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??

 

[Dave OCD]

Yes but what of the thousands and thousands of TT systems with much higher Ze , they all have the same problem but much worse than your sub 1 ohm reading.

 

[Ian1981]

Yes but what of the thousands and thousands of TT systems with much higher Ze , they all have the same problem but much worse than your sub 1 ohm reading.

By that reasoning what's the point of the DNO stipulating max ze figures on tn-c-s and tn-s systems.
They are supplying an earth for the system and their figures should be upheld In my opinion

 

[Dave OCD]

I'm not being argumentative Ian, just wondering if your perceived issue is really worth worrying about with the Ze that you have on this particular job. I just think a note stating DNO informed on your EIC would cover you fine.

 

[Andy-1960]

The only way around the problem if the DNO cannot improve the Ze is to fit a main incoming RCD at the point of the incoming supply, that way the DNO fuses will take care of overcurrent and the RCD will take care of earth faults.

 

[Ian1981]

I'm not being argumentative Ian, just wondering if your perceived issue is really worth worrying about with the Ze that you have on this particular job. I just think a note stating DNO informed on your EIC would cover you fine.

Apologies dave I don't mean to seem to be arguing, I do see what your saying though about the tt system scenario.
I've never really thought about it in that light.
Food for thought mate.

 

[Dave OCD]

No worries mate, basically just trying to help put your mind at ease.

 

[westward10]

With an all insulated head a TT system is not relevant, the DNO are not providing an earth so any fault requirement issues are not their problem.
With the OPs issue I am certain the fuse was reduced to 80A to allow an increased Ze.

 

[hightower]

Surely if your Zs readings are still under the limit you'll be alright? All you gotta do is meet disconnection times

 

[Lee42]

Hi
Did you power down the installation and remove the main cpc when taking the ze reading?

 

[Adie moore]

are you using a high current test as opposed to the non trip test as this will offer give lower readings.
edit, I mean the high current will give the lower reading.

 

[richy3333]

With the OPs issue I am certain the fuse was reduced to 80A to allow an increased Ze.

I agree. If the DNO are going to supply an earth they are responsible to maintain it at the correct values.

BTW why is the OP making alterations before assessing the installation is safe to do so? Reg 132.16 specifically makes mention of the supplier?

 

[GBDamo]

I agree. If the DNO are going to supply an earth they are responsible to maintain it at the correct values.

BTW why is the OP making alterations before assessing the installation is safe to do so? Reg 132.16 specifically makes mention of the supplier?

That is a very good question, a very good question.

The honest answer is that electrics was not the companies primary concern. Surveys were more concerned with pipe runs, equipment positioning and structural loads.

Electrics was seen as running a few cables in to make it all work. This is a very difficult mentality to change, someone had to blow the lid off Pandora's box, but changing it is with the NICEIC Approved Contractor status, CHAS and other safety and quality procedures and schemes in place.

I'm still looking at subbing out the electrical side prior to installation, a pre-condition of sale will be to have all the electrics in place and tested but getting reliable contacts nationally is not easy.

This is all part of the journey when getting involved with entrepreneurial start ups, you have to first find the skeletons, get agreement that they need dealing with then find someone with the will to resolve them.