RECENT C & G 2382 question
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Paul_C
Use the minimum size specified in the table, because you've consulted the supply undertaking and have been told there is no requirement to use a larger size.
Yes, we know that calling the DNO is likely to be a fruitless exercise and you could quite easily spend the whole afternoon on the phone and still not talk to somebody who even understands your question. But that's not really the point, is it? The question in the exam is about what BS7671 requires, and as the note to the relevant table makes clear, BS7671 requires that you consult the supply undertaking to see if the minimum size in the table is satisfactory or not.
So yes, answer B is correct if choosing the size in accordance with table 54.8 and its associated note is what answer B means. But that does not mean that answer D is wrong, because in any event it's necessary to carry out the course of action mentioned in answer D in order to carry out the course of action required by answer B. Answer B might give a course of action which goes into more detail, but that doesn't make answer D wrong per se.
Does BS7671 require consultation with the supply undertaking in order to decide the size of the conductor?
It's not a question of precision, it's a question of what gets you to the conductor csa.
B does, in an unqualified way.
Without qualification, D does not.
There's also the small matter that the regulations inform you that B is the right answer, not D.
Because D is not what the regulations say (remember what the exam is).
And IRL D might not get you the size. If you contact the DNO and ask if they have any special requirements and they say "No", then what?
Possibly.
But in this case there aren't two correct answers.
You couldn't.
So you'd contact the supplier, which is what you should glean from reading Table 54.8.
Presumably it depends upon impedances and therefore the distributor's estimate of the worst-case current that could flow to (a very low impedance) earth from their neutral cable under fault conditions.Just as a side note (which, I imagine, shouldn't take too long to get a clear answer)
Under what circumstances would a Local Distributor require a larger main protective bonding conductor?
However, maybe I'm missing something, but what may be a bit silly about all this is that, in practice, it could well be the impedance of the connection to earth via supply pipes etc. is usually going to be the main determinant of the current flowing from neutral supply to that earth under (supply) fault conditions, more-or-less regardless of the CSA of the neutral supply conductor. The figures in Table 54.8 require an MPB with a current-carrying capacity of roughly half that of the neutral supply conductor, which means that, ultimately, the MPB would melt before the supply neutral if the impedance of path to earth via the MPB was sufficiently low to result in a current of 'melting' potential - which I somewhat doubt would be the case.
Kind Regards, John
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No - you'd not be able to do that, because that's Answer B, and you've already rejected that as wrong.
So what are you going to do?
The DNO haven't told you, but according to you that's the way you get the size.
How can looking at the relevant table be the right thing to do if you've rejected that by saying that the answer "look at the table" is wrong?
How can you look at the table if the answer you have selected as correct does not involve looking at the table?
Stop trying to wriggle out of it.
You are flat out wrong, and once again displaying a worrying inability to just read what is written.
The note is not "associated with Table 54.8".
IT IS AN INTEGRAL PART OF TABLE 54.8
Answer D is "Contact the DNO". AND THAT IS ALL THAT ANSWER D DESCRIBES DOING.
The regulations do not tell you to only contact the DNO.
Therefore D cannot be correct.
It's not a question of more detail, it's a question of what the regulations tell you to do.
Unless the regulations tell you to only contact the DNO then Answer D is wrong.
If you want to cavil about the exact words used then please go ahead.
Because since BS 7671 does not require consultation with "the supply undertaking" then D can't be correct.
I think I probably should give up
Completeness and Correctness are two totally separate issues. The (correct) answers to a high proportion of questions are 'incomplete', in that it's very often possible to qualify those answers in some way, but that does not make them 'incorrect'.Kind Regards, John
In that case, B is also incorrect since, as you have written yourself (in very large letters) the implicit requirement to consult the supply undertaking is an 'integral part of' Table 54.8, hence an integral part of answer B.
So then we'd have a question with NO correct answers - just as bad as one with two correct answers!
Kind Regards, John
So therefore, B is the only correct answer. B tells you to look at 54.8, D does not.
Not really, but it wasn't mean't to be. I wrote that to provoke a bit of practical thought and comparison about the process of following the routes of B and D
Indeed, so again, B is the only way to find the answer. If you believe D is correct but the Local Distributor do not have any special requirements, then you do not know the required cable size.
The guidance suggested by answer D is not incorrect, as you say. However, in terms of the question, D does not in itself get you the answer if the Local Distributor says "no special requirements" so cannot be correct.
No, it doesn't
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Paul_C
Being incomplete does not make it incorrect.
If there are two answers, neither of which is incorrect, then both of those answers must be correct. The only issue is whether one is more precise than the other, not whether one is wrong.
Yes, answer B gives you the route to the actual stipulated size. But you need to do the things set out in answer D in order to follow the route of answer B. Answer D cannot, therefore, be wrong as such.
Do you have to consult the supply undertaking in order to decide upon the conductor size stipulated by BS7671? Yes. So answer D cannot possibly be incorrect, even if it does not give the full story.
So if one can use such a term, answer B might be "more correct," but that doesn't make answer D wrong. It's still a very badly worded set of answers if it can result in somebody providing a perfectly correct answer and failing to get a mark for that question, even though he understands perfectly what is required.
Yes, and answer D also says that you should contact the supplier.
Who's rejected it as wrong? That's the whole point being made here: There is no one correct answer out of the four options provided. Two of them are correct, just to differing degrees.
Doesn't alter the fact that answer D is still correct. I was only qualifying that after the comments about it way up the thread somewhere.
Do you select the size in accordance with table 54.8, including the note which says you must check with the supplier? Yes. So answer B is correct.
Do you select the size after consulting with the supply undertaking? Yes. So answer D is also correct, even though by itself it does not lead you directly to the actual size you need.
No, but they do require you to contact the DNO, which is what answer D says. So how can answer D be wrong?
But BS7671 does require such consultation. You've just said so - It's in the note with table 54.8. Unless you still want to try and argue that just because BS7671 does not use those two precise words that they somehow don't count.
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Paul_C
How do you figure that? We've just been talking about the very note in table 54.8 which requires it.
Presumably it depends upon impedances and therefore the distributor's estimate of the worst-case current that could flow to (a very low impedance) earth from their neutral cable under fault conditions.Just as a side note (which, I imagine, shouldn't take too long to get a clear answer
Under what circumstances would a Local Distributor require a larger main protective bonding conductor?
However, maybe I'm missing something, but what may be a bit silly about all this is that, in practice, it could well be the impedance of the connection to earth via supply pipes etc. is usually going to be the main determinant of the current flowing from neutral supply to that earth under (supply) fault conditions, more-or-less regardless of the CSA of the neutral supply conductor. The figures in Table 54.8 require an MPB with a current-carrying capacity of roughly half that of the neutral supply conductor, which means that, ultimately, the MPB would melt before the supply neutral if the impedance of path to earth via the MPB was sufficiently low to result in a current of 'melting' potential - which I somewhat doubt would be the case.
Kind Regards, John
Thats kinda what I was thinking aswell..
Even if D were the correct answer it wouldn't get you anywhere as in my experience anyone on the end of a DNO call hasn't got the foggiest idea what you are on about.
P
Paul_C
I noticed that
And what does that note say?
Very well - if you can't see that a procedure which when completed does not guarantee that it will produce a result when a result is needed cannot be the correct procedure to follow then I'll give up trying to explain such a fiendishly difficult concept to you and concentrate on something you might be able to cope with.
Do the regulations tell you that where PME conditions apply the main bonding conductor should be determined after consultation with the supply undertaking AND NOTHING ELSE?
Yes or no?
Multiple choice exams like that do not have more than one correct answer to each question. That's not how they work, and you really ought to know that.
Do the regulations tell you that where PME conditions apply the main bonding conductor should be determined after consultation with the supply undertaking AND NOTHING ELSE?
Yes or no?
Do the regulations tell you that where PME conditions apply the main bonding conductor should be determined after consultation with the supply undertaking AND NOTHING ELSE?
Yes or no?
Do the regulations tell you that where PME conditions apply the main bonding conductor should be determined after consultation with the supply undertaking AND NOTHING ELSE?
Yes or no?
Nobody who understands perfectly could possibly select the wrong answer.
If he knows how the exam works, what it's testing and how to read the questions and how to read the regulations he'll pick the right answer.
The only people claiming that it's badly worded are the ones who cannot properly read the questions and/or the regulations.
You have.
It is a multiple choice question, with 3 wrong answers and one right one. That's how they work.
By selecting D as the right answer you are automatically rejecting A. B and C as incorrect. That's how it works.
If you don;t even understand how multiple choice questions work then you are not qualified to discuss this issue, so please go away and stop wasting everyone's time.
That's not possible because the multiple choice question does not work like that.
Do the regulations tell you that where PME conditions apply the main bonding conductor should be determined after consultation with the supply undertaking AND NOTHING ELSE?
Yes or no?
Do the regulations tell you that where PME conditions apply the main bonding conductor should be determined after consultation with the supply undertaking AND NOTHING ELSE?
Yes or no?
D gets you ONLY contacting the DNO, AND NOTHING ELSE.
Do the regulations tell you that where PME conditions apply the main bonding conductor should be determined after consultation with the supply undertaking AND NOTHING ELSE?
Yes or no?
I don't want to argue that at all.
Do you want to argue that the regulations tell you that where PME conditions apply the main bonding conductor should be determined after consultation with the supply undertaking AND NOTHING ELSE?
Yes or no?
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