How did they calculate this ?

[Gerhard.Oosthuizen]

It appears I need some work on this and what I was taught in my apprentice days may not be correct.... anyways,

So here is my process... back to basics so here is a every day sorta situation...

A 2.5mm flat twin and earth cable which has a 1.5mm CPC (Earth) on a 20A CB. How long can I run this cable? ( assume a 10A load at the end)

If i do the Rmax = v / 2 x I I get 5.75 ohms.
If I then apply it to Recc = p x L / A
(A being 1.5mm˛ as doing line to E fault) I get a length of 386 meters...

If I do the Touch Vtage formula so as to keep it to 30V ( in book says do calcs at 30V )

TV= 2 x I x R
30 = 2 x 20 x R
30= 40 x R
R = 0.75 ohm

Then again the Recc formula to calc length
Recc = P x L / A (A being 1.5mm˛)
0.75 = 0.0223 x L / 1.5
Legth = 50meters

Then on page 168 of Ed 3.0 has the formula for the max length of the Ecc (CPC, Earth)
And when i do that with the 0.8 factor out i get 48meters (closer to the TV side of 50 meters)


Then the bloody table above also has very low Ohm ranges haha.

So anyone wiser than me on this know the best answer or method to determine that question ?

Also in section 8 it says that if the Recc can not be checked (long wonder lead) then the method for the loop test must be done which then included the Rmax formula which is 5.75ohm, I know it then has a loop so "double distance) and the phase and Earth are diffenrft CSA so not sure how to get that to a single Earth restaince number to try do the math...

I am down a rabbit hole at the moment with all this as need to prove to current boss that there may be an issue in the design but I want to make sure I am not to far gone and lost...

Edit: Vd excluded at this time, as could be a 5A load but this is just for some math and getting out the rabbit hole.


Edit again: had a brain wave at 1 this morning when small was awake that the TV formula I use is the same as the Rmax formula.... so the diffenrce then being the voktage of either 230v or 30v
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This was one of my previous queries. Touch voltage is now 25 volt as per SANS 10142-1:2020. Edition 3.
This tables with the reduced ohm values is due to this lower touch voltage.
You will find it at 8.6.5.4 of the code mentioned above. 25 volt touch voltage under short circuiting conditions not exceeding a period of 5 seconds.
Thanks
Gerhard

[Dylboy]

This was one of my previous queries. Touch voltage is now 25 volt as per SANS 10142-1:2020. Edition 3.
This tables with the reduced ohm values is due to this lower touch voltage.
You will find it at 8.6.5.4 of the code mentioned above. 25 volt touch voltage under short circuiting conditions not exceeding a period of 5 seconds.
Thanks
Gerhard

Yes ya saw that post earlier and see it is now 25V, thanks for that thread as can now do it better.

Had discussions with boss awhile ago how in the real world must not exceed 25V but when calculating It is 30V and always thought that was wrong.

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