Testing with a CU change

[Welchyboy1]

You can not do R1 +R2 = Zs -Ze as for the parallel paths

If we are talking domestic, mostly twin and earth cabling

You can calculate this fairly accurately, if your in any doubt, if you take your ZS then remove your ze, then have an approximate idea of cable run you can look at the resisstance per meter


Rings are easier to confirm with the exception of a long spur from the furthest point

i only usually find parallell paths in a domestic setting on immersion circuits/ring circuits with heating/boilers etc on and houses with supp bonding

Commercial/ind jobs using a lot of metal work are of course
another matter

Obviously r1 and r2 on TT systems domestic or otherwise i would measure in all circumstances

 

[Toneyz]

Gets easier the more you use it

Don't like the emoji's however there is no bad spelling Pete

 

[DPG]

And where's the dumb emoji? Only kidding, calm down.

 

[Charlie_]

Calm down

 

[HappyHippyDad]

It definitely doesn't feel write to determine R1+R2 by subtracting Ze from Zs.
From a mathematical point of view it works just fine and is 100% correct with respect to rearranging the equation Zs = Ze + (R1+R2), but your Zs is influenced by other earthing paths that your Ze is not influenced by so you could get a biased answer.

Although... that would be true of the initial equation as well! Ze could be 100Ω on a TT. Zs could be much lower than that if the bonding conductors have a good pathway to earth, ∴ in this case Zs ≠ Ze + (R1+R2).

I enjoyed all those mathematical symbols, it's been a while! I miss good old fashioned maths where the answer is black and white.

 

[HappyHippyDad]

#
Sorry the wrong way around ZS = ZE (R1+R2) must leave the red alone sorry Ant you are right my apologies.

That is also wrong Pete. Ze(R1+R2) means Ze x (R1+R2). Pedantic I know but I like maths to be correctly written, not too worried about englush!

 

[Charlie_]

I am also not a fan of determining R1+R2 using Zs.
It’s useful, yes but also misleading..
IMO R1+R2 is to determine the resistance of the respective cables. Furthermore, if the length of the circuits needed to be verified then using Zs - Ze would give incorrect values.

 

[Pete999]

That is also wrong Pete. Ze(R1+R2) means Ze x (R1+R2). Pedantic I know but I like maths to be correctly written, not too worried about englush!

OK, ZS=ZE+(R1+R2) You pedant you, but correct

 

[telectrix]

I thought zs=Ze+ (R1+R2)...but Ze does not = Zs-(R1+R2) or something like that possibly maybe......I’m feeling really uncomfortable with the new look.....the only other time I’ve felt like this was when they spruced up my local! Took me ages to get over that....

they done same with my local back in the 90's. knocked down walls, stables, ripped out woodworm infested bar and all the grime. refurbished into a food orientated, child friendly, monstrosity. what used to be nice friendly pub where I got barred on the Friday and reinstated on the Monday ( massive drop in takings) then became a plastic copy of what a pub might be.

 

[Flanders]

How do you calculate your R1+R2 as that should only be done by measurement only other than at the design stage?

Agree But you also don't calculate R1+R2 and design stage either , you calculate Zs from by inquiring what the Ze will be and adding that your R1+ R2 readings, As others have said you can't back calculate the R1+R2 but I believe loads sparks who do it, As for the ops question, I do all the test except Zs which I calculate .

 

[Ian1981]

Agree But you also don't calculate R1+R2 and design stage either , you calculate Zs from by inquiring what the Ze will be and adding that your R1+ R2 readings, As others have said you can't back calculate the R1+R2 but I believe loads sparks who do it, As for the ops question, I do all the test except Zs which I calculate .

Of course you can calculate your R1+R2 at design.
It’s not what I would record on the EIC as I would measure it during initial verification however when deciding what cables to use then using the mv/a/m values at 20 degrees and multiplying by 1.2 will give me a ball park figure of design, added to the Ze I can see what my Zs should be for compliance with disconnection times

 

[TJ Anderson]

Agree, we can and do just that. I'm sure you meant the OSG Table L1 (R1+R2)/Metre Values as opposed to mV/A/m for the calculated value though

 

[Ian1981]

Agree, we can and do just that. I'm sure you meant the OSG Table L1 (R1+R2)/Metre Values as opposed to mV/A/m for the calculated value though

Yep not the volt drop mv/A/m oops

 

[Richard Burns]

Though Volts/ amps does give resistance so they are valid figures for resistance!!

 

[magnoliafan89]

I always do zs but dont bother filling in R1+R2 unless i actually test it. I never see the point people calculating R1+R2 or even zs on a test form if you dont test dont bother.
I know lots of people do but meh.

Was taught the calculation at college and nowadays at the am2 you have to calculate the R1+R2 before actually then testing by using the formula R1 reading + the R2 reading and dividing by 4.

 

[TJ Anderson]

Though Volts/ amps does give resistance so they are valid figures for resistance!!

Yeah, definitely. I typo'd too, meant table I1.

If you take any of the mV/A/m values and ÷ 2.4 it will give you the values of (R1 + R2) / Metre in Table I1.

 

[Richard Burns]

Yeah, definitely. I typo'd too, meant table I1.

If you take any of the mV/A/m values and ÷ 2.4 it will give you the values of (R1 + R2) / Metre in Table I1.

I had never actually thought of it like that but that is much simpler!
I looked at that and thought what is he talking about then actually thought and went, oh yeah!
I was thinking volt drop is over the length of circuit for both line and neutral and at 70°C, so twice the length so I can take the R1+R2 values for same size cpc and line in table I1, or divide by 2, however I need to correct from 70°C to 20°C so also divide by 1.2.
However using the single conductor value in I1 and multiplying by twice 1.2 (2.4) or vice versa is a lot more straightforward.