Discuss TNS Ze 0.8Ohms in the Electrical Forum area at ElectriciansForums.net

GBDamo

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Did a lot of miles today and was pondering.

This is not a real scenario.

Not necessarily "how often does a Ze make an installation impossible?" but more..

If, the measured Ze is say 0.30 Ohms and you then design your installation to keep your Zs's within BBB max allowances, does the DNO have a responsibility to maintain the measured Ze or can they simply state 0.8 Ohms.

For an example you install a 10KW shower.

At time of installation the Ze is 0.3 Ohms.

Your R1+R2 is 0.1 Ohms, Zs of Approx 0.4 Ohms.

10mm T&E on a B50 Zs limit = 0.69 Ohms.

So, job done, cert issued and notified.

Stay with me....

6 months later there's a change of tenancy and the landlord has an EICR done. I nkow

Now the Ze is 0.6 Ohms and my installed circuit is measured as 0.7 Ohms and is listed as a C2 on the cert.

What happens, will the DNO just state "0.8 Ohms not our problem"
 
TL;DR
Changing Ze, who's responsible?

Vortigern

Advent Win
If the Ze is within stated values as supplied by the DNO I tend to think they will not take any action. Sometimes it is possible to talk with someone and discuss that change and there would be a slim possibility they may check their earth. They are required to maintain the earth as required under ESQCR (24) but whether they would see a legal responsibility to do so in the scenario described I think is remote. And the figures you have shown suggest a change from requirements of .01 ohms so would that be a C2? But anyway the 80% rule applying would mean a significant non compliance I agree. Difficult as to how they would propose to remedy such a C2
 

Leesparkykent

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Wouldn’t be a C2 as it would be RCD protected being a shower so the RCD would provide the earth fault protection. No code necessary.
 

multimick

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why are we as sparks so pedantic about things,in my opinion a discrepancy of anything like 0.05 ohms is irrelevant due to all sorts of things ,different meters ,tightness of croc clips, unclean conductor connections etc etc .the dno in my experience would take no action.
 

GBDamo

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  • #5
I knew i was asking for trouble by making the numbers so tight, schoolboy error.

Its a hypothetical scenario so imagine the change to be significant.
 

davesparks

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I knew i was asking for trouble by making the numbers so tight, schoolboy error.

Its a hypothetical scenario so imagine the change to be significant.
If the change is significant then the DNO will do something about it.
 

GBDamo

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Wouldn’t be a C2 as it would be RCD protected being a shower so the RCD would provide the earth fault protection. No code necessary.
I've seen 1667 Ohms listed as max Zs on EICRs before where RCDs are present but thought it was frowned upon, relying on an RCD for fault protection.
 

Midwest

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Might be wrong, but aren’t those values given ‘typical’ values not maximum, and the DNO may or may not try and reduce them, if it exceeds the typical value.
 
I've seen 1667 Ohms listed as max Zs on EICRs before where RCDs are present but thought it was frowned upon, relying on an RCD for fault protection.
An RCD is a permitted device for earth fault protection on a TN system, I dont have the BBB to hand for the reg number but it is (obviously) in the section on fault protection for TN systems! and clearly states the maximum Zs in the table for RCD's applies (41.5 off the top of my head ??). A lot of electricians simply dont accept it, but technically, where a 30ma RCD protects a circuit the maximum permitted Zs is 1667 ohms regardless of the earthing system.
 
An RCD is a permitted device for earth fault protection on a TN system, I dont have the BBB to hand for the reg number but it is (obviously) in the section on fault protection for TN systems! and clearly states the maximum Zs in the table for RCD's applies (41.5 off the top of my head ??). A lot of electricians simply dont accept it, but technically, where a 30ma RCD protects a circuit the maximum permitted Zs is 1667 ohms regardless of the earthing system.
411.4.5, 411.4.204, and table 41.5
 
It is not 1667 ohms under the BBB, it is 7667 ohms. 230/0.03=7667
Half agree?
411.4.204 says "the maximum values of earth fault loop impedance in Table 41.5 MAY be applied", not MUST be.
However, I believe the calculation is (230 X 0.95)/0.03 = 7283.3.
But who cares? Never going to get close to any of these values in a healthy circuit.
 

happyhippydad

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A good question!
I cant imagine the DNO doing anything about a Ze of 0.8ohms on a TNS as its within limits. If it's changed from say 0.3ohms to 0.8ohms which then pushes your Zs over the limit I think that is going to be down to the electrician to sort out. I'm not saying that's fair but it's the only real option.

I also didn't think you could rely on an RCD for fault protection if it is TNS or TNCS. Zs on these systems would almost become irrelevant if we did as they would always be below 1667.
 

Vortigern

Advent Win
Don't forget, when you examine your manual for your MFT there is on most MFT a factor of 6 places for continuity tests. i.e. a figure of 0.09 ohms could in real terms be 0.03 or 0.15. Thus when applying found figures the accuracy is highly suspect.
 

GBDamo

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Half agree?
411.4.204 says "the maximum values of earth fault loop impedance in Table 41.5 MAY be applied", not MUST be.
However, I believe the calculation is (230 X 0.95)/0.03 = 7283.3.
But who cares? Never going to get close to any of these values in a healthy circuit.
It would be nice if they produced a book that gives clear guidance and limits, maybe review it and ammend it every year or two, it would be costly mind but it would remove the ambiguity.
 
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