R1 + R2 expected readings

[Lt1990]

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

 

[Rockingit]

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

 

[ethernet]

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

 

[ethernet]

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]

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]

Just pace out your run

Baby feet of giant. Or mouse feet in mm.

 

[ethernet]

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]

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]

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]

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]

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]

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.

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]

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]

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

What calculations do you use

read above.

 

[Vortigern]

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.