TN-S high Ze

[AdieB]

Testing at a house today. The supply is a TN-S. Earth taken from lead incomer cable. First test with my Megger 1730 gave me Ze of 3.5 Ohms, way above the 0.8 Ohms. Cleaned up the earth clamp and tightened it up and improved the reading to 1.7 Ohms. Tested from the live in the fuseboard to the lead sheath of the incoming supply with no earths attached and again achieved 1.7 Ohms. As this Ze value is the best that can be achieved what is my next action. Contact the local DNO and ask them to sort it, or Install an earth rod and change to TT system. This has obviously happened before top some body. Anyone help ------PLEASE.
AdieB

 

[Andy78]

Testing at a house today. The supply is a TN-S. Earth taken from lead incomer cable. First test with my Megger 1730 gave me Ze of 3.5 Ohms, way above the 0.8 Ohms. Cleaned up the earth clamp and tightened it up and improved the reading to 1.7 Ohms. Tested from the live in the fuseboard to the lead sheath of the incoming supply with no earths attached and again achieved 1.7 Ohms. As this Ze value is the best that can be achieved what is my next action. Contact the local DNO and ask them to sort it, or Install an earth rod and change to TT system. This has obviously happened before top some body. Anyone help ------PLEASE.
AdieB

Yes call the DNO. It's their earth and they have to maintain it. Don't start making this into a TT, you only need to consider this when no reliable earth has originally been supplied.

 

[oldtimer]

A house ? Like where is a terraced house in the city or a house in the country?

 

[Ducksy]

What did the DNO say?

EDIT thought you said you contacted them.

 

[MDJ]

Bang an RCD in and you will be fine, in the mean time you could contact the DNO and ask them to improve the reading, 1.7 ohms isn't the end of the world and far better than a pin in the ground giving a 30 ohm reading lol.

 

[telectrix]

OP gives a possible insight. " tightened the clamp" . was this then a DIY TN-S?

 

[AdieB]

The house is in a residential area. The clamp is an old type which goes around the sheath of the cable with a nut and bolt either side it was slightly loose. I also re-sited it on the cable as did not want to keep tightening and then go bang, this achieved a better Ze. The clamp is fit for purpose just old. There is a 17th edition fuseboard with 100Amp main switch and then two separate RCD's controlling 6 and 4 ways respectively.

 

[MDJ]

The house is in a residential area. The clamp is an old type which goes around the sheath of the cable with a nut and bolt either side it was slightly loose. I also re-sited it on the cable as did not want to keep tightening and then go bang, this achieved a better Ze. The clamp is fit for purpose just old. There is a 17th edition fuseboard with 100Amp main switch and then two separate RCD's controlling 6 and 4 ways respectively.

Well what do you think then?

 

[AdieB]

I think the DNO should come out and improve the reading. But as I said when I tested from the live in the fuseboard and the sheath of the incoming cable the best I could achieve was 1.7 Ohms so I cannot see how they could give me the required 0.8 Ohms. This is why I have asked the question to see if someone has had this problem before and solved it or advise as to the best way forward.

 

[Guest55]

i'd probably tighten clamp bolt and retest.
but i dont expect many to agree with my insane thinking.
;-)

 

[MDJ]

Rcds, thats will put your mind at rest.

 

[TaffyDuck]

I think the DNO should come out and improve the reading. But as I said when I tested from the live in the fuseboard and the sheath of the incoming cable the best I could achieve was 1.7 Ohms so I cannot see how they could give me the required 0.8 Ohms. This is why I have asked the question to see if someone has had this problem before and solved it or advise as to the best way forward.

But what you cant do is to check the connection back at the sub station can you. Call the DNO, they have a responsibilty to ensure the Ze is within regs.

And yes I have had this on a number of occasions and Western Power sorted it out

 

[GLENNSPARK]

Yes call the DNO. It's their earth and they have to maintain it. Don't start making this into a TT, you only need to consider this when no reliable earth has originally been supplied.

yes...rodding it will give no real advantage here.....it would be unlikely that TT would give a similar value....and as you say...its distributer earth...so they can sort it....

 

[Silly Sausage]

I hope the OP Zeroed his Instrument correctly!

 

[GLENNSPARK]

I hope the OP Zeroed his Instrument correctly!

thought this was more about the fluke though Archy...
i know you can null out for loop with those....

 

[Buckfast]

As TaffyDuck has already said fault could be anywhere between mains intake position and sub station. Fault could easily rise way above existing 1.7 ohms. Get DNO out to it. Generally find they respond very quickly to earthing faults on distribution system. Generally couple of hours max.

 

[GLENNSPARK]

As TaffyDuck has already said fault could be anywhere between mains intake position and sub station. Fault could easily rise way above existing 1.7 ohms. Get DNO out to it. Generally find they respond very quickly to earthing faults on distribution system. Generally couple of hours max.

but he needs to get put straight through to the duty engineer....
not some dopy, obnoxious bint on the end of the phone....

 

[Buckfast]

Can't disagree with that. arty3:

but he needs to get put straight through to the duty engineer....
not some dopy, obnoxious bint on the end of the phone....

 

[NickD]

Bang an RCD in and you will be fine, in the mean time you could contact the DNO and ask them to improve the reading, 1.7 ohms isn't the end of the world and far better than a pin in the ground giving a 30 ohm reading lol.

So if you were to put RCDs in to meet disconnection times on a TN system that doesn't meet maximum Zs, what I wonder is the position then with certificating the work, particularly in terms of the should-virtually-always-say-None "Deviations from BS7671" box? I know it'd be kosher on a TT system as you have little choice, but on a TN system where Zs is above max...

 

[oldtimer]

I have had this situation before and the DNO asks do you have RCD protection if yes manyana manyana if not we will be in tomorrow