IR compliance - trainee (and mentors!) quiz

[happysteve]

Been reading up for my 2394/5 exams next week. Learnt some interesting stuff over the last few days. Thought I'd pose a simple quiz, as this was something that I misunderstood, and I'd be interested to find out if others do too (or if it was just me!) Just for fun, trainees and mentors can all join in.

Every final circuit off two separate sub boards has a measured insulation resistance (on each final circuit) between line and earth of 3M (3 mega-ohms). The DB1 has just 2 final circuits, DB2 has 5. Which is correct?

For those on tapatalk who can't see the poll:

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 you want to post up your reasoning/workings then go ahead!

 

[trev]

3 mega-ohms

You've just dropped a point if you write that in an exam. Megohms.
Sorry for the pedantry but you have to get the terminology right mate. Get into the habit now before the exams kick in. This isn't like school where you get a point if they know what you mean.

 

[telectrix]

You've just dropped a point if you write that in an exam. Megohms.
Sorry for the pedantry but you have to get the terminology right mate. Get into the habit now before the exams kick in. This isn't like school where you get a point if they now what you mean.

so happy added an "a" where he should not, and you missed a "K", so it's happy 1, trev 1. at half time.

 

[trev]

so happy added an "a" where he should not, and you missed a "K", so it's happy 1, trev 1. at half time.

Touché. Stuff trapped under some of the buttons mate

 

[happysteve]

Alreight! Trev's stuck his neck out an givven his answer, an learned me on my grandma, good on yer, Trev! And you're spot on, stuff like that matters to the C&G examiners.

Have a ponder, have a read of yer notes or the BGB or GN3 or OSG, what do you reckon the rest on yer? There's no bother what you reckon, no-one's gonna judge you here in the trainee section, I'll stick my hand up and say I've had it wrong for pretty much the past year, just wondered if it's just me or if it's a widespread misconception, that's all

 

[trev]

I deliberately didn't post my reasoning up as it could have swayed people one way or another but I'll put it up when we're done if necessary. It's pretty simple and there's some red herrings in the question.

 

[jimmyray]

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?

 

[trev]

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?

"Every final circuit off two separate sub boards has a measured insulation resistance"
Meaning every circuit has been individually measured.

 

[jimmyray]

Hi trev, think I'm overcomplicating things! In that case, I'll stick with 'A', as they are all above the minimum acceptable value of 1Mohm.

 

[trev]

I see what you're doing there Jimmy but you have to work with what you're given and in this case if you're going to be right the term "measured globally" should have been used imho.
I could be wrong of course (there's always a 15 billionth time lol)

 

[jimmyray]

Cheers Trev, I ordered the Kitcher book on testing a couple of days ago, and haven't had a chance to flick through it properly yet, so will hopefully get up to speed with the testing side of things!

 

[trev]

It'll come Jimmy, while you're on get GN3

 

[happysteve]

Ok, let's expand on this a bit. Putting a question in a poll, you have a very, very limited number of characters, I think it's 80, that's less than half a tweet.

The intention of the question is: you measure the insulation resistance of each individual final circuit, and what do you know, each final circuit individually measures 3 megohms. There are 7 final circuits in total: 2 are supplied from DB1, and the other 5 are supplied from DB2. What I'm trying to get at, is, in your opinion, whether each of the DBs comply with BS7671.

And jimmyray, you are spot on mate with your calculation of what the global IR measurements would be

 

[trev]

Yes he is but the question is poorly worded (there's a surprise from C&G lol) but as long as you put your assumption in writing based on the information given then there's no way on earth you could lose marks for it.

 

[happysteve]

Anyone else want to have a punt at this? Poll closes Sunday evening, after which I'll comment on my thoughts and reasoning. I'm just really interested in what folk think, and their interpretation of the regs and guidance notes etc.