading.

What's you're thinking? Not sure of the relevance. To be perfectly honest if it's not required on the Cert then it's not something I've been trained to do or do do.

The system has been confirmed as TNCS by the DNO, as stated above.

 

[westward10]

What's you're thinking? Not sure of the relevance. To be perfectly honest if it's not required on the Cert then it's not something I've been trained to do or do do.

The system has been confirmed as TNCS by the DNO, as stated above.

Just a comparison I do when I get odd readings because if it is TN-C-S as they say you should get a similar reading, if not then something is amiss.

 

[GBDamo]

How close to the transformer were you when carrying out the measurement?

About 200-250 meters, this is the last system on the line, with two spikes between the cut-out and transformer, all this checked by the DNO...allegedly.

 

[GBDamo]

Just a comparison I do when I get odd readings because if it is TN-C-S as they say you should get a similar reading, if not then something is amiss.

I thought as much but as stated the DNO has confirmed it's TNCS.

Not sure my MFT is clever enough to do this, unless you attach the leads to the L - N -E as stipulated it comes up with an error and will not carry out the test. But I see the logic in what your suggesting.

 

[westward10]

Just connect both leads to neutral and the other to line.

 

[Ian1981]

Yeah I had a rural installation where the ze stipulated on tncs from DNO was 0.5 ohms.
Thankful it wasn't problematic as my zs values where still within tolerances.
It was all rcd protected via 30mA anyway.

 

[7029 dave]

IMO, it is their issue, call them up again. You can not change the Ze measurement. Good luck.

 

[GBDamo]

Just connect both leads to neutral and the other to line.

Will try, just PM me your address to mail the charred remains to........

 

[westward10]

Will try, just PM me your address to mail the charred remains to........

Never going to happen, black and green to neutral and red to line I do it all the time.