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Topic Title: R2
Topic Summary: R2 test
Created On: 07 July 2014 03:15 PM
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 09 July 2014 02:02 PM
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AJJewsbury

Posts: 11462
Joined: 13 August 2003

No requirement to check continuity on radial live and neutral conductors or any mention of obtaining resistance readings at all in the regs.

What about:

612.6 "a test of polarity shall be made and it shall be verified that ... (iii) wiring has been correctly connected to socket-outlets and similar accessories." and

612.9 "Where protective measures are used which require a knowledge of earth fault loop impedance, the relevant impedances shall be measured, or determined by an alternative method."

- Andy.
 09 July 2014 02:18 PM
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Parsley

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Unfortunately GN3 makes several references to recording R1+R2, including using the measured values + Ze to obtain Zs. The model circuit test result schedule on page 401 of 7671 and the one in GN3 page 93 has data entries for R1+R2 or R2 in ohms with the accompanying note on GN3

"*6 Continuity
Where Test Method 1 is used, enter the measured resistance of the line conductor plus the circuit protective conductor (R1 + R2). See 10.3.1 of the On-Site Guide or 2.7.5 of GN3. During the continuity testing (Test Method 1) the following polarity checks are to be carried out:
(a) every fuse and single-pole control and protective device is connected in the line conductor only
(b) centre-contact bayonet and Edison screw lampholders have outer contact connected to the neutral conductor
(c) wiring is correctly connected to socket-outlets and similar accessories. Compliance is to be indicated by a tick in polarity column 11. (R1 + R2) need not be recorded if R2 is recorded in column 7.
*7 Where Test Method 2 is used, the maximum value of R2 is recorded in column 7. See 10.3.1 of the On-Site Guide or 2.7.5 of GN3."

But as Geoff has highlighted there is no actual requirement in 7671 to record these values and the 7671 model MIWC only has a tick for earth continuity. Maybe IET will review and update the guidance notes to match 7671. Personally I think the trade organisations, software provider and test instrument manufacture's have a lot to answer for.

Regards

Regards
 09 July 2014 03:21 PM
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sparkingchip

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What about:

612.6 "a test of polarity shall be made and it shall be verified that ... (iii) wiring has been correctly connected to socket-outlets and similar accessories." and

612.9 "Where protective measures are used which require a knowledge of earth fault loop impedance, the relevant impedances shall be measured, or determined by an alternative method."

The loop tester generally does both jobs.

Andy
 09 July 2014 03:29 PM
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AJJewsbury

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The loop tester generally does both jobs.

Doesn't spot N-PE reversal... (although the RCD might if there is one). Besides, do you really want to wait for a live test before checking that the metal frontplate of the polished chrome socket is really connected to the c.p.c. rather than L?
- Andy.
 09 July 2014 05:35 PM
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sparkingchip

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I had a quick look at the books whilst eating sandwich for lunch earlier, the wording of the OSG indicates that the R2 test with a wander lead is better than a R1+R2 test. It also emphasises the importance of visual inspection.

Rest assured I do obtain a full set of readings as appropriate to the job in hand!

Andy
 09 July 2014 05:51 PM
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phantom9

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Ok I'm calm. Just got back from NAPIT Expo 2014 in Liverpool (2hrs journey) and had a really nice gentle day.

Example: Take a cable and install it as a radial circuit. Connect a socket to the end of it, feed it from a 16A mcb. A simple circuit feeding one accessory. If we switch on and plug a lamp in or a vacuum cleaner and they work then we know that L and N conductors are intact and continuous. They must be because the socket works. However, we don't know for sure that the cpc is continuous. So we can test continuity by making a loop out of the cpc and one of the other two conductors. A finite reading will confirm it is continuous.

Putting it plain and simply, that is what an R1+R2 test is. It doesn't require a value, it doesn't need further investigation it simply gives us a way of confirming that the cpc is continuous. An r2 test on a ring circuit does exactly the same thing, however a ring is not as straight forward to test because we can verify by end-to-end measurements that there is a continuous loop in each conductor but we need to do further tests to see that conductors are in the correct terminals and no reverse or cross polarity exists. Figure of 8 testing will allow some indication of crossed polarity but it is essentially what we are testing, continuity of the conductors. The test reading values are used for other reasons. this in a nutshell is what I am trying to state. in simple basic terms that is what we are testing for and why we are testing. The vast majority of electricians try to see it from the wrong angle. They try to see it that the test reading must be obtained to put in the box on the certificate. Well, yes, but it is not what the test is for. Putting a number in a box is just evidence that you have carried out the test (unless you have just written a number in the box to satisfy the need to put a number in the box .) The magnitude of the number will be used to verify other parameters about the circuit and tell an intelligent reviewer whether the number in the box is made up or as expected. Knowledge of resistance values of copper conductors and estimated lengths of conductors and a simple maths check will tell the intelligent reviewer whether the spark is taking the p. A correctly installed circuit will always produce good results becuase all the elements of the circuit are known variables. We know the resistance per metre of copper, we know the csa of the conductor, we know its length, we know roughly what Ze will be and ultimately adding the two together we know roughly what Zs will be and in a correcly installed circuit it will always be good. If someone hacks the circuit around and destroys the integrity then R1+R2 is probably not going to be possible but we can only do what we can physically do to ascertain the integrity of the cpc in these situations.

Are we all understanding now?

Edited: 09 July 2014 at 06:00 PM by phantom9
 09 July 2014 07:21 PM
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geoffsd

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Should I post or not - it's too simple.
Edit - ohms comes out as question mark.



I am not sure what the argument is but -
I'm either teaching Granny to suck eggs or have completely missed something in the thread.

these measurements can confirm the circuit is good.

With Phantom's single socket radial - 2.5/1.5 cable.

At the CU, Join R1 and R2 - measure at socket, 8? (for easy maths)
Join Rn and R2 - 8?, should be the same.
Join R1 and Rn - 6?,
From these you confirm that polarity is correct and
R1 and Rn are 3? each therefore
R2 must be and should be 5? - 1.66 x R1 or Rn.
You can also deduce that there are no loose connections on more complex circuits (unless exactly the same amount in both R1 and Rn - unlikely).
You can add to Ze for true Zs unaffected by parallel paths which could be removed in the future.
Then measure to find it less.

You can enter any or all of the values on the certificate.


It's even more fun for ring finals as you also get to divide by four.
 09 July 2014 09:00 PM
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sparkingchip

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On the other hand I have a couple of Megger LT320 loop testers that have a R1+R2 test facility on them.

You take a reference reading at the distribution board to get Zdb, the meter remembers this and when you test elsewhere it automatically deducts Zdb to give you R1+R2 to the point you are testing at.

However it is of course a live test, but presumably it gives you a impedance reading rather than a resistance reading so it includes the nasty bits that RCD's can add to the result.

I can't say I actually use this method of confirming CPC continuity, but the option is on the loop meter.

Oh, I forgot to say the guidance is that the R1+R2 method should only be used on twin and earth etc. R2 should always be used on conduit systems etc.

Andy
 09 July 2014 09:51 PM
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leckie

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There seems to be two separate debates going on here.

One is using the R1+R2 or R2 testing to establish continuity of the CPC.

The other is using R1+R2 measurement to assist in establishing Zs.
 09 July 2014 10:28 PM
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AJJewsbury

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Example: Take a cable and install it as a radial circuit. Connect a socket to the end of it, feed it from a 16A mcb. A simple circuit feeding one accessory. If we switch on and plug a lamp in or a vacuum cleaner and they work then we know that L and N conductors are intact and continuous. They must be because the socket works.

Mind you, it would still work if L and N were reversed, or even (in the absence of an RCD) N and PE reversed. Or if supply L was connected to socket N and PE and supply N to socket L (i.e. old east european style TN-C-S with a polarity reversal).

However, we don't know for sure that the cpc is continuous. So we can test continuity by making a loop out of the cpc and one of the other two conductors. A finite reading will confirm it is continuous.

But only continuous as far as some connection to your chosen reference live conductor. The c.p.c. might be severed from the supply and shorted to L (e.g. typical nail damage, or old east-european style....) and it would still give a finite (if slightly smaller) R1+R2 reading at the load end. You'd have to remove your temporary link and re-test to show open circuit to eliminate that possibility.

there are an awful lot of nasty possibilities out there - fun isn't it!

Putting it plain and simply, that is what an R1+R2 test is. It doesn't require a value, it doesn't need further investigation it simply gives us a way of confirming that the cpc is continuous.

I see it slightly differently (as you might have guessed) - the regs 'continuity test' and 'polarity test' don't require a value (just a yea or nay would do) - I agree, but other test (e.g. Zs and for certain approaches R1) most certainly do require a specific value....

If we had to do separate tests for polarity, c.p.c continuity, Zs etc at every point, we'd have an awful lot of testing to do. I believe that the R1+R2 test was an attempt to satisfy several requirements with a reduced number of actual tests - e.g. if you'd already proved that the c.p.c. was intact then a single R1+R2 test on a point would both prove correct polarity and give you an actual R1+R2 value you could add to Ze to verify that Zs would be satisfactory. You'd then just have to do a single live Zs test on each circuit (rather than at every point) to ensure that after your temporary link had been removed the permanent connections had been properly re-made. (Or if you prefer, if you'd already proved polarity then the R1+R2 test verifies both c.p.c continuity and allows Zs calculation.) I think the confusion comes from the assumption that the single R1+R2 test can prove both continuity & polarity at the same time.

- Andy.
 09 July 2014 10:37 PM
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sparkingchip

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"There seems to be two separate debates going on here.

One is using the R1+R2 or R2 testing to establish continuity of the CPC.

The other is using R1+R2 measurement to assist in establishing Zs."

Yeah! and the original poster only wanted to know if one or two boxes should be filled on the certificate.

Have a look at the completed examples in the OSG and you will see only one box is filled in!

Andy
IET » Wiring and the regulations » R2

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