ze 12 ohms

[JohnW2]

In the case of a fully open circuit earth sheath (rotted through) where the earth current (test current for Ze) would flow across the rotted gap and via parallel paths, would the PME earthing (along the rout) not lower (slightly or greatly) the earth return resistance?

Oh - are you talking of the path from rotten end of the cable sheath (assuming it's in contact with earth) via the earth ('soil') and PME electrodes and hence back to neutral? If so, I guess that would lower the Ze a little - although I would have imagined that the weak link (high impedance) in that chain would probably be the connection from the rotten sheath to the 'earth' (soil).

Kind Regards, John.

 

[sparkticus]

Oh - are you talking of the path from rotten end of the cable sheath (assuming it's in contact with earth) via the earth ('soil') and PME electrodes and hence back to neutral? If so, I guess that would lower the Ze a little - although I would have imagined that the weak link (high impedance) in that chain would probably be the connection from the rotten sheath to the 'earth' (soil).

Yes exactly, in addition to the current flowing across the open (rotted) sheath (via the surrounding earth) there could well be significant parallel paths from the house end of the open sheath to the nearest PME link forming a path to neutral.

 

[sparkticus]

In the case of a fully open circuit earth sheath (rotted through)

Don't forget that the lead sheath is usually the earth, not the steel tape armouring (though the two are bonded together). It is not that common as obviously if the lead is damaged dampness can get into the paper insulation.

Actually I was not sure of that and thanks for clarifying. It does make me wonder why we get complete open circuit sheaths sometimes though some of them are VERY old I guess some have been mechanically disturbed or actually hit from time to time.

 

[westie101]

If it is underground it would be a short lived situation before the cable faulted through water ingress.
We do see it in some very old joint designs and occasionally after building work has disturbed a cable near the cut-out (DIY service alterations?)

 

[westie101]

But going back to the original post, contact the DNO to attend and investigate!

 

[sparkticus]

But going back to the original post, contact the DNO to attend and investigate!

Ah yes, reality check - good point

 

[ban-all-sheds]

If a property has an existing earth we are obliged to maintain it.

So how does that work then? (And with reference to sparkticus' tale of the house in Bristol.)

If you've been providing a TN-S earth, and that goes bad, and you can't convert it to TN-C-S because of lack of PME, how does saying "it's now TT" meet the obligation to maintain an existing earth provision?

 

[ebee]

That has been my understanding too.

The supplier was not always obliged to provide an earth but where they did it's service must be maintained.

 

[westie101]

how does saying "it's now TT" meet the obligation to maintain an existing earth provision?

Simple, it doesn't!

 

[sparkticus]

We have a paper trail on that particular event. I'll dig it out this evening and see what the details were. I may be wrong but I don't think the DNO ever repaired the TN-S or installed/funded the TT conversion but memory may not be serving me well so will dig out the paperwork.

 

[sparkticus]

Now I have more details. Slightly different than we had "remembered"

-Property was TN-S
-2006: Could not obtain Ze within 0.8ohms (actual Ze 41ohms)
-2006: DNO investigated and they converted installation to TT
-2009: Property became "vailable for PME" < DNO words
-2009: Property converted to TN-C-S/PME free of charge after DNO reminded that it was originally TN-S

We do not know when the TN-S failed since high Ze was only detected prior to rewire.

 

[sparkticus]

One other thing. In 2008 at the end of the street, an old factory/commercial unit was knocked down and new blocks of private flats where built.

Could they (the DNO) have taken this opportunity of "rewiring" part of the street supply?

 

[swift2]

PrenticeBoyofDerry - 1553 set on Loop high, 2 wire test ( red and green probes used) disconnected conductor from terminal clip around cable pressed to make sure tight, red probe in 25mm incoming tail terminal of 100 amp main breaker in db. After that reading tool the clip of the green test lead and then put it on the neutral/earth block on pme.

Flameport - I cant gurantee it is definately tncs it looks like one and there is no other means of earth apart from equipotional bond on gas not even the water is bonded. The block is there between the neutral and earth at the cut out, connection was tight.

westie101 - I think that there is a very good possibility it is that or on DNO side the neutral may not be tightened.

EFLImpudence - What do you think it could be ?

 

[westie101]

westie101 - I think that there is a very good possibility it is that or on DNO side the neutral may not be tightened.

Quite possible, have you reported the high reading to them?

Just a thought, is it overhead or underground fed and what sort of supply cable (PVC concentric/Lead sheath/pvc singles)?

 

[EFLImpudence]

I was thinking what you have just said - loose or high-resistance Neutral.

Have you done PSCC test? If reading is the same as Ze then definitely Neutral fault. Although if, as I asked, it is one-piece N/E block then you have.