Exam question

Zs=Ze + (R1 + R2)  = answer A doesn't it? I know it's early and I haven't woken up yet.


Z.

yes i agree that's what i get, but! the answer hes given is D, i think its a typo by his tutor!!

This could be a catch question. Remember the 80% "rule"? 1.65 * 0.8=1.32. 1.12 + 0.2 = 1.32!

One needs to think quite hard in exams, they ARE out to get you. The key is the "maximum tabulated velue", which is not the same as the OSG for example. The 80% is also arbitrary of course.


David CEng

Thanks Dave,


We did try that, but still cant get to the O.20 OHMS they are saying is the correct answer

Which 0.2 can you not see? I have written it out above! There is probably another trick there which is the accuracy of the R1+R2 measurement. Is 1.12 absolute, or is it perhaps 1.13? This question is about test, and test is not a very quick bit of adding up! This might come straight from the old 2391, or perhaps even the new one. The other answers are the result of various common mistakes and misunderstandings.

Dave,


Thanks,


I can see how you've done that,!

Differs from the explanation Dave gives, and 0.52 is what i originally worked out,

I have a further comment. The answers in multiple choice exams are never one correct one and 3 (or whatever) random numbers or answers. It is always quite possible to derive all the answers by making various errors, even if working them out may be difficult. The answers may have several parts (as I suggest above with measurment accuracy) and require a level of understanding which is not immediately obvious. They are actually quite difficult to design, which is why real multiple choice back papers are often not available.Remember, an exam is not there to be easy, it is to set a defined standard of understanding.


Take  the 18th edition exam, with a reasonable level of knowledge it is quite possible to look up all the answers in the BBB within the allowed time. Thus there is no excuse to not getting 100%. However the average candidate struggles with the basic knowledge to know exactly where to look and what to reference in the index, list of tables etc. Thus it takes them more than the allowed time to complete the exam, and they get lower marks. Even with an excellent knowledge some questions are difficult because the answer is not found from the index etc., you need to know where to look fairly exactly. It is debateable whether the pass mark is high enough, because it is possible to pass with just the index and very little knowledge.


In the above question the answer c is probably misreading the 1.13 number as 1.3, and forgetting the 80%, someone will do that. There are many ways to get the wrong answer, yet be convinced it is right.


Good luck and be careful!


David CEng

Thanks again Dave,

Fully agree with your comments,

I think its just a very poor exam question. If its objective is (at least partially) to test the candidate's awareness of ambient versus max temp cable resistance, it just fails. What is a "tabulated" value? A value taken from a table in BS 7671? From the OSG? From a manufacturer's data sheet? Does that table already factor in Cmin and/or the 20% derating? Who knows?