IR compliance - trainee (and mentors!) quiz

[Pete999]

I put 'A', without looking at the books! Just had a quick read, and worked out the combined insulation resistances for the separate DBs. I came out with DB1 = 1.5 Mohms, which is above the required 1Mohm minimum acceptable value to satisfy BS7671, but as it is under 2Mohms, would require further investigation. With regards to DB2, I calculated total Insulation Resistance of 0.6Mohms, which is below acceptable value, so doesn't comply to BS7671. Any good?

"A" you are testing two separate DBs with a result of 3 Mohms

 

[trev]

"A" you are testing two separate DBs with a result of 3 Mohms

The way I read it was that all circuits have been individually tested and have a result of 3 megohms
As usual, a C&G question that's as clear as mud

 

[happysteve]

Sorry for the confusion. can't blame C&G for this, the "clear as mud" thing's all my fault - they are a bit protective about their past papers so this is a simplified version off a more complicated question I got sight of but couldn't copy.

Forget the "how to answer a City and Guilds question" thing for now (but yep, that's definitely an Art!). I understood the question, which was essentially, "Calculate the reading you would get if you did a global IR test of the circuits with the following values, showing your workings." The follow up question was, "does this DB comply with BS7671?" There was a comment in the examiner's report that threw me a bit, so I went back to the BGB, GN3 and the OSG and read the relevant sections very carefully, then the penny dropped.

Thanks Pete for chipping in!

 

[trev]

So you gave us half a question.
That's hardly fair!
Git!

 

[happysteve]

So you gave us half a question.
That's hardly fair!
Git!

Eh?!?

I simplified a longer question so it fitted into 80 characters and rather that split it into two bits (1. Calculate the global IR; 2. Do the DBs comply?) I put it as one. Jimmyray did a very nice job of calculating the global IR of each DB (DB1=1.5M, DB2=0.6M).

Alright then, maybe this is how I should have phrased it:

"You undertake a global insulation resistance test on two sub boards. The measured global insulation resistance of DB1 is 1.5M, DB2 measures 0.6M. After further investigation, you determine that no individual final circuit of either board has an insulation resistance of less than 3 megohms. Which of the following statements is true?

A: Both DBs comply to BS7671.
B: DB1 complies with BS7671, DB2 does not.
C: DB2 complies with BS7671, DB1 does not.
D: Neither DB complies with BS7671."

If the question were phrased in this way, would it affect your answer?

 

[trev]

Eh?!?
If the question were phrased in this way, would it affect your answer?

Absobloodylutely mate, I'd have done sums and come up with the right answer.

 

[Pete999]

So you gave us half a question.
That's hardly fair!
Git!

Double git lol

 

[happysteve]

Well then, for my muddiness, I apologise The aim was not to deceive or try and be clever, it was to test a theory about a common misconception.

 

[Pete999]

Well then, for my muddiness, I apologise The aim was not to deceive or try and be clever, it was to test a theory about a common misconception.

See me after school and pick up some extra homework, I will be contacting your supervisor for further punishment. No gold stars for you this week double lol

 

[happysteve]

See me after school and pick up some extra homework, I will be contacting your supervisor for further punishment. No gold stars for you this week double lol

Awww! I was having a good week up to then, too!

 

[jimmyray]

Good effort Steve! Can't do better than 100%, really well done mate.

 

[Simonslimline]

Seconded, very well done mate. :shades_smile:

 

[happysteve]

Cheers guys! The next two exams (closed book, written) won't be as easy, I know that much!

Any last punts? Quiz closes in 10 mins or so...

 

[happysteve]

I reckon the answer's B.

Some of those who answered "A" may have fallen victim of my "clear as mud" question, in which case, sorry about that.:blush5:

Jimmyray got it spot on:

Just had a quick read, and worked out the combined insulation resistances for the separate DBs. I came out with DB1 = 1.5 Mohms, which is above the required 1Mohm minimum acceptable value to satisfy BS7671, but as it is under 2Mohms, would require further investigation. With regards to DB2, I calculated total Insulation Resistance of 0.6Mohms, which is below acceptable value, so doesn't comply to BS7671. Any good?

The main thing is, to meet the requirements of BS7671:

612.3.2 The insulation resistance measured with the test voltages indicated in Table 61 shall be considered satisfactory if the main switchboard and each distribution circuit tested separately, with all its final circuits connected but with current-carrying equipment disconnected, has an insulation resistance not less that the appropriate value given in Table 61:

(From Table 61):
Up to and including 500V ... ; Test voltage (dc): 500V; minimum insulation resistance: 1.0 MΩ
​

The main bit is the bit in bold red. There is nothing in BS7671 that says, "If your global IR is less than 1MΩ, don't worry about it if all individual circuits are all over 1MΩ." Nor is there in GN3, or the On-Site Guide. If your global IR is less than 1MΩ (with distribution circuits disconnected, but with all final circuits connected), then the distribution board concerned does not comply with BS7671.

Now, I'd always believed that, as long as each individual circuit was >1MΩ you were alright. I could imagine a scenario where I measured the global IR of a board, it was a bit low, so I'd have to measure the IR of each individual circuit to see if it complied.

My understanding now tells me that, in the event of being unable to conduct a global IR on a board and having to test circuits individually, I need to use the parallel resistance formula to check whether the board complies or not when they are all connected.

A note from GN3 (p77, Section 3.10.3 (b) which is concerned with periodic testing):

The inspector will need to measure the values of insulation resistance for a given distribution board and then take a view based on his/her engineering judgement as to whether the results obtained are acceptable. It should be noted that distribution boards with large numbers of final circuits will generally give a lower insulation resistance value than distribution boards with fewer final circuits.
​

So for a periodic inspection and test you have a little more "wiggle room". For initial verification: not so much.

Is the above correct? And if so, was it just me who's had it wrong all this time?

 

[happysteve]

Forgot to say, this is the little nugget that alerted me to this, from the 2395 Examiner's report from February 2013:

When it came to identifying whether the results complied with the requirements two common wrong answers were offered by some candidates. The first being that the acceptable value was either 2 MΩ or 0.5 MΩ when 400/230 V system was involved and the second being that as long as the individual circuits were above 1 MΩ the installation was acceptable.
​

I was ok on the first bit, but the second "wrong answer" threw me as I might well have written that.