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

 

[wildgoose]

If you selected a 3 wire configuration for measuring, then it uses a smaller current and is less likely to show-up a problem on a poor joint.

Use the 2 wire measurement as it's more likely to identify the problem!

 

[Stroppy]

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

You think that's insane thinking? I was going to suggested smearing the clamp with cold custard (with nutmeg, obviously) and getting on your knees and praying to as many gods as you can think of.

 

[oldtimer]

agreed...its cheap n slovenly..
theres no reason why a well designed system with a stable, reliable means of earthing cannot meet disconnection times without the magic `RCDs`...it bloody well should be doing anyway...

Thing is I worry as depending on an RCD yet I think we have all had to return 1 or 2 faulty units

 

[NickD]

You think that's insane thinking? I was going to suggested smearing the clamp with cold custard (with nutmeg, obviously) and getting on your knees and praying to as many gods as you can think of.

Cold custard? Don't forget to adjust tabulated max Zs for BS custard by 0.8

 

[Engineer54]

Thing is I worry as depending on an RCD yet I think we have all had to return 1 or 2 faulty units

I think most here would shudder at the amount of RCDs (of all types) that were returned to manufacturer for replacement on my last and previous projects!! I'm expecting very little change on this project either. And, were talking of a top of the range manufacturer here too!! It's certainly opened my eyes on the reliability of RCD devices!! lol!!

 

[NickD]

What kind of faults?

 

[oldtimer]

What kind of faults?

Emm like not tripping when test button being pushed or failling x1 or x5 trip

 

[Stroppy]

Cold custard? Don't forget to adjust tabulated max Zs for BS custard by 0.8

Good point, I forgot that the tables for Birds Standard custard are tabulated for 20[SUP]o[/SUP]C.

 

[baldsparkies]

1. Check your readings and test equipment are accurate. (You dont want egg on your face)
2. If its higher than 0.8 inform the DNO they have a possible earthing issue.
3. Rcd's are no substitute for a solid low resistance earth return path (End of)

And remember your 1.7 reading is a no load test. It indicates possible failure of the earthing system for reasons not defined. Under fault conditions the current flow could result in all kinds of nasties. Sure 30ma with an rcd on the end but what if ???

 

[driverman]

Inform the DNO......suggest they remove old earth clamp and fit a new HEP clamp and whilst there make it a TN-C-S (if available) system @ their cost.

 

[Edd]

Call the DNO, wait 45 mins on the phone. DNO will turn up and give you the option of digging up the street or to go PME..
When you choose the PME option (and you will) they will leave and say it needs to be booked in by the meter supplyer as they dont do it themselfs.

make sure gas and water bond is up to size.
Good luck

 

[dean.diy]

I liked that. Is there a Linguaphone tape available too. (NickD post #34)

 

[Engineer54]

Inform the DNO......suggest they remove old earth clamp and fit a new HEP clamp and whilst there make it a TN-C-S (if available) system @ their cost.

Why would they remove the existing clamp, renew with a Hepworth clamp, and then PME the system??

 

[Jay Sparks]

I think most here would shudder at the amount of RCDs (of all types) that were returned to manufacturer for replacement on my last and previous projects!! I'm expecting very little change on this project either. And, were talking of a top of the range manufacturer here too!! It's certainly opened my eyes on the reliability of RCD devices!! lol!!

Youd never guess!!!lol