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Topic Title: Zs=Ze + (R1+R2) question
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Created On: 12 March 2015 10:58 PM
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 12 March 2015 10:58 PM
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unitynick

Posts: 5
Joined: 11 March 2015

I cannot find this anywhere in the yellow book and it is driving me crazy as i have a question on it on the exam similator , any help very much appreciated. Sorry to trouble you.
 12 March 2015 11:11 PM
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sparkingchip

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So what is the question?

Andy
 13 March 2015 12:04 AM
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unitynick

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It just had 4 formulas to choose the correct formula from, i just wanted to find it in the book to check but could not.
 13 March 2015 12:47 AM
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IronFreely

Posts: 322
Joined: 06 November 2014

Well... Zs = Ze + (R1 + R2) is correct... Anything else got you stumped?
I seem to remember that if you're looking to find Zs calc written down its in the on site guide 10.3.6 (section 10, guidance on testing)
 13 March 2015 08:08 AM
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dickllewellyn

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Joined: 19 March 2010

If this is for the test and inspection exams, there may be a temperature correction to consider. Amendment 3 is the book you want for those exams.

If it's a book of formulas you want, the electricians design guide (looks like OSG but grey cover) is the mind boggler you're after.

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 13 March 2015 02:39 PM
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unitynick

Posts: 5
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I am doing the exam just to update from 16th to latest 17th but i thought all answers where in the yellow book ? Do i need any extra books to take the exam as i also saw questions on the guidance notes, which i think arnt in the yellow book.
 13 March 2015 02:46 PM
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marclambert

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Hi, did the exam in February.
There is a question (possibly more than one) where the correct answer is not given. I had a formula of Zs n Uo/Ia X Cmin.
It's a format or font fault when the questions are uploaded. I have raised with C&G. In the case above the n should be of course <= . See can't do it on here either but I mean less than or equal to of course. Only BS7671 is permitted.
regards
Marc
 13 March 2015 05:26 PM
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IronFreely

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Originally posted by: marclambert

Hi, did the exam in February.

There is a question (possibly more than one) where the correct answer is not given. I had a formula of Zs n Uo/Ia X Cmin.

It's a format or font fault when the questions are uploaded. I have raised with C&G. In the case above the n should be of course <= . See can't do it on here either but I mean less than or equal to of course. Only BS7671 is permitted.

regards

Marc


I think you mean Zs = Uo x Cmin/Ia
Though what's important with this equation is appreciating its existence rather than using it in design calculations, this is because the Zs tables in the yellow version of bs7671 alrwady take the minimum voltage factor (Cmin) into account.
 14 March 2015 12:07 PM
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phantom9

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It says in the yellow book that Cmin is 0.95. It would have been more logical, if fixing a value for Cmin, to use 0.95 in the formula and a simple note to say what its there for. Crazy.

Not being disrespectful to the OP, but that formua is pretty basic and fundamental to every day practice. It is taught in C&G courses early on. I feel sure some electricians still don't appreciate what the formula represents. Maybe the standard for being a spark should be raised?
 14 March 2015 12:57 PM
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IronFreely

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I agree phantom, the other way of looking at it is to change the so called 80% rule of thumb to a 76% rule of thumb, that's why I think the onsite guide figures are misleading - when you're new (like our OP) it can be a mind boggle and people may take a while to realise that BS7671 and OSG differ in that the OSG tables allready have this 80% rule of thumb applied to them but BS7671 are the figures obtained at an opporating temperature in a lab... Then just to make it a little more complicated you'll find temperature correction factors in a whole other set of tables in Apendix 3 which relates to current carrying capacity of conductors.
To the OP, phantom makes a valid point in that Zs calcs are part of the daily grind, it's one of the core calculations (along with ohms law) that needs to be written inside your eye lids. Although you can test Zs on a live circuit it is generally only done at socket outlets... Usually, for let's say a lighting circuit, you will test Ze at the consumer unit, test R1+R2 together using the short lead method and add them together, it becomes every day basic stuff quite quickly.
 14 March 2015 01:12 PM
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geoffsd

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I too agree but that is what the OSG is - a version of the regulations for dummies who can't work out anything.
It is a list of worst case scenarios to ensure safety.

As for Cmin, it is not really a correction factor as there is no circumstance where it can NOT be used.
It is, in effect, a reduction of nominal voltage to 218.5V (230x95%) when calculating maximum Zs for OPDs
That the figure should actually be 216.2V (230x94%) is yet another puzzling anomaly.
 14 March 2015 02:54 PM
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IronFreely

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Indeed, I just find it strange that they put the maths in the book as like we all know the new Zs tables are correct with Cmin in mind.
 16 March 2015 10:51 AM
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AJJewsbury

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That the figure should actually be 216.2V (230x94%) is yet another puzzling anomaly.

Two different voltages though. As we include Ze in our calculations, we should base the voltage on what's being kicked out by the transformer - as we're taking into account the impedance of the supply network. The 216.2V is the lower limit at the consumer's supply terminals - so necessarily has to include voltage drop in the supply network, due to it's resistance. To 216.2V and Ze is to double-count the network resistance.
- Andy.
 16 March 2015 01:44 PM
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mapj1

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Arguably a new design should not be sailing that close to the wind that 216/218 of the answer makes the difference between magneitic instant trip and thermal slower time trip.
In that sense minor corrections for things like temperature of cable, tolerance of supply and so on are academic. And in the world of fuses they are, as its a smooth curve between breaking in0.19secs, to 0.21 secs and 0.39 secs to 0.41secs.
It's a bit unfortunate that the binary nature of circuit breakers at the trip/no trip of the magnetic part seem to introduce a binary thinking, and a focus on stuff that probably is not that critical. I'm sure no one worried over much about the tolerances on fusewire.
The whole assumption of a 'zero resistiance fault at the furthest socket', like 'lets assume the horse is a cube' is so inapproprioate as to be at best a rough guide to deciding if the ADS being sized correctly.
Measured with micrometer, marked in chalk but cut with a felling axe (metaphoriocally - that's not really how I do wiring.).

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regards Mike
 16 March 2015 08:20 PM
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geoffsd

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I did not really understand the explanation and would be grateful for an explanation..

I'm not quibbllng over 2.3V but I thought the new Cmin figure was to account for the minimum allowed voltage if it should happen, i.e. 216.2V, not to mention that there could still be a further 5% (or more) voltage drop in the consumer's installation.

Had anything happened to cause this new Cmin to be necessary and how have we managed to date?
 17 March 2015 09:55 AM
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AJJewsbury

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I thought the new Cmin figure was to account for the minimum allowed voltage if it should happen, i.e. 216.2V

I thought it was more for building in a safety margin to allow for changes in the supply network - tap changes at the transformer - perhaps having imperial cables replaced with metric ones that aren't exactly the same size, or being reconnected to a different distribution main in the street as the DNO tries to balance loads, or replaces a transformer - that kind of thing.

Extra loads in the neighbourhood dragging the voltage at the consumer's end down a bit won't of itself affect the loop impedance or the driving voltage at the transformer.

- Andy.
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