question about ring calculation...

[ricicle]

The R1+R2 value is only exactly 1/4 of the r1+r2 value if the conductors are of equal CSA. If they are not then it is approx. 1/4.

No - it's always ¼.

What you measure at any point when cross-connected for testing is indeed only approximately ¼, but when not connected like that, i.e. in normal use, R1+R1 will vary dramatically, just like it would with a radial, depending on how close you are to the origin.

But the value you need to know is the worst case, which will be at the mid point of the ring and there R1+R2 is exactly (r1+r2)/4 no matter how different r1 and r2 are.

I was talking about the tested values. r1 added to r2 will always be 4x the cross connected values of R1 +R2 if they are the same CSA. If not then it is only approx although the difference is linear.

 

[ban-all-sheds]

But the context of this thread, i.e. what it started with, was R1+R2 in the context of fault loop value, calculated from the r1 & r2 values of a known length of 2.5/1.5 T&E to check if it would be too high for a B32.

And that is (r1+r2)/4...

 

[ricicle]

Perhaps I was off on a tangent............

 

[ericmark]

Page 258 section 6.1 does give a formula for correction of the mV/A/m which will give you slightly more length
Using tp = 70
Ca = 1
Cg = 1
Lb = 26 (That’s 20 amp centre point and 12 amp even spread)
It = 40
Then Ct = 0.923
So permitted volt drop = 11.5 volt and volt drop per meter = 0.018 x 0.923 = 0.016614 dividing this into 11.5 volts gives 26.62259 times 4 = 106.4904 meters maximum 2.5mm cable allowed in a ring main.
As to TN-S calculations with 30ma RCD the length is so high not even worth working out?