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Topic Title: zs
Topic Summary: zs is conflicting
Created On: 27 July 2010 10:05 PM
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 27 July 2010 10:05 PM
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adamlitt23

Posts: 36
Joined: 14 December 2008

Evening all
I am a concerned about zs as I understand it we on the certificates we put the max values from chapter 41 of the red book and on the schedule of test results under the heading remarks de-rating factor of 0.8
zsx0.8

Due to temperature differences we must then times this figure by 0.8 which gives us corrected value for comparison
So long as our measured value is lower then the corrected value
The temp factors and max zs come from the OSG chapter

All comments pls welcome
 27 July 2010 10:17 PM
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Martynduerden

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And in English?

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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 27 July 2010 10:20 PM
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adamlitt23

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what do u mean
and it all about the use of max zs figure and there reporting
the use of 0.8
 27 July 2010 10:24 PM
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scottyscott

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Hi adamlitt23, you may have to rephrase the question - if it is a question?

Scott
 27 July 2010 10:27 PM
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Martynduerden

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Originally posted by: adamlitt23

what do u mean


Your not a consultant are you don't steal my lines!

and it all about the use of max zs figure and there reporting

the use of 0.8


Where oh there.

If you want an answer you need to as a question, not simply string words together in a funny order and hope we can decipher you question.

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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 27 July 2010 10:35 PM
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deleted_2_tony30

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well if it asks for the max zs in bs7671. put those figures in that you find in bs7671.
not any other book,guide etc
tony
 27 July 2010 10:42 PM
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adamlitt23

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Lets start again
Pg 104 onsite guild
Temperature in the room is at 25 degrees pg 104 OSG 1.06
Maximum earth fault loop impedance for a B16 60898 MCB is 2.32ohms pg 103 OSG
2.32x1.06=2.45 ohms which is the corrected value
Now what i would like to understand is on a electrical installation certificate what figure would record under the max ZS VALUE and how if at all any correction factor would be needed to on the measured value

Or pls can someone explain how to go about this
 27 July 2010 10:55 PM
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ant1uk

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In the max zs box you put the max tabulated value in BS7671 (the big red book) without using the correction factors which are in GN3 and OSG
 27 July 2010 10:56 PM
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John Peckham

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Adam

It would be helpful if you stopped using text speak and use the Queens English on this forum as it is full of us old farts. I am not a good example of excellence in grammer or spelling by the way!

There are some exceptions but put simply using the Rule of Thumb rather than temperature measurement.

1. On the schedule of circuit details as it says on the top of the form if you are using NICEIC type forms you put in the maximum value of Zs as written down in the tables in BS7671:2008 Part 4.
2. On the schedule of test results you write down your measured results as displayed on the meter.
3. You then, using the rule of thumb, multiply the Maximum Zs value from BS7671 by 0.8. This is your compensated value. You don't write this down on the PIR or EIC.
4. You then compare your measured result to the compensated value of Zs. It has to be equal to or less than the compensated value to achieve ADS.

Hope this helps?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 27 July 2010 11:00 PM
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scottyscott

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think I'm getting you...I record the highest Zs measured value for the circuit. the tables are used to refer to - in the OSG the Zs max have already had the 0.8 x

hope I'm right
 27 July 2010 11:08 PM
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adamlitt23

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from start to finish how would u go abot this
many thanks
 27 July 2010 11:09 PM
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John Peckham

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Adam

Did you see my post above?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 27 July 2010 11:42 PM
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AJJewsbury

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You've got to admit it's a bit of a daft way of going about producing a report for the client though ... "recorded measurement 1.0 ohms", "documented maximum 1.1 ohms" - therefore fail because 1.0 (invisibly divided by 0.8) = (a not mentioned) 1.25 which exceeds 1.1. And then we gripe when people don't take notice of the reports
- Andy.
 28 July 2010 01:13 AM
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Phillron

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Where it asks for maximum Zs write down the adsjusted values and state on the top of the column 80 %
It makes the comparisons visnble to whoever
 28 July 2010 06:49 AM
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ebee

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Yes it would be easier to note the corrected values as the target rather than the tabulated values in order to compare against the measured values.
It would simply be a comparison of two figures for anyone to see.
Lots do it that way even knowing it is wrong.
At least it's sensible though.

Andy hit the nail reet on the head.

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 28 July 2010 09:13 AM
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GeoffBlackwell

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I was pulled up on this by one of the great and the good a couple of years ago. In an earlier time I would have been his boss, but things change .

I had always indicated the adjusted value of Zs when completing contractor's forms from whatever scheme they registered with. I do what I like on my own forms but that is another story. He insisted that my former employer wanted the unadjusted figure on their forms (they have certainly gone downhill since I left ).

Still we shouldn't get too upset - after all the form is not designed to be of any use to the customer is it. Even the latest efforts fall way short on the accessibility front.

It wouldn't be hard to improve things for a PIR - all we need to do is separate the document into a 'Condition Report' written, as far as possible, in plain non technical language, and including recommended remedial actions - together with a technical report to give guidance to contractors who may wish to quote for remedial works.

No conflict of interest there for me - but I don't do contracting .

Regards

Geoff Blackwell
 28 July 2010 01:30 PM
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AJJewsbury

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Actually I can see an opportunity here.

Comparing meter reading with "80% tables" is easy, but logically a bit flawed as it presumes that the conductors are always at 10 to 20 degrees - which they might well not be. Really we should be taking the meter readings, correcting according to the actual temperature and then comparing with the BS 7671 original tables.

That's going to be a bit tedious though.

But most loop meters are digital these days, so it wouldn't be hard for the meter to do the adjustment itself (e.g. divide by 0.8) before displaying the answer - it could even incorporate a little digital thermometer so it could directly read the actual ambient temperature and adjust the factor accordingly.

We could call them "18th Ed meters" and charge a small premium Maybe even imply that older meters without this new facility are dangerous and probably illegal!

- Andy.
 28 July 2010 02:01 PM
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John Peckham

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GB

On my initail assessment the AE spent some time going through my folder of PIRs. He said he was pleased to see I had inserted the 100% values of Zs on the schedules of circuit details although he added it make no difference anyway.

My EasyCert software gives you the option of using 80% values or 100% values for the schedules of circuit details, it then automaticaly inserts the the value when the CPD type and rating are entered. It then automatical compares this vale with your measured value and highlights excess values. This feature is not much use if you have correctly set the forms up for 100% max. Zs values. I have asked for a change to be made to the software but nothing yet.

Like you I have my own forms which enter the 100% values of Zs for particular CPDs and then compare the measured values to 80% of the entered values.

Andy

I would settle for a loop tester that actualy reads a precise (+ or - 5%) measured value of Zs everytime down to supplies of 25kA and close to the transformer.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 28 July 2010 02:34 PM
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slittle

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Oh come on John, We all want a meter that reads properly close to supplies but......

Stu
IET » Wiring and the regulations » zs

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