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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