Is the only reason you do not calculate r1 r2 by deducting Ze/Zdb from Zs is that it is down to parallel paths. The reason I am asking is, when the apprentices are doing their trade test, they are encouraged to short the circuit they are working on to the earth bar via a crocodile clip to get r1 r2. Surely this is picking up parallel paths also? Or is there is another reason?
CAN ANYONE OFFER A DEFINITIVE ANSWER TO THIS QUESTION. PLEASE READ AND ANSWER ACCORDINGLY......................
Is the only reason you do not calculate r1 r2 by deducting Ze/Zdb from Zs is that it is down to parallel paths.
No, I think that GK has explained it earlier today.
You are checking for CPC continuity, i.e. R2 is let's say, around 0.5 Ω (obviously, depending upon the length of the circuit). You can do this one of two ways. First, by connecting your meter to both ends of the CPC using a 'wander lead', you measure R2 directly. The second method is easier: join line and CPC in the DB and measure between them at the far point. This gives you R1 + R2. You put one or the other on your generic schedule.
At this stage, you have yet to energise the installation, so you are working safely. So that is the main reason for obtaining R1 + R2 that way.
If there are parallel paths, they may be included in your R2 value. You can reduce the chance of that happening by joining line and CPC before you put the CPC in the earth bar.
Ze is one of the first live tests, which you do at the intake. Having measured that, and knowing R1 + R2 for all of your circuits, you can calculate all the values of Zs and check that against observed values.
It is all about working as safely as possible.
Surely this is picking up parallel paths also?
Yes and no. Parallel paths can occur in a number of different ways - yes ideally you'd measure R1+R2 (or R2) without any parallel paths at all - but that very difficult in practice. Not having the c.p.c. connected to the earth might eliminate some of the parallel paths, but other may well remain. On the other hand, testing just the final circuit bit of the loop separately from the supply side can reduce the effect of some parallel paths, even if the c.p.c.s remain connected.
A while back people were trained to disconnect both L and c.p.c. and connect them together to do a R1+R2 test ... but that leaves the single most important joint in the circuit untested ... so whatever's suggested there's always a downside. The more you get into the regs the more you realise they're based less on perfect logic and more on compromise and weighing one disadvantage against another.
- Andy.
BTW - I think you mean R1 and R2 - rather than r1 and r2 ... upper/lower case is significant - the latter mean something rather different (end-to-end continuity of a ring).
Surely this is picking up parallel paths also?
So, for an EICR, are we saying that, as it includes parallel paths, it's a more effective assessment of the real-world conditions than the original certification? Is that an issue? Certainly for initial verification removal or parallel paths gives the best result ... but as you've said in another way, the real-world conditions are far more favourable to our protection than the worst-case envisaged by Chapter 41 of BS 7671 ?
This thread has gone a way I never thought possible, I'm now getting pulled up for grammar......... LOL..... Can I refer back to the original question...... and take Initial Verification out..... If we can do live testing ZE/ZDB and ZS, why is it frowned upon to back calculate to put in R1 R2 (hope I got it correct this time)? This is all I'm asking nothing more? As I have already stated that the FICA asks them to use a crocodile clip from the circuit they are testing on the earth bar so you are using the earth for the whole installation. I AM ASKING WHAT IS THE DIFFERENCE FROM DOING IT THAT WAY OR LIVE TESTING AND BACK CALCULATING. NOTHING MORE I AM ASKING WHY???????????????????
Ro, have we not answered your question? If something is unclear, please say so in order that we may try again.
AS ABOVE
I do feel that the whole thread has ended up as being 'at cross purposes'.
On the one side we have technical explanations about the quality of the different approaches.
On the other side we have (or want) administrative explanations needed for the apprentice training.
We had two approaches (to taking a measurement) that appear to give 'different' results, with a 'correction' allowed one way, but not, apparently, the other way.
It's the "administrative" 'excuse' for the 'not the other way' that's required.
If I read the threads correctly, it is because we shouldn't make things look 'better' than measured, plus other factors based on the different ways the tests are set-up (short at the board vs at the far end, etc).
And we are looking for a solid apprentice "yes/no" level clarity. No wishy washy qualitative senior engineer stuff 
. Is that a fair assessment?
Well done Philip, that is exactly what I'm looking for, an idiot proof answer to the original question which, going by this thread is very hard to get.
OK then, if you want to calculate R1+R2 from Zs-Ze, fill your boots! But what is the point? Are you just being a slave to an empty column on a test sheet?
If we are only talking ADS, then once you have Zs, R1+R2 is totally superfluous.
Read the question at the top. I do not want to do anything. I'm asking a question that seems impossible to answer going by this thread. Unless of course I am missing something.
Read the question at the top. I do not want to do anything. I'm asking a question that seems impossible to answer going by this thread. Unless of course I am missing something.
the question has, I think been answered about ten posts up, and the answer is 'no' - its not the only reason.
That is because there is a safety reason for doing this, though the risk balance is different for existing circuits vs brand new ones, and inspections of unknown or only partly accessible installations may require more inventive approach.
Also it would be better if your apprentices actually verified the continuity of circuit CPC rather than a CPC that may be partly shorted along its length, as that paralell route may be hiding a break or high Z joint in the CPC.
For insulation tests however, all fault paths, not just to CPC should be explored, and that favours the MET and CPC to be connected.
regards Mike.
Is the only reason you do not calculate r1 r2 by deducting Ze/Zdb from Zs is that it is down to parallel paths. The reason I am asking is, when the apprentices are doing their trade test, they are encouraged to short the circuit they are working on to the earth bar via a crocodile clip to get r1 r2. Surely this is picking up parallel paths also? Or is there is another reason?
THIS IS THE ORIGINAL QUESTION, PLEASE SHOW ME WHERE IT HAS BEEN ANSWERED?????
AGAIN I'M NOT ASKING HOW TO TEST OR WHAT SHOULD BE DONE TO DO IT SAFELY.
Read the question at the top. I do not want to do anything. I'm asking a question that seems impossible to answer going by this thread.
Is the only reason you do not calculate r1 r2 by deducting Ze/Zdb from Zs is that it is down to parallel paths.
No. At initial verification, you measure either R1 + R2 or R2 on its own before the installation is energised.
Strictly speaking, as GK has written below, ZS = ZDB + (Z1 + Z2), but I would not quibble about impedance as opposed to resistance, particularly at the domestic level.
Or is there is another reason?
Aside from safety and parallel paths, the errors in the measurement of EFLI are greater than the measurement of resistance.
A further reason is that it is not required for periodic I&T. Continuity of the CPC may be confirmed by ZS at accessible sockets, etc.
THIS IS THE ORIGINAL QUESTION, PLEASE SHOW ME WHERE IT HAS BEEN ANSWERED?????
It has been answered a number of times. I understand it might not be an answer you want ... but it's been answered.
s the only reason you do not calculate r1 r2 by deducting Ze/Zdb from Zs is that it is down to parallel paths.
I will have a go again.
BS 7671 does not require a value of (R1+R2) to be recorded by any of the normative requirements in Chapter 64 or Chapter 65.
In the form in Appendix 6, the (R1+R2) column is intended to record the results of a Continuity Test, (this is stated above the column) if, and only if, you carry out:
1. Continuity of Protected Conductors by that method; OR
2. if you carry out a continuity of ring final circuit conductors test.
In all other cases, yes you are free to calculate (R1+R2) in the manner you suggest, and do with it what you like, but it shouldn't go in a "continuity" column, as you didn't measure it that way (using a continuity tester) - you did a measured Zs test the result of which goes in column 27!
And finally, to be clear ... if you do if the other way round, it is absolutely valid to calculate Zs from a measurement of ZDB added to a measured (R1+R2), and write that in column 27, because BS 7671 requires Zs to be 'measured, or determined by an alternative method' (Regulation 643.7.3.1).
AGAIN I'M NOT ASKING HOW TO TEST OR WHAT SHOULD BE DONE TO DO IT SAFELY.
Thank you for shouting at us when we are trying to answer your questions. We are not aware of your experience, and therefore are providing the best advice we can given the questions asked (which is all the information we have to go from.
Further, please understand that you are asking why things aren't considered 'best practice' and it is absolutely the case that the safe way of doing tests is taken into account as to what forms 'best practice' ... therefore I feel it's very valid to answer your questions by considering safety in electrical testing !
No. At initial verification, you measure either R1 + R2 or R2 on its own before the installation is energised.
I might add that 643.7.3.1 states, "An electrical continuity test shall be carried out according to Regulation 643.2 before carrying out the earth fault loop impedance measurement."
So before you even know Ze and Zs, you already know R1 + R2 or R2.
Just to say that the extra/2nd part of the reply isn't in the notification of the first part of the reply.
I suspect that's a 'feature' of the message board not to inform folks of the edits, which can lead to confusion.
It can be difficult (cf 2nd part) when perceptions of the Q&A are not the same.
edits, which can lead to confusion
Yes, agreed on the whole, but I share GK's exasperation.
perceptions of the Q&A are not the same
I cannot see why.
perceptions of the Q&A are not the sameI cannot see why.
Which part of the Catch 22 of these sorts of confusions...
It reminds me of when my son was taking a prelim/mock exam that has levels - this was a mid level 'intermediate' exam and was asked what 'coal' was and he said a hydro-carbon (which is the 'higher' level answer) and was marked down because the answer expected was simply 'fuel' at that lower level. - same question two different expected answers.
As the local knowledge progresses interpretations can be applied, but at the basic levels there are often 'right and 'wrong' answers and trying to guess which level the question is at can be tricky.
That's also why multiple choice questions work at that level (one choice has to be right); Meanwhile open questions are really tricky to judge (I got zero marks at the weekend for saying a 'first aid kit' was useful for an outdoor first aid incident!)
I see in another thread we have the "What is a circuit" question needing a lot of blinkers to decide the context.
It reminds me of when my son was taking a prelim/mock exam that has levels - this was a mid level 'intermediate' exam and was asked what 'coal' was and he said a hydro-carbon (which is the 'higher' level answer) and was marked down because the answer expected was simply 'fuel' at that lower level. - same question two different expected answers.
I would say that was a poorly designed test. The marking scheme should have covered both answers, but I see what you mean.
It reminds me of sex in Morningside (a polite part of Edinburgh). Sex, as in what the coal comes in. :-)
It reminds me of when my son was taking a prelim/mock exam that has levels - this was a mid level 'intermediate' exam and was asked what 'coal' was and he said a hydro-carbon (which is the 'higher' level answer) and was marked down because the answer expected was simply 'fuel' at that lower level. - same question two different expected answers.
I would say that was a poorly designed test. The marking scheme should have covered both answers, but I see what you mean.
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