Insulation Resistance Testing

[erickench]

Hi Guys, I'm getting ready to take a state civil service test and I'm using a Passbook to prepare for the test. I came across an interesting question which I can't say that I agree with the answer as given in the answer key. Here goes:

The best way to determine the insulation resistance of a new wiring system installed in conduit would be to measure

A. between individual wires only
B. from each wire to conduit only
C. between individual wires from each wire to conduit
D. from the hot legs to the neutral

My choice was 'C' because you are looking for leakage between a wire and ground in this case being the conduit. But I could be wrong. The answer key gave 'D' as the answer which doesn't make any sense to me. What do you think?

 

[WIMaster]

Does the question state what type of conduit?
As I remember when I took my states' master test there were a number of questions that could have been answered all of the above, but that was not a choice. You need to read the question carefully and pick the BEST answer.

 

[erickench]

No there is no indication of what type of conduit was used or whether it was part of the EGC. The answer key doesn't make sense. Maybe it's the procedure to connect the hot leg to the neutral when doing the test?

 

[G._S._Ohm]

My choice was 'C' because you are looking for leakage between a wire and ground in this case being the conduit.

I guess I would decompose each answer like you did.

What will each test tell you?
In what ways would each test give you differing answers?
Is there one test that tests for many different fault conditions?
What are all the possible fault conditions?
How likely is each fault condition?

I don't see that much difference between these answers so one strategy is to amplify whatever differences there are.

However it turns out you might be able to make a case for this question to be thrown out. This, especially if you show more diligence in decoding this question than the test maker showed in making up this question.

 

[RichB]

My thoughts after reading it a couple of times are:
1. A is not going to tell you anything except weather you have a short between conductors--assuming both ends open

2. B would give a good answer except you don't know what type of conduit--PVC would be useless here

3. C See one and two above.
4. D--- would give a resistance to ground--as long as it is a grounded system

So I feel the "best" answer would be D. However I do agree that it is a very poorly worded question and would try to get it re-worded or thrown out if I was involved with it.

 

[jbrown]

Keeping in mind since they included the neutral in the equation of choices, remember proper system bonding automatically includes the conduit as part of the phase-to-neutral test.
'A' is similar to 'D' in concept except it says individual wires "only". If there is, or you test between a neutral and a grounding (bonding) conductor, you'll end up with a failed test.
'B' & 'C' is basically the same question, just worded differently to test for shorts to ground but leaves out phase-to-phase testing.
'D' is the best, but not perfect choice. Had the answer included hot-leg to hot-leg testing also, then it would be my perfect answer.