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?
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.
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.
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