High ze readings. Even higher zs?

[Bsharp]

testing a TNS installation today, found a high incoming ze of 1.14ohms, in my head putbthis down to incoming earth issue as all terminations tight, but when I test any subsequent circuit I’m getting much greater zs readings approx 4.0 ohms. I was expecting high earth loops as my incomer was high but not this great, it does seem lower when linking out rcds and using two lead test, but using three leads I’m getting these high result?? Has anybody had similar or have any suggestions. The DNO have been informed so I’ll see what happens.
Any advice appreciated.
Cheers.

 

[Bsharp]

Their explanation as is normal with the DNO in my experience is either a damaged joint to the property or a damaged joint further down which could mean entire street etc. With ZE’s which don’t comply but there only course of action is an RCD which is pointless as consumers circuits are all rcd protected anyway (17th edition board) so disconnection times achieved by RCd, but as you say doesn’t resolve the supply earth issue.

 

[telectrix]

DNO have supplied the earth. it's their duty to maintain it. kick up a fuss. don'tbe fobbed off by some tart who doesn't know a TN-S from her arse. demand to speak to the duty engineer.

 

[Bsharp]

Telectrix, that’s the kind of attitude I like and will be doing just that tomorrow morning.

 

[westward10]

Did you loop test between line and neutral.

 

[Bsharp]

No line-neutral test, but I was thinking maybe a neutral fault or neutral-Earth fault because as I said with rcds linked out so I am able to use a two lead test (no connection to neutral) the zs is pretty close to calculated zs+(r1+r2) whereas when using three lead test on final circuits the results are greatly higher I.e
Ze -1.14 (high but damaged Sheath/earth would explain)
R1+r2 0.35 ohms
Calculated zs 1.49 ohms
Zs (two lead test) 1.61 ohms
Zs (three lead test) 4.0 ohms
I appreciate the three lead (trip lock/soft test) would exentuate the fault but seems a vast difference to me.

 

[happysteve]

Just out of interest, what make/model of earth fault loop tester (presumably MFT) are you using? Some makes are better than others at no-trip Zs measurements through RCDs - on some you get "RCD uplift" of an ohm or so. I suspect the higher the Ze - particularly if it's variable - the more likely you'll get a discrepancy. Using the alternative method of measuring Zs, when there is an RCD present, is often the only way to get a reliable reading.

My concern about your high Ze is that it might be indicitive of a developing fault, one that's getting worse. It might be 1.14Ω this week, but if it's symptomatic of a breakdown in the lead sheath of the supply cable, it might well suddenly go much, much higher. You'll then be relying on main protective bonding for your RCD to work (assuming you've got metal pipes/anything else extraneous).

The DNO are within their rights not to supply a means of earthing if they don't want to. But I'm very surprised at their answer which seems to effectively be, "Yes, we're supplying an earth, but it's not a very good one, so bung an RCD on it."

 

[Bsharp]

Megger MDT 1553. A’s you say a calculation of the zs would be preferable now, as they have fitted front end RCD so can no longer by pass it for two lead test. Or possibly to fit an eart spike and convert to TT. But as you say I’m surprised that’s how they’ve left it, I’ve not been back to the job yet so I’m only going on what the home owner tells me.

 

[Toneyz]

Is the RCD time delayed/ type S ?.

 

[Bsharp]

I haven’t seen the RCD they have installed yet but a presume just a standard 100ma

 

[Percyprod]

In North West if we is between 0.8 and 1.6 then main fuse de rated and the faulty joint or whatever repaired. Between 1.6 and 20 a temporary RCD would be fitted and again the fault repaired. Over 20 if no RCD on consumer unit supply would be disconnected. The fault repair would be within 7 days, but usually done within one or two days

 

[Bsharp]

Thanks for the information I’m in North Wales so I’ll see tomorrow how things have been left with homeowner.