Ze=1.87 TT or TN-S?

Bizron · May 26, 2020

diditrain said:
It looks like you have 3 conductors on your supply cable, but who knows what is going on inside the cutout. You could try contacting your DNO to see if they have a record of the supply type to the property. ESQCR regulations require them to maintain the earth, if they are providing you with one.

Thanks for that
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telectrix said:
agree with all of above. looks like the cable coming up to the outside box is lead sheathed , the sheath being the TN-S earth. maybe a bad connection at that box or at cut-out.

can you see if you get similar readings at neighbours fed from same outside box?

pc1966 said:
It certainly does not look right! Can you confirm that your 2 lead / 3 lead tests were both done at the same location? If you repeat the test do you get the same results (i.e. did a high current check alter the low current reading)?

While going from high to low current can lead to a loss of accuracy at low-Z it is not going to explain more than a fraction of an ohm, same for "RCD uplift" which is usually 0.5 ohm or so and probably not an issue with newer MFT equipment anyway.

My suspicion is a bad connection on the earth line at CU/MET/DNO but you need to rule out the customer-side of the system (CU & MET) as a problem.

yes all at same location main earth disconnected, terminations sound. Test done 3 times. My company does not allow any tampering with DNO kit so we’ll have to pass it on. Cheers
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Strima said:
99.99% that's TN, as has been suggested I would request a couple of quick tests done on neighbours properties to narrow down the problem and inform the DNO.

Take a look at page 24 of the attached, you could get the DNO out prety much straight away to sort this out.

MOCOPA-guide-version-3.5.pdf

Thank you for that I’ll have a read of that later. I assumed it was TN-S as there are 3 cables and looks like the armour is the making the 3rd cable but the reading made me question it.