High Zs on ring final circuit

[robo83]

Good morning,

I was recently at a job and had a discussion with another Electrician regards a high Zs reading on a ring final circuit.

R1+R2 measured 1.68ohm
Ze 0.18
Tncs

This puts the circuit over the permitted zs. I begun seeking out the issue.

However the other electrican said although this was above the permitted zs it can be noted on the cert as 'zs doesn't not comply but rcd protection on the circuit so1667ohms permitted'

Can somebody enlighten me please. Ive sent this on TT Installs but wouldn't expect this to be allowed on a tncs?



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[stevethesparks]

Sounds like its either a poor design or a fault, and neither should be accepted because rcd protection is in place. RCD's fail from time to time.
What is r1, r2, and rn?

 

[Pete999]

Good morning,

I was recently at a job and had a discussion with another Electrician regards a high Zs reading on a ring final circuit.

R1+R2 measured 1.68ohm
Ze 0.18
Tncs

This puts the circuit over the permitted zs. I begun seeking out the issue.

However the other electrican said although this was above the permitted zs it can be noted on the cert as 'zs doesn't not comply but rcd protection on the circuit so1667ohms permitted'

Can somebody enlighten me please. Ive sent this on TT Installs but wouldn't expect this to be allowed on a tncs?



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What was the Measured Zs and what OCPD are you using?

 

[Octopus]

R1+R2 measured 1.68ohm

Are you 100% sure about this figure? At every socket?

 

[davesparks]

Good morning,

I was recently at a job and had a discussion with another Electrician regards a high Zs reading on a ring final circuit.

R1+R2 measured 1.68ohm
Ze 0.18
Tncs

This puts the circuit over the permitted zs. I begun seeking out the issue.

However the other electrican said although this was above the permitted zs it can be noted on the cert as 'zs doesn't not comply but rcd protection on the circuit so1667ohms permitted'

Can somebody enlighten me please. Ive sent this on TT Installs but wouldn't expect this to be allowed on a tncs?



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Without the Zs reading how on earth do you expect us to be able to offer a helpful comment?


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[robo83]

Hi,

R1 = 0.29
Rn = 0.30
R2= 1.30

Measured zs 1.48

The socket in question was a spur, the rest of the ring was highest reading 0.42 r1+r2

OCPD 32amp was on the circuit.

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[hightower]

So there's surely an issue with your CPC (it should be around 0.5 ohms for 2.5/1.5mm T+E) on that spur. I would suggest it needs resolving before signing off.

 

[robo83]

We checked sockets, re tightened everything.

Changed mcb to a 20amp.

We are back there tomorrow, will re check.



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[hightower]

Actually, the r2 reading is an end to end rfc test, so your high reading is somewhere on the ring, not the spur.

 

[Octopus]

Hi,

R1 = 0.29
Rn = 0.30
R2= 1.30

Measured zs 1.48

The socket in question was a spur, the rest of the ring was highest reading 0.42 r1+r2

OCPD 32amp was on the circuit.

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I am not trying to be difficult but your notes don't make sense. Are you saying the only socket where the Zs is high is at the spur? If so remove the socket and test to the cable.

 

[Pete999]

Hi,

R1 = 0.29
Rn = 0.30
R2= 1.30

Measured zs 1.48

The socket in question was a spur, the rest of the ring was highest reading 0.42 r1+r2

OCPD 32amp was on the circuit.

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32Amp cb what sort b,c??

 

[robo83]

I am not trying to be difficult but your notes don't make sense. Are you saying the only socket where the Zs is high is at the spur? If so remove the socket and test to the cable.

Hi Murdoch,

That's correct and I will be tomorrow. , I noticed the high Zs prior to board change as I did pre tests and had the discussion with another Sparks who said he notes on his certs rcd protection. I said wasnt correct which brings me here.

I put the the circuit onto a 20amp tybe B until tomorrow as I've got to return with test cert.

Everything else was spot on apart from the other sparks who was there. No test cert, no Sleveing on cpc. No rcd on outdoor lighting, also the swa was just run along garden.

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[robo83]

Hi Pete,

Type b mate

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[Octopus]

Hi Murdoch,

That's correct and I will be tomorrow. , I noticed the high Zs prior to board change as I did pre tests and had the discussion with another Sparks who said he notes on his certs rcd protection. I said wasnt correct which brings me here.

I put the the circuit onto a 20amp tybe B until tomorrow as I've got to return with test cert.

Everything else was spot on apart from the other sparks who was there. No test cert, no Sleveing on cpc. No rcd on outdoor lighting, also the swa was just run along garden.

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If sockets are rarely used, the contracts are often poor. Plug your tester in and out a few times to see if the reading varies.

 

[robo83]

Thanks Murdoch,

I mention before I have mainly done commercial so old school houses are become the norm.

One thing for sure, if I'm not sure the yellow book appears. I also post here to seek everyone's opinions.

Many thanks

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[Pete999]

Thanks Murdoch,

I mention before I have mainly done commercial so old school houses are become the norm.

One thing for sure, if I'm not sure the yellow book appears. I also post here to seek everyone's opinions.

Many thanks

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Not sure I understand this reply, is that due to tapatalk?

 

[Octopus]

Thanks Murdoch,

I mention before I have mainly done commercial so old school houses are become the norm.

One thing for sure, if I'm not sure the yellow book appears. I also post here to seek everyone's opinions.

Many thanks

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Did you not test properly doing commercial? Testing is testing irrespective of the type of installation

 

[ian.settle1]

Hi,

R1 = 0.29
Rn = 0.30
R2= 1.30

Measured zs 1.48

The socket in question was a spur, the rest of the ring was highest reading 0.42 r1+r2

OCPD 32amp was on the circuit.

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So the socket in question is a spur of the ring main and not part of the actual ring main?

 

[robo83]

Did you not test properly doing commercial? Testing is testing irrespective of the type of installation

Did mainly metal munching on rail projects. Domestic is another ball game and I'm sure you would respect that although testing is testing it's a bit different when u had Bob the builder playing games compared to a fresh install.

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[robo83]

So the socket in question is a spur of the ring main and not part of the actual ring main?

Yes Ian that's correct.

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[stevethesparks]

But you still have a high r2 on the end to end results as already pointed out by HT.

 

[bensk]

is the spur part of the ring or spurred off? surely something to do with that link if the rest of the ring has low r2

 

[Pete999]

If it was part of the Ring it wouldn't be a spur, would it?

 

[robo83]

It's definitely a spur.

I'm back there today so will sort it.

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[telectrix]

first thing i'd do is investigate the high r2 value. that's end/end of the \RFC and nothing to do with the spur. then i'd do a hi-current Zs bypassing the RCD and MCB to eliminate any internal resistance errors.

 

[robo83]

Thanks Telectrix

If the r1+r2 was fine throught the circuit except the spur. But still getting a high r2 value end to end.

Surly that would mean there is a loose connection on the ring where the spur has been taken?



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[hightower]

Tel, could you explain the process of bypassing the RCD and MCB for the high current test. Would you just link the circuit line to the load side of the main switch temporarily? Anything to keep in mind for this - obviously I would say it's safest to do load free.

 

[telectrix]

r2 is nothing to do with the spur. it could be a poor connection anywhere on the ring. and please differentiate betweem r and R. it's confusing if you mix them up.

 

[tony mc]

Tel, could you explain the process of bypassing the RCD and MCB for the high current test. Would you just link the circuit line to the load side of the main switch temporarily? Anything to keep in mind for this - obviously I would say it's safest to do load free.

Bypassing the RCD/MCB


ISOLATE the circuit in question and double check that it is isolated!!


Link the CPC and Line at one of the socket outlets on the circuit in question


At the CU with your tester set to High ZS Loop place one probe on the outgoing side of the isolated circuit being tested and the other on the Line supply on the main switch.


Note reading on tester and REMOVE LINK BEFORE MAKING CIRCUIT LIVE AGAIN!!

Hope this makes sense!

 

[telectrix]

Tel, could you explain the process of bypassing the RCD and MCB for the high current test. Would you just link the circuit line to the load side of the main switch temporarily? Anything to keep in mind for this - obviously I would say it's safest to do load free.

just been posted .. #29. by tony mc. and richard burns has posted an excellent diagram of the procedure .

 

[hightower]

Ah yep, that makes a lot more sense to do it that way.

 

[DPG]

Did you find the fault(s)? If so what was it. Daz

 

[robo83]

There was a socket behind a fitted set of draws that I wasn't showen. Had to remove draws and back pannel, was a bugger. I re-tightened connections and it sorted it.


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[DPG]

Nice one. Did that sort the high r2 value? What about the spur? Daz

 

[robo83]

The R2 for the ring dropped, also for the spur. I changed the faceplate for the spur and waggo'd the connections behind the draws.

To be honest, it was my own fault. I couldn't find the where the spur was taken from when I tested, I should have known there was another socket but the lady didn't know. Only the husband did when I returned.

Lesson learned.

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[westward10]

If you are measuring end to end for the cpcs that R2 reading is way too high, sounds like continuity is breaking down which may well not be at one of the sockets. How old is the circuit, if it goes back to the early seventies the cables may incorporate 1.0 cpcs. Maybe the R2 is correct and the R1 is incorrect. But from your continuity readings the Zs should be approximately 0.58 ohm for any accessory connected direct to the ring final, the Zs of any spurred outlet will depend on its proximity of connection to the ring final.

 

[Pete999]

The R2 for the ring dropped, also for the spur. I changed the faceplate for the spur and waggo'd the connections behind the draws.

To be honest, it was my own fault. I couldn't find the where the spur was taken from when I tested, I should have known there was another socket but the lady didn't know. Only the husband did when I returned.

Lesson learned.

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Robo, please don't think I'm having a did, certainly not my intention, but " I waggod the connections" what exactly do you mean by that? as you said you had change the faceplate.

 

[richy3333]

Hi Murdoch,

That's correct and I will be tomorrow. , I noticed the high Zs prior to board change as I did pre tests and had the discussion with another Sparks who said he notes on his certs rcd protection. I said wasnt correct which brings me here.

I put the the circuit onto a 20amp tybe B until tomorrow as I've got to return with test cert.

Everything else was spot on apart from the other sparks who was there. No test cert, no Sleveing on cpc. No rcd on outdoor lighting, also the swa was just run along garden.

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not really an issue if it's self burying cable like many use around here

 

[robo83]

Robo, please don't think I'm having a did, certainly not my intention, but " I waggod the connections" what exactly do you mean by that? as you said you had change the faceplate.

Hi Pete,

I removed the double socket that was situated behind the draws and joined the cables and fitted a blanking plate.

I thought best, as the socket was not accessible.

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[Pete999]

Hi Pete,

I removed the double socket that was situated behind the draws and joined the cables and fitted a blanking plate.

I thought best, as the socket was not accessible.

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Right many thanks was a bit confused but not now

 

[robo83]

It's fine Pete,

I appreciate your advice and guidance.

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[Pete999]

It's fine Pete,

I appreciate your advice and guidance.

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No Worries mate