Ring Main resistances

[farmerian]

Is there a maximum r1 rn and r2 value for a ring main ?
I no the maximum ZS is 1.10ohms for a 32amp type B curve breaker but is there anything in regs for the end to end resistance of the ring ?

 

[Risteard]

I no the maximum ZS is 1.10ohms for a 32amp type B curve breaker

I presume you mean using the 80% rule of thumb as that is not the maximum Zs for that device ((230 / 32 / 5) * 0.95 Ohms).

 

[Simonslimline]

The 1.10 Ohms is the on site guide value @ 80%. BS7671 value is 1.37 Ohms.

 

[farmerian]

Maybe I got the max zs value wrong for that protective device,, but what I am asking is they anything in regs for end to end values for a ring circuit ?

 

[yellowvanman]

Is there a maximum r1 rn and r2 value for a ring main ?
I no the maximum ZS is 1.10ohms for a 32amp type B curve breaker but is there anything in regs for the end to end resistance of the ring ?

Haven't got Yellow OSG with me at the moment, but Table 7.1 (i) of the Green OSG gives some maximum cable lengths for ring circuits with varying protective devices, so in effect giving max r1, rn & r2. I guess the Yellow version values with differ from the Green book due to the new 0.95 factor!

 

[uksparks]

Well, I'd say your Zs, so comply would have to be say 1.1 or less, so I suppose the worst case scenario for R1+R2 would have to be no more than 1.1 - 0.35 which would be the lowest out of the supply types.

i suppose this is hypothetical and calculated opposed to measured anyway. So suppose you did that you R1+R2 would have to be no more than 0.75,

im probably totally wrong but I suppose if you were to calculate it on paper for a TNCS supply type I'd say your max figures would be the following based on 2.5/1.5

R1+R2 = 0.75
r1 = 0.56
rn = 0.56
r2 = 0.94

its only a stab in the dark and probably wrong.

 

[polo1]

Recommended max floor area for a RFC is 100m2 iirc. There is no maximum figure for r1,r2 etc as such, as ultimately, it's achieving less than the max Zs which will be the determining factor.

 

[Risteard]

Recommended max floor area for a RFC is 100m2 iirc.

That was simply a historical requirement. The 17th Edition removed it so there is no 100m^2 maximum anymore.

As said if the circuit is designed for 0.4 second disconnection and non-excessive voltage drop then that is the limit.

 

[uksparks]

Just to add to my post, I think off the top of my head 2.5 has a resistance of 7.41 mopm I think...

so so take my calculated R1+R2 of 0.75 ohms x 1000, divide by 7.41 that gives you 75m

 

[Lucien Nunes]

and probably wrong.

You're out by a factor of two. The maximum value of r1+r2 cannot be more than 4 times the maximum R1+R2, because the highest resistance will occur halfway round the ring where it is served by two legs of half the total length in parallel, each of (r1+r2)/2.

 

[farmerian]

Thanks for all the replys

 

[uksparks]

You're out by a factor of two. The maximum value of r1+r2 cannot be more than 4 times the maximum R1+R2, because the highest resistance will occur halfway round the ring where it is served by two legs of half the total length in parallel, each of (r1+r2)/2.

Oh yes, why did i divide it by 4? No idea, I know it's two. Thanks for the correction.

i was on the right lines but out if that makes sense.

 

[Pete999]

Mr Pedant on sorry ring main??? RING FINAL CIRCUIT RFC