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Topic Title: R2
Topic Summary: R2 test
Created On: 07 July 2014 03:15 PM
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 07 July 2014 03:15 PM
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gchar

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Do you required a R2 test if already carry out a R1 + R2 to put in the required spaces
 07 July 2014 03:26 PM
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mapj1

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Generally, no. Unless you expect there to be something that means they are not related simply to testing the same wiring.

Now as an example of something where it might not always be simple, consider the wiring to a thermostat that controls a heater, in the case where the supply goes to the heater and then live goes out to the thermostat and comes back switched on or off.

Knowing R1 +R2 (Live plus earth loop round resistance) at the heater, tests the thermostat wiring in the R1 (live and switch) path, but the earth path may go to the heater direct from the supply, not via the thermostat.

If the thermostat needs a CPC, then R2 should be checked there also, as you would not know any other way if the earth connection between heater and thermostat was OK.

Of course this additional information does not fit properly onto the official forms, and ends up being noted on a separate bit of paper or very often not actually noted at all - and on a bad day not even checked

-------------------------
regards Mike
 07 July 2014 05:16 PM
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AJJewsbury

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Do you required a R2 test if already carry out a R1 + R2 to put in the required spaces

You only need to record R1 or R1+R2 (see heading at the top of the columns). But don't loose sight of what it is you're trying to achieve. While knowing R1+R2 is a step towards proving that the value Zs will be adequate, it doesn't of itself prove correct connections (e.g. L and PE not been reversed). The R1+R2 test doesn't prove polarity (L is really connected to L) unless you already know the c.p.c. is correctly connected. R1+R2 doesn't prove the c.p.c. is correctly connected if you don't already know that the polarity is correct.

- Andy.
 07 July 2014 10:17 PM
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phantom9

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Generally no? Absolutely no! Do one or the other, not both. What is the R2, or R1+R2 test designed to prove? Both achieve the same result, why do it twice?
 07 July 2014 10:23 PM
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phantom9

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Originally posted by: AJJewsbury

Do you required a R2 test if already carry out a R1 + R2 to put in the required spaces


You only need to record R1 or R1+R2 (see heading at the top of the columns). But don't lose sight of what it is you're trying to achieve. While knowing R1+R2 is a step towards proving that the value Zs will be adequate,
- Andy.


What? You are kidding! It is nothing to do with that! It is to prove the continuity of the cpc. That's it. Whatever else you pretend to do with it is of no consequence for the actual reason for the test. I am sorry to be hard on you for that but making stuff stuff up just to post a reply. It has nothing whatsoever to do with proving the value of Zs that is extremely poor advice and way off the mark.
 07 July 2014 10:37 PM
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mapj1

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Well, my dear P9, consider my example of the heater and metal bodied thermostat where power calls in at the heater, and there is as bit of T/E between heater and the thermostat.
Where would you measure R1 +R2 or just R2 to get the worst case fault current - heater or thermostat and ensure the CPC reaches both places ?
I'd do two tests, one of each flavour , but I suppose you could do two of the same - but I can't see any way where one single figure covers all the bases correctly.
Sensible & technically ordered answer only please, and bonus points for how much you would actually record, and how.
We need to remember the OP is non-native,and may not 'get' your humour..

-------------------------
regards Mike
 08 July 2014 02:10 AM
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gchar

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Thank you map1
 08 July 2014 02:11 AM
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gchar

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Thank you to all your replies
 08 July 2014 07:51 AM
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phantom9

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Dear mapj. The purpose of the R2 test, or the R1+R2 test, is very simple. It is to prove the continuity of the cpc. R2 is a direct measurement end-to-end using a wandering lead to make a continuity loop. R1+R2 uses one of the other conductors in the cable to make a continuity loop. How you carry out the test is of no consequence. You can place as many what ifs in the equation as you want to, it makes no difference to the ultimate aim of the test. It is purely to ensure that the cpc is continuous.

I appreciate that it is difficult sometimes to remove parallel earth paths and ensure that the conductor under test is isolated from other earth paths but the basis of the test is proving continuity of a conductor.

AJJs comment that it is "a step towards proving that the value of Zs will be adequate" is surprisingly naive. I don't understand why people read so much more in to a test than it warrants.

Regards
 08 July 2014 08:52 AM
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Parsley

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Phantom,

For what it's worth I think Andy's and Mike's responses to the OP's question are perfectly suitable.

Regards
 08 July 2014 08:58 AM
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leckie

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That's funny Phantom.... I had this strange idea that Ze + (R1+R2) = Zs.

So one stage of finding the value of Zs would be to establish Ze and then stage two would be to measure R1+R2

So in fact, if being used to assess Zs then the R1+R2 measurement would be to check the value of both the line and cpc conductors combined resistance.

So then not just the cpc continuity.

But the measurement of R2 is to establish the continuity of the cpc, as per Map's example.
 08 July 2014 08:59 AM
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GeoffBlackwell

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I am not challenging Mike or Andy's responses - but on the question of the purpose of the test - I agree with phantom9 here.

I have commented on this topic many times and I have no intention of repeating all of that - do a search if you are interested.

I will just summarize some of the salient points.

1) BS7671 - 612.2.1 simply requires a continuity test. It does not specify that a resistance value should be determined. Test equipment to BS EN 61557 or a suitable alternative should be used. I have said before that you can (and I have) construct a suitable tester based around a bell and battery set.

2) BS EN 61557 test equipment often has an allowable operational uncertainty of up to 30% of the fiducial value. This is the case for resistance measurement.

I used to have an avatar that featured Beaker from the Muppetts - the point was if you think you can measure low values of resistance using BS EN 61557 instruments on-site accurately - you really should be wearing a white coat and have Beaker as your assistant .

3) BS 7671 only requires a continuity test, but the EWR requires that earthing arrangement have sufficient strength and capability to discharge electrical energy to earth. (Regulation 8).

4) A continuity test does not prove strength and capability (note that a loop impedance test is equally useless in this regard) - you need to use one of the 'use once' test instruments shown in my current avatar for that .

If you are too scared (or many be that should be sensible) to do that you are left with combining the evidence from your tests with a thorough inspection to make your assessment of the earthing systems in the installation.

Regards

Geoff Blackwell
 08 July 2014 09:29 AM
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Parsley

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Geoff

You are correct, but unfortunately the OP probably has a NIC type form to fill in that asks for a R2 or R1+R2 circuit impedance values and probably states at least one column needs to be completed. If the OP's using a software package it won't like a tick it will want a numerical value to be entered.

I believe Andy was trying to explain the need to be careful when using R1+R2 to prove polarity and the need for inspection as well as testing.

Regards
 08 July 2014 09:38 AM
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leckie

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Hi Geoff

So that's for testing continuity of the CPC, etc. But we are required to record a value of Zs for max disconnection times of CPD's, and one method is to measure the value of R1+R2 and add it to Ze. So if you are using that method, as opposed to direct measurement, then the value of R1 and R2 are required. So the R1+R2 test may not be just to test for the continuity of the CPC.

Edited: 08 July 2014 at 10:15 AM by leckie
 08 July 2014 10:11 AM
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AJJewsbury

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The purpose of the R2 test, or the R1+R2 test, is very simple. It is to prove the continuity of the cpc.

What if the c.p.c. is been crossed with L? What will the R1+R2 test show? Does it really prove c.p.c. continuity on its own? How do you show that Zs will be adequate when you're in a situation where the permanent supply isn't available during testing? (e.g. mobile unit or new development where the supplier requires a 'dead tests' certificate before connection). As leckie pointed out, Ze + R1 +R2 is an acceptable approach.

BS7671 - 612.2.1 simply requires a continuity test.

For continuity yes, but the regs also requires us to verify Zs (612.9), and in practice the result of R1+R2 is often used as part of that process (two birds with one stone kind of thinking). For instance one well known organisation recommended doing R1+R2 at every point, but a live loop test only at the point on each circuit with the highest R1+R2 (to minimise live working) - without knowing the value of R1+R2 that process breaks down. Some even suggest always using Ze+R1+R2 and eliminate live working Zs altogether.

Then with the ring circuit's version of the R1+R2 test ((r1+r2)/4), you should consider the measured values as it's unlikely that there'll be any convenient terminals at the exact mid-point of the ring to do a Zs loop test at - the worst-case point of the circuit. (Notwithstanding Geoff's comments about the precision of instruments and so whether at a near-by socket would be 'good enough', in most circumstances at least.)

- Andy.
 08 July 2014 10:16 AM
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leckie

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That's what I was trying to say Andy but you explained it better than me!!
 08 July 2014 11:39 AM
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GeoffBlackwell

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I really am not going through all this again - I have already written volumes on it .

Andy said
(Notwithstanding Geoff's comments about the precision of instruments


The precision of testing methods is key - if you choose to add measured values of R1 + R2 to a measured of Ze - what are your worst case errors - you are making two measurements that can be 30% out - possibly in different directions, and you are adding a resistance to an impedance - naughty in large installations .

Is that good enough to verify shock protection ?

Now that's enough from me - fight it out between you .

Regards

Geoff Blackwell
 08 July 2014 11:58 AM
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AJJewsbury

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The precision of testing methods is key - if you choose to add measured values of R1 + R2 to a measured of Ze - what are your worst case errors - you are making two measurements that can be 30% out - possibly in different directions, and you are adding a resistance to an impedance - naughty in large installations .

Is that good enough to verify shock protection ?

I don't disagree Geoff, but might it be a case of making the best of a bad situation with the tools that are available to us? Is measuring Ze and R1+R2 separately any worse than measuring Zs directly - especially if the latter means using a no-trip setting? To some extent it's fruit-machine numbers both ways. If we already knew the length & c.s.a. of the conductors we could work out their resistance from tables and so would only need to verify they were properly connected on site, but for an existing installation with no proper records and hidden cable runs, what other options are open to us?

- Andy.
 08 July 2014 12:26 PM
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John Peckham

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Andy

I have a proper earth loop impedance tester that comes in a large nice polished wooden box that was given to me by my old NICEIC Area Engineer. I have had a look at it and read the information that comes with it but I am to scared to try it out, it does a high current test for 5 seconds! It will also do a neutral earth loop test.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 08 July 2014 12:33 PM
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GeoffBlackwell

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Andy said
I don't disagree Geoff, but might it be a case of making the best of a bad situation with the tools that are available to us?


Tell that to Mrs Brown when little Johnny is lying dead on the kitchen floor because your best guesstimate proved to be wrong

If its an EICR why pretend that you have verified something when all you have really done is 'have a stab at it'.

As I said above - fight it out between you.

Regards

Geoff Blackwell
IET » Wiring and the regulations » R2

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