Inspection & Testing Exam Success Question on Insulation Resistance

[johninlondon1]

In the book one of the questions (page 49) the question states 'state the effect on insulation resistance of an increase in conductor CSA.' In college as Lecturer was going through this he said it was an increase, when I queried this and said I thought his answer was wrong and there was no change, I just replied that maybe the book was wrong. Can anyone give an indication of who is right, the lecturer or the book and why?

 

[Sidewinder]

john,

Can you please post the whole question?

However, on initial thoughts from the information you have given, I would say it would decrease.

Why?

OK bigger CSA of conductor, larger surface area of insulation.

Larger area for leakage paths.

Leading to reduced IR.

 

[ProDave]

It's the sort of stupid academic question that bears no relevance to real life. (a bit like the how to strip t&e thread)

Insulation resistance of modern cable of any size will be greater than most testers can measure, hence a reading of >299 Mohm

So unless the cable has a fault, that's what you will read. And discussion of minute variations are just a distraction.

All IMHO

 

[Sidewinder]

Yes & no Dave,

they test the students understanding of the underpinning knowledge, i.e. the scientific principles behind the results and the readings, which helps them to understand and interpret odd readings and results, thus giving them a better understanding of electrical science.

 

[Manator]

I agree, could you try and post the complete question, Dave when training people it is always better to give them information above what is really required, so they have a full understanding of why, what, when and where. I do however agree, it is almost always never needed, but knowing why is always better than not knowing.

 

[Hefferl]

The question in full is :

State the effect on insulation resistance of

i) An increase in cable length

ii) an increase in conductor cross-sectional area

iii) additional cables connected in parallel

The answers given are

i) Decrease

ii) None

iii) Decrease

I agree with the book in that the CSA will have no direct effect upon the insulation resistance.

This is somewhat of a trick question in that it attempts to confuse the candidate between the effect of CSA on insulation resistance with the effect of CSA on conductor resistance.

 

[Sidewinder]

OK, then I disagree with the book, and if you model this correctly I believe that the explanation I give above will be shown as correct.

This is borne out by the changes for Z values given in the larger CSA cables in 7671.

All of the questions given will result in a decrease in IR.

 

[Hefferl]

I do agree with what you're saying about an increased surface area of insulation resulting in increased leakage paths, but in theory you could produce a cable with an increased conductor CSA but same insulation surface area (Thinner wall of insulation). I appreciate that this thinner wall would in turn effect insulation resistance but I think the point of the question is demonstrate that it is not the properties of the condcutor that effect the insulation resistance it is in fact the properties of the insulation itself.

OK, then I disagree with the book, and if you model this correctly I believe that the explanation I give above will be shown as correct.This is borne out by the changes for Z values given in the larger CSA cables in 7671.

All of the questions given will result in a decrease in IR.

 

[Sidewinder]

However, on an insulated cable one cannot separate.

A limitation of multi-choice!

 

[green-hornet]

That was a nice civil debate, carried out by two informative members.

 

[johninlondon1]

Thanks for all the replies, when I first read the book (6 months ago), I assumed that insulation of the cable would be in proportion (more of less) to the CSA of conductors and obviously the thicker the Conductors the thicker the insulation used and therefore the same high insulation resistance in proportion to the cable. So I agree with the book answer but obviously the lecturer just photocopied the questions without answers and assumed from his thinking that he was right. Had I sat this exam and failed by one point on getting this question wrong I would have been annoyed. Ps I got 93% correct so must be doing something right.

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Thanks for posting complete question Hefferi.

 

[Technician]

This has reminded me of a practice in which lower

insulation reads on large motors were acceptable

because of the larger surface area of windings that

were in close proximity to each other (I carefully

avoided using the word contact) through the layers

of baked insulation.

Above a certain threshold ( I forget what it was in HP)

a lower insulation read was admissible, but for those

below it, a higher criteria was demanded, 1 Megohm

minimum.