Expected R1+R2 for a spur

[RM11541]

hi all,

quite a common situation i imagine. and the question out of curiosity, more than anything else.....

if you have a ring-final, the expected resistance at any socket outlet on the ring should be approx (r1+r2)/4...yes? assume you're planning to run a spur off an outlet, how would you estimate the new R1+R2 (obviously at the spur)? is this just (r1+r2)/4 + (tabulated R1+R2 {osg table i1} x length of spur circuit)?

if not that simple, how do folks estimate max run for zs to still be acceptable?

cheers,

rm.

 

[telectrix]

sounds about right,but i've not fully awake yet.

 

[SparkyChick]

Hi rm,

Yes, that formula looks about right for calculating R1+R2.

But the max Zs you are aiming for will depend on how the spur is connected to the ring.

Direct connection and your target max Zs will be that of the OCPD supplying the ring, but if you have a fused connection unit, the max Zs limit is that of the fuse inside the FCU.

 

[RM11541]

ahhh ok.....so having the fcu allows for a much higher zs by the looks of it.

 

[SparkyChick]

Adding an FCU can alleviate concerns about Zs breaching it's limits (the same logic applies to distribution circuits), but don't forget there are other factors that will affect the maximum cable length.

 

[Richard Burns]

Remember that the (r1+r2)/4 is the maximum R1+R2 at the mid point of the ring, as you get closer to the DB the resistance will drop. Therefore if you had a short spur from the first socket on the ring then the R1+R2 may well be lower than at the mid point of the ring.
Calculating the actual R1+R2 of the spur would require you to know the lengths of cable in the different legs on the ring at that point as well as the length of the spur.
Probably easier just to measure Zs?

 

[RM11541]

thanks all for the advice...

appreciate that there are other factors that might affect cable length....i'm assuming main concern would be voltage drop?

if a spark had to install a new spur off an existing ring-final, he/she would want to be reasonably sure (before starting work) that the new install would be able to meet regs.

so in this sense, the estimating of R1+R2 (rather than measuring) is slightly academic.

 

[Richard Burns]

Volt drop would be a concern if the spur was a long length. You would be looking at something like a 30m length of cable from db to end of spur. It is recommended that a spur should be no more than 1/8th of the end to end length of the ring, so in general it should be OK for anything within that range but a spur run out from the midpoint of a large ring could pose problems.
For a new ring with a spur then you would know the lengths of cable and be able to calculate the expected Zs, for a new spur on an existing ring then measuring Zs at the point of the spur and adding the R1+R2 of the spur would give you a reasonable idea.