Zs too high ???

EFLImpudence · 20 Mar 2012

Customer had extension wired by 'less than reputable electrician'

Inspected by LABC electrician who merely states "Zs too high on sockets".
There were no values stated on his report, perhaps he had a Megger and used no-trip setting and got even higher value.

The highest one was 0.68Ω.

Whilst this may be higher than expected, though approximately half the maximum allowed, is it reasonable to just state "too high" and offer the customer no advice other than get someone to 'repair'

I discovered why it was that high but could not 'repair'.
It was an extension to the circuit of the half the bungalow.
The original ring (now two thirds of the circuit) is wired in 2.5mm² with 1.0mm² cpc. and long.

Any views?

securespark · 20 Mar 2012

I think they need to justify it.

"Too high" is a bit limp really.

As long as the extension is wired correctly (ie, end to end OK and it is a true RF circuit) and it has RCD protection PEB's etc... then I can't see a problem.

Is it a C type breaker?? Maybe that's why.

1john · 20 Mar 2012

Ask for a regulation number, also the reading he got and the reading in the regs that is permitted, see what comes back. Hes missing out details and you say there are not problems (which I believe) but perhaps this guy has taken a test on a socket which may be concealed somewhere that you cant find and perhaps the homeowner is unaware. It may be something as simple as a socket in the loft for an areal booster and this may even be fed from the lighting circuit which would explain the high reading.

PrenticeBoyofDerry · 20 Mar 2012

If the reading is 0.68Ω and can I assume 32A type B BS60898?
and this is the highest reading on circuit.
It is compliant.
Have you taken r1+r2 and Ze readings?
LABC electrician, needs to be asked some questions?
are sure he is an electrician?

EFLImpudence · 20 Mar 2012

Thanks for replies. That's what I thought.

Yes, B32 60898. All RCD. Ze 0.07Ω. Zs 0.68Ω is the highest

Only three new DSOs in extended lounge.

All r1, rn & r2 end to end match.
(I even separated old and new cables and measured each end to end - all match and add up to total)
It's just that at first r2 'seems' high because of (unexpected) 1mm² cpc but it is visible in CU and I noticed.

R1+R2 + Ze matches measured Zs (near enough - good old Fluke)

Have to go back tomorrow because "IR on lighting circuit 'too low'"

mikhailfaradayski · 20 Mar 2012

LABC sparks really needs to raise his game and start being more specific.
what next 'wiring is too wirey'?

EFLImpudence · 20 Mar 2012

To be honest, it was very wirey.

PrenticeBoyofDerry · 20 Mar 2012

The last LABC inspection I had to endure, the inspector was a plumber and not an electrician!

JohnW2 · 20 Mar 2012

Thinking alound (as asking questions), as an outsider ...

If Zs is low enough to satisfy disconnection times, then that it surely 'it' as far as that consideration is concerned, and I'm almost certain that no-one could find any regulation which said otherwise.

However, if given a known Ze, the Zs of a circuit is 'unexpectedly high', even though low enough for disconnection times, that will usually (unless problem is in CU devices or upstream of that) mean that R1+R2 is also 'unexpectedly high' That presumably raises a concern that there might be a poor connection which needs to be investigated and rectified. After all, just 0.25 of 'unexpected impedance' at a connection would, at, say 20A, mean that 10W was being dissipated in that connection - plenty, I would have thought, to create a potential fire risk. Is that correct?

The question, of course, is whether R1+R2 really is 'unexpectedly high' in relation to the cabling concerned.

Kind Regards, John.

mikhailfaradayski · 20 Mar 2012

JohnW2 said:
Thinking alound (as asking questions), as an outsider ...

If Zs is low enough to satisfy disconnection times, then that it surely 'it' as far as that consideration is concerned, and I'm almost certain that no-one could find any regulation which said otherwise.

What if the Ze was <0.04 ohm?

However, if given a known Ze, the Zs of a circuit is 'unexpectedly high', even though low enough for disconnection times, that will usually (unless problem is in CU devices or upstream of that) mean that R1+R2 is also 'unexpectedly high' That presumably raises a concern that there might be a poor connection which needs to be investigated and rectified. After all, just 0.25 of 'unexpected impedance' at a connection would, at, say 20A, mean that 10W was being dissipated in that connection - plenty, I would have thought, to create a potential fire risk. Is that correct?

The question, of course, is whether R1+R2 really is 'unexpectedly high' in relation to the cabling concerned.

Kind Regards, John.

All valid guesswork, but thats what it is, guesswork. Hence why i suggested that the spark be more specific.

JohnW2 · 20 Mar 2012

mikhailfaradayski said:
JohnW2 said:

If Zs is low enough to satisfy disconnection times, then that it surely 'it' as far as that consideration is concerned, and I'm almost certain that no-one could find any regulation which said otherwise.

Click to expand...

What if the Ze was <0.04 ohm?

I don't understand how that affects what I said - can you explain? I presume that if one appears to have a very low Ze, one needs to think about where the supply is coming from (distance to transformer) and, if necessary, question the accuracy of one's measurement - after all, with a TN supply in 25mm², 0.04Ω would only allow for about 23m to the transformer.

mikhailfaradayski said:
All valid guesswork, but thats what it is, guesswork. Hence why i suggested that the spark be more specific.

I agree totally in the context of this thread - the LABC electrician has clearly given unsatisfactory and inadequate information. However, as I said, I was thinking alound and inviting comments, as an educational exercise (for me).

Kind Regards, John.

mikhailfaradayski · 20 Mar 2012

John, with respect, should you wish to use this forum for your own educational purposes, i see no problem with that, (I believe everyday should be a school day) but can you put your thoughts and postulations into your own threads instead of constantly hijacking other peoples posts and taking them on a tangent which results in the main message of the OP being lost.

I imagine there have been a number of threads where there may have been more valid information still to come, but when people have to sift through lots and lots of excessively verbose, irrelevant (to the OP) musings, they begin to lose the will to live, let alone bother to bring forth any more worthy info.

Please note forum rules 1 and 6.

regards

Mik

mikhailfaradayski · 20 Mar 2012

JohnW2 said:
mikhailfaradayski said:

JohnW2 said:

If Zs is low enough to satisfy disconnection times, then that it surely 'it' as far as that consideration is concerned, and I'm almost certain that no-one could find any regulation which said otherwise.

Click to expand...

What if the Ze was <0.04 ohm?

Click to expand...

I don't understand how that affects what I said - can you explain? I presume that if one appears to have a very low Ze, one needs to think about where the supply is coming from (distance to transformer) and, if necessary, question the accuracy of one's measurement - after all, with a TN supply in 25mm², 0.04Ω would only allow for about 23m to the transformer.

start your own, appropriately titled thread to discuss this and i will happily explain

mikhailfaradayski said:
mikhailfaradayski said:

All valid guesswork, but thats what it is, guesswork. Hence why i suggested that the spark be more specific.

Click to expand...

I agree totally in the context of this thread - the LABC electrician has clearly given unsatisfactory and inadequate information. However, as I said, I was thinking alound and inviting comments, as an educational exercise (for me).

Kind Regards, John.

see last post

JohnW2 · 20 Mar 2012

mikhailfaradayski said:
John, with respect, should you wish to use this forum for your own educational purposes, i see no problem with that, (I believe everyday should be a school day) but can you put your thoughts and postulations into your own threads instead of constantly hijacking other peoples posts and taking them on a tangent which results in the main message of the OP being lost. I imagine there have been a number of threads where there may have been more valid information still to come, but when people have to sift through lots and lots of excessively verbose, irrelevant (to the OP) musings, they begin to lose the will to live, let alone bother to bring forth any more worthy info.

You make a valid point. I usually (as in this thread) delay injecting any 'musings' until after 'the main message of the OP' has been done and dusted (which was achieved within about 5 responses in this thread), but I will experiment with a different strategy.

Kind Regards, John.

stillp · 20 Mar 2012

EFLImpudence said:
To be honest, it was very wirey.

A few years ago a supplier of Fieldbus equipment from the great playgroup across the Atlantic wrote the following in his promotional literature:

"Ultimately, the installation labour associated with wiring represents wire’s only truly significant drawback as a technology. A technology intended to replace wire must retain as many of the desirable features of wiring technology as possible, while reducing installation labour and cost as much as possible."

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Zs too high ???

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