best practice continuity

[sambotc]

If doing end to end continuity testing on a radial circuit (2391 test board) is there a requirement to carry out R1+R2 as well, and if so what does it prove?

I carried out end to end on the board today as some of the circuit used armour as CPC etc and I was told I should have tested R1+ R2 as well, just wondering if this is a requirement?

I was under the illusion R1 + R2 was used more for convenience, what am I missing?

 

[SirKit Breaker]

R1 + R2 is a convenient way of testing the CPC of a long circuit, and by doing this you also confirm polarity, because R1 being you line conductor, R2 being your CPC, when joined together if you operate switches you will see the reading going open circuit, then closed circuit assuming your switches are in the line conductor as they should be. This should have been explained to you on your theory course.

Cheers...........Howard

 

[sambotc]

So if end to end is carried out on all conductors, there is no reason to carry out R1+R2? I realise this will very often not be the case, but on the test rig the leads reached so I didn't do R1+R2 as it seemed pointless.

 

[SirKit Breaker]

Sam,

I take it you have GN3, this explains it well, but i will try and simplify.

First test on a ring final circuit.
Continuity of ring conductors.
r1,rn,r2, end to end values, and make a note of them.
Cross connect opposing ends of r1 and r2, to make a figure of eight arrangement, then take a continuity reading at each socket outlet from the front of the socket using a plug in breakout box, not at the terminals.
At each socket outlet you will get a reading, this reading should on a ring with no interconnections be the same at every point, and should be a quarter of the value of r1 end to end value, and r2 end to end value when added together. If all your sockets have the same reading, then you have a proper ring, when you test at a spur the value will increase, and it will increase further if there is a spur on a spur. This method also confirms polarity if everything is wired up properly. You then repeat the process but joining r1 to rn and do the same again. These two tests will also highlight any loose/bad connections.
This is the test procedure as per GN3. The long lead method only confirms your CPC continuity.
Hope this helps.

Cheers.............Howard

 

[telectrix]

OP stated radial. if so, then he's already measured R1 and R2, so just add together.

 

[spark 68]

If I remember on the 2391 practical you had to follow the procedures as laid down in GN3.
I know the cables on the test rig were short enough to measure end to end, which they wouldn't be in real life except for the very smallest of circuits, so you had to show you understood the procedure and how you should test in real life.

As Howard said this would show that the CPC was intact, that the polarity was correct, and also give you your R1+R2 value.

 

[sambotc]

Thanks, yeah I understand the principles of it, and I have showed that I understand R1+R2 on another circuit on the test board, but the way it was explained to me today was that it had to be done which I found hard to understand for the above reasons.

GN3 states both methods as being acceptable and the EIC's I use state either or on them, but the test schedule's we were using today were the one's from BS7671 and are more simplified.

I just wanted to double check I wasn't missing something but you've confirmed I haven't!

Thanks

 

[tigerpaul]

Usually on the test sheets you need to fill in a resistance value for r1 + r2, OR you can fill in a value for just r2.

r2 is for when you use the wandering lead method.

In practice though is it usually better to measure r1+r2 for the reasons mentioned earlier, and also to avoid having wandering leads laying all over the place.

 

[spark 68]

Usually the wandering lead or R2 only method is the preferred way on EICR's and the like where the installation has already been in service (powered up), When I did my 2391 we were filling in an EIC as if it was a new installation.

The reason for this is that they are testing you on all aspects and procedures to see that you understand fully both the sequence and the reasons for the tests.

 

[dagrat]

On an EICR we are encouraged not to disconnect equipment wherever we can avoid it,and so the r2 method is the preferred method, however the r1r2 method is far more satisfactory on a new install.
The college will almost certainly require you to exhibit understanding of both methods.

 

[Amp David]

Why are you wanting to measure end to end on a radial?

 

[dagrat]

Mmmh, just read over the thread again and there do seem to be few 'wires crossed' here

 

[SirKit Breaker]

OP stated radial. if so, then he's already measured R1 and R2, so just add together.

So he did........i spent ages typing all that crap out.

 

[sambotc]

Why are you wanting to measure end to end on a radial?

as said the radial circuit in question utilised the armour of SWA as the CPC into a metal enclosure. It's on a test board so the leads reach either end. Why wouldn't I do end to end?

 

[Amp David]

Because aren't you supposed to test as you would in real life?

Why wouldn't you just link out you line and CPC at the end of the radial and measure your r1 r2 at the DB?

 

[sambotc]

Sorry i've re-read my original post and I didn't add enough detail to the circuit, it was a 3 phase isolator and there was nothing connected to it. No CPC other than the enclosure, and armour to disconnect. I would have had to have made a link to carry the test out.

I carried out R1R2 on other lighting radials so that they could see I was aware of the technique used. When he questioned why I hadn't written down the R1R2 test method on the right up, i explained and was told you should always perform R1R2 as the result will be different than the calculated R1 + R2.

I made a decision as I would expect to in a similar situation or scenario in the real world, and felt a bit peeved. I get a bit tired of doing these kind of assessments and being judged on what they want you to do (without telling you) rather than accepting that you have used a bit of common sense and assessed the situation rather than follow the book.

That is unless I am missing something which I will gladly hold my hand up to if it is the case?

 

[robsparky1975]

When you do the 2391 test follow to the book,this will make your chances of passing improved,will can all aplly common sense but the exam situations are not made this way so follow GN3 and you can not go wrong,besides its good practise to use the R1+R2 method as said earlier to prove polarity,even if its making you fed up for having to do a link or such like,its getting through the exam and passing it that counts.They want to see competence in testing,if done so pass is easy.

 

[Amp David]

Another query I have is whats a calculated r1 r2 reading?

Are you measuring the length of the circuit then using the values given in regs for the resistance of said cable to achieve this?

 

[spark 68]

As Rob sparky said do it to the book in the exam as this is what they are expecting.

When I did mine I had to do an IR test on a TP+N+E cable, and because I did not have enough links I had to do all 10 tests, a bit tedious to say the least, I would have done this differently in the field, but as it was under exam conditions this was what I had to do.

 

[sambotc]

Another query I have is whats a calculated r1 r2 reading?

Are you measuring the length of the circuit then using the values given in regs for the resistance of said cable to achieve this?

No, measuring R1, Rn and R2 then adding R1 to R2 together.

 

[Amp David]

Ah I see.

 

[spark 68]

The thing is with the 2391, they are so pedantic, both with the written and the practical, it is better not to deviate in any way if you want to pass.

@ To the OP, so did they fail you because of this ?, if you don't mind me asking.

 

[sambotc]

Nah no fail, not sure if you can fail it? The bloke walked off and left me to it!

It was only when he was looking over the write up and questioned the no R1R2 etc etc

I asked the question for real life purposes not the exam, I know you have to do what they want to see unfortunately, but the assessor was leading me to believe it should be done on site as well, and that R1,Rn and R2 was somehow inferior, which is why I posted originally.

 

[spark 68]

Just to clarify, which course are you doing ?, because they can and do fail candidates on the 2391.

I asked the question for real life purposes not the exam

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I don't really see the need to do the R1+R2 as seperate measurements in real life either for all of the reasons already said, mainly because you cover 3 items with one test, and quicker is better, as well as more convenient.
But I don't actually see what is wrong with measuring them seperately and adding the results together either, other than you will have more testing steps to do.

In fact GN3 states other testing methods are not precluded, so long as the same end results are obtained.

However on courses it is better to just do it by the book as we have already said, and not 'rock the boat' so to speak.

 

[sambotc]

Its the EAL diploma testing and inspection, equivalent to C&G 2391 so I was led to believe, they just use EAL as the certification body. Got my written test paper on the 26th.

Yeah don't get me wrong I wasn't sat there arguing my point with him, I saved that for here lol I just questioned the logic as I was more interested in making sure I hadn't missed something obvious regarding the benefit of R1R2.

As i've said, in a real world scenario where the cable length was longer than my test leads, i'd combine as it makes sense, just effectively using the line conductor as a wander lead whilst also checking polarity and R1 continuity at the same time, but if it were a situation like the board where you could reach with the leads end to end, I don't see why you would want to disconnect conductors etc when it could be carried out without having too.

 

[dagrat]

I think testing r1+r2 is by far the more efficient method than testing separately, as has already been stated. However you appear to have an excellent understanding of the requirement for the test so I'd be very surprised if on this practical the assessor would mark you down for this.
In real life the benefits of the combined test would provide more results in one step, although the specific test result would be the same

 

[Guitarist]

Another query I have is whats a calculated r1 r2 reading?

Are you measuring the length of the circuit then using the values given in regs for the resistance of said cable to achieve this?

You could calculate the r1+r2 in this way for checking purposes. Rarely used though as it's hard to know exactly how long your circuit is unless measured out during installation.

 

[dagrat]

Actually the table for resistance of copper conductors can give a reasonably accurate measurement of length of Cable and r1+r2, but not the first choice method as per previous posts

 

[robsparky1975]

So if you pass this exam i hope that you would get a City & guilds 2391 certificate,would the eal have 2391 written on it,only saying this as agency donks for instance only know certain quals i.e 2391,2382 etc.

 

[Rob Smith 643]

I'm relatively new to inspection and testing but not to installation. Its a hard road but an interesting one and the forum does help. I'm confident with my inspection and testing procedures and know the tester inside out as it were. I often get perplexed by results even when they are well within tolerance/guidelines/tables etc, particularly when the measured values don't mirror the calculated/expected values. Some would say why care but I'm a bugger for needing to understand why. Most dwellings I have 'I & T'd' during installation and upon completion give Ze readings in the region of 0.12 to 0.17. On my last lighting install the highest measured R1+R2 was 1.79 (landing light fitting) however the Zs taken at this lighting point was 1.09. This gives me a calculated value of 1.91 versus a measured value of 1.09. This is not the first time and I doubt it will be the last but I would be obliged for some theoretical, experienced based explanations to help me understand. Cheers, Rob.