R1 + R2 expected readings

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That's a pretty long and varied length of string question...... you're going to have to narrow that down!!
 
ethernet said:
Pg 218 of your onsite guide. Length of circuit x resistance ÷ 1000 will give you a rough idea of your R1/R2. Just pace out your run
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In most cases that is your starting point. If you just installed a circuit then you should know (at least roughly speaking) how long the run was so multiply that by the table values for the choice of cable and your readings ought to be pretty close to that.

If you don't know as doing EICR, etc, then from your R1+R2 measurement divide by the table value and see what length it gives you. Does it look sane for the property? Admittedly there could be bizarre routes that are longer (or even shorter, perhaps just appearing so if some parallel earth is present) than you might expect to be used, but if gives you a good starting point.

If it looks vaguely sane then add the (ideally) measured Ze (or at least the typical limits of 0.35 ohm / 0.8 ohm for TN-C-S / TN-S) and compare it to the OCPD Zs values to get an idea if it is likely to meet the disconnection time.
 
Thanks for the replies
Does anyone have a photo of table in the on site guide as I’ve misplaced my book so need to get a new and only have the 18th edition blue book at minute
My new company only require us to do dead testing only where as im used to doing both dead and live and been able to see if R1 + R2 is right by the live the testing side of things.
 
My meter wont work for live testing of R1/R2! Anyway before testing R1 or R1/R2 I always tell any trainess with me to calculate if first so that you know what you should expect before you begin testing. If there is a significant difference you know something is wrong. You can easily google resistance table for copper cables. Calculating also applies to RFC tests i.e. r1 r2 and rn as well as R!/R2
 
Vortigern said:
My meter wont work for live testing of R1/R2! Anyway before testing R1 or R1/R2 I always tell any trainess with me to calculate if first so that you know what you should expect before you begin testing. If there is a significant difference you know something is wrong.
Click to expand...
Good advice, as you might have an install fault or it might be as simple as MFT probes not correctly nulled and if you know what to expect, you know when to double-check things.
 
Lt1990 said:
and only have the 18th edition blue book at minute
Click to expand...
It's possible but rather hard work from the blue book (see page 409)

Volt drop per meter is given in miliVolt / Amp / metre
Volts = Current X Resistance.
Resistance = Volts / Current.

So that volt drop number is also resistance / meter. But it's for Line and Neutral and you want Line and Earth. It's also not temperature corrects.

So for 2.5mm twin and earth....

Take the 2.5 number (18), divide by 2 as you only want one core's worth of resistance.
Then take the 1.5 number (29) and also divide by 2.

Add that all together. Then divide by 1.2 (temperature correction). That then gives you the number in the OSG

The MUCH easier way is to invest in buying the ProCerts app. Seriously recommended to the degree that I barely use the OSG.

Lt1990 said:
im used to doing both dead and live and been able to see if R1 + R2 is right by the live the testing side of things.
Click to expand...
It's early and I'm not caffeinated but that is not making too much sense. The common factor to both tests is the resistance of the cable, with or without faults. Maybe I'm missing you're point.
 
Vortigern said:
My meter wont work for live testing of R1/R2! Anyway before testing R1 or R1/R2 I always tell any trainess with me to calculate if first so that you know what you should expect before you begin testing. If there is a significant difference you know something is wrong. You can easily google resistance table for copper cables. Calculating also applies to RFC tests i.e. r1 r2 and rn as well as R!/R2
Click to expand...
What calculations do you use
 
You can calculate from resistance of copper tables the expected rn and expect that r1 will be about the same then multiply by 1.67 in the case of modern cable (2.5/1.5=1.67) so a reading r1 = 0.60 ohm becomes 0.6 x 1.67=1.002 therefore you expect around 1 ohm reading then when crossing over E to L and vice versa r1 + r2 = 1.6 ohm / 4 = 0.40 apart from that as above estimate size of RFC or radial use tables and multipy etc. as above. I usually get the assistant/trainee to do all this in their head, no calculator and most of the sizes we use, the resistance tables are remembered. So typically 2.5 T&E is 19.5 mohm round it to twenty times the m. estimated i.e. 20m 20 x 20 =400/1000 = 0.40 roughly, close enough for the purpose.
 
Last edited:
timhoward said:
It's possible but rather hard work from the blue book (see page 409)

Volt drop per meter is given in miliVolt / Amp / metre
Volts = Current X Resistance.
Resistance = Volts / Current.

So that volt drop number is also resistance / meter. But it's for Line and Neutral and you want Line and Earth. It's also not temperature corrects.

So for 2.5mm twin and earth....

Take the 2.5 number (18), divide by 2 as you only want one core's worth of resistance.
Then take the 1.5 number (29) and also divide by 2.

Add that all together. Then divide by 1.2 (temperature correction). That then gives you the number in the OSG

The MUCH easier way is to invest in buying the ProCerts app. Seriously recommended to the degree that I barely use the OSG.


It's early and I'm not caffeinated but that is not making too much sense. The common factor to both tests is the resistance of the cable, with or without faults. Maybe I'm missing you're point.
Click to expand...
i use easy certs and the old version I had at my old company has to highlight high readings in red but this current version doesn’t do it
And in regards to testing I am only doing dead tests basically so would have liked a table to just check results
 
F144AE05-A63C-4724-9E36-CF8EBECFD68C.png

Could I use this table
 
No don't use that table, it;s ok for reference but you need to use the resistance tables, google them. The whole point being is to work out for yourself the expected measurements as a reference point. When I put in a RFC I know how much cable I have used so say it was 63 m I calculate off of that. Those charts are sort of end user you need to get into the mechanics of how they made those charts.
 
I've already explained (or tried to) how to work this out for yourself. If you don't understand, then feel free ask more questions.

Again, I'd highly recommend getting an app.
The one (and several other people on here) use has simple tables that are accurate, and also the max Zs values, among many other helpful things.
One off payment of £18.

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buzzlightyear said:
say we have R1+R2 = 2.35 ( hypothetical figure )

R2 = 1.67 x R1 ( 2.5/1.5mm )

so R1 + ( 1.67xR1 ) = 2.35

( 2.67xR1 ) = 2.35

R1 = 2.35/2.67 = 0.88

R2 = 2.35 - 0.88 = 1.47

( or R2 = 1.67 x 0.88 =1.47)
Click to expand...
Buzz - congratulations my friend! That's the clearest and most coherent post you have ever made on this forum ;-)
 
Vortigern said:
No don't use that table, it;s ok for reference but you need to use the resistance tables, google them. The whole point being is to work out for yourself the expected measurements as a reference point. When I put in a RFC I know how much cable I have used so say it was 63 m I calculate off of that. Those charts are sort of end user you need to get into the mechanics of how they made those charts.
Click to expand...
I am doing eicr tests so it will be just a rough guess to how much cable has been used for calculations
 
How much would yous let the reading be over before you were to investigate it further as obviously a number of things could effect readings but for like a radial socket in a house on 16amp carrying 6 sockets say the reading was just over 1ohm (just a random figure) would yous want to investigate that further for been high reading
 
Lt1990 said:
How much would yous let the reading be over before you were to investigate it further as obviously a number of things could effect readings but for like a radial socket in a house on 16amp carrying 6 sockets say the reading was just over 1ohm (just a random figure) would yous want to investigate that further for been high reading
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If you had a Ze of 0.75 ohms and an R1+R2 of 1 ohms for a circuit protected by a B16 breaker, would the supply disconnect in under 0.4 seconds under fault conditions?

ethernet said:
And thats all the op needs, is a reference point. If you pay for the procerts app, they use the same resistance values in their tables to use as a quick calculation onsite.
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In this case I respectfully disagree, as the OP is not just 1st fixing and dead testing, he is carrying out EICR inspections. A good understanding of electrical theory is very important to determine what is safe and what isn't.
 
What are you measuring and getting the figure 1 ohm From the circuit you descibe I would expect something around .3-.6 ohm max. I would expect age and installation technique would vary that figure if it was 1 ohm fine (ish) but would get my spider sense running looking for poor termination at sockets for instance. The critical figure at each socket would be the Zs and its compliance for ADS and if the 1ohm figure was still there for say R1 it is unlikely I would work through all the sockets to re-terminate not enough money or time for that.
 
timhoward said:
In this case I respectfully disagree, as the OP is not just 1st fixing and dead testing, he is carrying out EICR inspections. A good understanding of electrical theory is very important to determine what is safe and what isn't.
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Okay, if i had a radial circuit wired in 6mm/6mm swa and the measured R1/R2 was 0.16ohms. What table or resistance value would you personally use to calculate the length of that circuit as a reference point, and what length would you calculate?

Since you disagree with using the onsite tables and the procert one, im curious what you would use?

Im assuming the op is an approved electrician if he's doing eicrs, so should have a sound knowledge of electrical theory
 
ethernet said:
Okay, if i had a radial circuit wired in 6mm/6mm swa and the measured R1/R2 was 0.16ohms. What table or resistance value would you personally use to calculate the length of that circuit as a reference point, and what length would you calculate?

Since you disagree with using the onsite tables and the procert one, im curious what you would use?
Click to expand...
I think we have ended up slightly at cross purposes. I have no issue with anyone using OSG, BS7671, Pro Certs, or any other reliable reference of the resistance of copper in order to work out expected R1+R2 values.

My emphasis was intended to be on the "all the OP needs".
I'm a little bit concerned that anyone is having to undertake EICRs and asking some of the questions above, apparently without being provided with the latest regs book or OSG.
I don't mind using this forum to further my own understanding and in turn to try and help other people increase their understanding, but sometimes it feels like people are put in impossible situations by their employers and are expected to run before they can walk.
 
Two things concern me about this thread:
1. The OP is conducting EICRs but not doing any live testing.
2. The OP is conducting EICRs without knowing basic things like how to calculate R1 and R2 values.
3. Sorry there is no third thing
 
loz2754 said:
Two things concern me about this thread:
1. The OP is conducting EICRs but not doing any live testing.
2. The OP is conducting EICRs without knowing basic things like how to calculate R1 and R2 values.
3. Sorry there is no third thing
Click to expand...

The third thing for me is that the OP doesn't seem to see the problem of the first two points.
 
I have a good understanding on electrics and testing and what I should be looking for and testing
The company I now work for only require us to do dead testing and then calculate the zs readings
All I was wondering is if they was a table about I could use as a guide for r1 r2 readings so I know my readings are correct or slightly high and I need to do abit of further investigation on circuits
Previously I’d do dead and live testing and I’d be able to check check the r1 r2 results by the zs readings
 
If I had my own way I’d be doing dead and live testing but when the company doesn’t want us doing it and to just calculate zs results then I’m going to do what I’ve been told to
loz2754 said:
Two things concern me about this thread:
1. The OP is conducting EICRs but not doing any live testing.
2. The OP is conducting EICRs without knowing basic things like how to calculate R1 and R2 values.
3. Sorry there is no third thing
Click to expand...
 
Does nobody do any live testing?
Who signs the certs?
 
Lt1990 said:
All I was wondering is if they was a table about I could use as a guide for r1 r2 readings so I know my readings are correct or slightly high and I need to do abit of further investigation on circuits
Click to expand...
For EICR purposes, the two things that come to mind are how close the calculated Zs is to the limit for the device, and a sense of how big the property you are in is and whether a reading seems plausible. The more you do the easier that gets.
e.g. if you're re in 2 up 2 down and you get 1.5 ohms for an immersion heater circuit then a bit of head scratching is due.

The other thing you can do is try RN+R2 or use a wander lead to obtain a 2nd frame of reference, and whether it's R1, R2 or both that have a loose connection.

I'm assuming that you are allowed to do Ze, Zs@DB and RCD tests, and your company only prohibits live Zs readings?
 
This is a 'modern' take on Zs testing that you don't do it live except for socket outlets where there is no risk of exposed live parts but as @timhoward asks do you carry out Ze and rcd tests.
 
timhoward said:
For EICR purposes, the two things that come to mind are how close the calculated Zs is to the limit for the device, and a sense of how big the property you are in is and whether a reading seems plausible. The more you do the easier that gets.
e.g. if you're re in 2 up 2 down and you get 1.5 ohms for an immersion heater circuit then a bit of head scratching is due.

The other thing you can do is try RN+R2 or use a wander lead to obtain a 2nd frame of reference, and whether it's R1, R2 or both that have a loose connection.

I'm assuming that you are allowed to do Ze, Zs@DB and RCD tests, and your company only prohibits live Zs readings?
Click to expand...
Yea I generat know what I’m looking at what seems high and things but I just wanted something to work towards incase I was been slightly generous and over cautious chasing faults that didn’t really need further investigation.

And yea so we do ze zsdb And rcd tests
It’s literally just only zs readings that we can’t do
 
ipf said:
This is so basic for someone qualified. Whether dead or live testing, you should know the methods of carrying them out (and working them out) without thinking.
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I do know the methods for carrying them and things,
I was just asking to know so I knew if I was over cautious or to generous when it came to the readings
 
You are either going to be carrying Zs on lights or sockets principally. Spurs come into it as well. I always do live tests at sockets, spurs and lights. I use the lightmate by Kewtech for Zs test. On courses it has been put to me this is a Lim, as you have to go up a ladder and H&S forbid this in many scenarios. I just get on with the job and test it all. When I put the Zs reading for each circuit I like to be sure I did actually read it with my MFT, I can't imagine any other way to operate. Maybe I am missing something, but I don't think so. Convince me! I mean how do you fill in the forms? Yes calculation, but I am not convinced that is kosher. For instance you can find that there are big variances between R1/R2 + Ze calculation and Zs measured with an MFT.
 
Vortigern said:
You are either going to be carrying Zs on lights or sockets principally. Spurs come into it as well. I always do live tests at sockets, spurs and lights. I use the lightmate by Kewtech for Zs test. On courses it has been put to me this is a Lim, as you have to go up a ladder and H&S forbid this in many scenarios. I just get on with the job and test it all. When I put the Zs reading for each circuit I like to be sure I did actually read it with my MFT, I can't imagine any other way to operate. Maybe I am missing something, but I don't think so. Convince me! I mean how do you fill in the forms? Yes calculation, but I am not convinced that is kosher. For instance you can find that there are big variances between R1/R2 + Ze calculation and Zs measured with an MFT.
Click to expand...
@Lt1990 is only carrying out works in the way as suggested by his company.
 
westward10 said:
Have you ever asked why seems like someone has carried out a risk assessment and deemed it is not safe.
Click to expand...
Where I previously worked we carried out full tests but I’ve only recently started for this place and from what I can gather it’s literally just down to them not wanting us to work live.
They are health and safety mad
 
Vortigern said:
You are either going to be carrying Zs on lights or sockets principally. Spurs come into it as well. I always do live tests at sockets, spurs and lights. I use the lightmate by Kewtech for Zs test. On courses it has been put to me this is a Lim, as you have to go up a ladder and H&S forbid this in many scenarios. I just get on with the job and test it all. When I put the Zs reading for each circuit I like to be sure I did actually read it with my MFT, I can't imagine any other way to operate. Maybe I am missing something, but I don't think so. Convince me! I mean how do you fill in the forms? Yes calculation, but I am not convinced that is kosher. For instance you can find that there are big variances between R1/R2 + Ze calculation and Zs measured with an MFT.
Click to expand...
I agree with you they is differences from calculations and actually measured results and I use to always do live tests at previous company
But I’m just following what this company requires us to do and that is calculations for zs results and not to do live testing apart from ze and rcd tests
 
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