R1 + R2 expected readings

Lt1990 · Nov 4, 2022

Has anyone got a table of expected R1 + R2 readings that give a guide to what readings we should be getting when testing

Rockingit · Nov 4, 2022

That's a pretty long and varied length of string question...... you're going to have to narrow that down!!

ethernet · Nov 4, 2022

Lt1990 said:
Has anyone got a table of expected R1 + R2 readings that give a guide to what readings we should be getting when testin

ethernet · Nov 4, 2022

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

pc1966 · Nov 4, 2022

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

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.

buzzlightyear · Nov 4, 2022

ethernet said:
Just pace out your run

Baby feet of giant. Or mouse feet in mm.

ethernet · Nov 4, 2022

buzzlightyear said:
Baby feet of giant. Or mouse feet in mm.

well, my long legged strides are roughly a metre, but my apprentice has stumpy legs so i usually halve the number he gives me.

Lt1990 · Nov 4, 2022

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.

Vortigern · Nov 4, 2022

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

pc1966 · Nov 5, 2022

Lt1990 said:
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

The resistance of copper did not change with AM2

pc1966 · Nov 5, 2022

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.

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.

timhoward · Nov 5, 2022

Lt1990 said:
and only have the 18th edition blue book at minute

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.

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.

Lt1990 · Nov 5, 2022

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

What calculations do you use

buzzlightyear · Nov 5, 2022

timhoward said:
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

Lt1990 said:
What calculations do you use

read above.

Vortigern · Nov 5, 2022

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.

Lt1990 · Nov 5, 2022

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.

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

Lt1990 · Nov 5, 2022

[ElectriciansForums.net] R1 + R2 expected readings

Could I use this table

ethernet · Nov 5, 2022

Lt1990 said:
View attachment 103371
Could I use this table

As your an electrician. Surely you can answer that yourself!

Lt1990 · Nov 5, 2022

ethernet said:
As your an electrician. Surely you can answer that yourself!

Yea but I’m just after double checking things out so I know I’m following the right stuff as still fairly new to it all aswell

ethernet · Nov 5, 2022

Lt1990 said:
Yea but I’m just after double checking things out so I know I’m following the right stuff as still fairly new to it all aswell

Use this table to work it out

Vortigern · Nov 5, 2022

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.

buzzlightyear · Nov 5, 2022

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)

timhoward · Nov 5, 2022

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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timhoward · Nov 5, 2022

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)

Buzz - congratulations my friend! That's the clearest and most coherent post you have ever made on this forum ;-)

Lt1990 · Nov 5, 2022

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.

I am doing eicr tests so it will be just a rough guess to how much cable has been used for calculations