How do I calculate the resistance of a spur without phisically testing ? ..

[Barrington1990]

Boffins of the electrical world ... How can I calculate the resistance of 48m ring 2.5/1.5 .. with a 8m spur .. I can quiet easily do this without a spur in the equation but , my test papers require my doing so with an 8m spur. . how does this affect the zs tests ... Thanks v much ...

 

[James]

If I were doing that question,

I would first work out the values for the furthest point of the ring, 24m with equal cable lengths of the ring back to supply. (worst case scenario)

assume the spur is taken from that point.

then calculate the values for the spur itself (8m)

resistances in series are simply, added together to find the total resistance.

have a go at it with the above mentioned in mind.

 

[Wilko]

Hi - if I’ve understood correctly - calculate the spur component as if it was an 8m radial of 2.5/1.5 and add that value to the ring plus Ze.

 

[Strima]

Work out the R1+R2 for the ring. Work out the R1+R2 for the spur. Add them together.

Edit: Then add the Ze.

 

[Lucien Nunes]

It doesn't say whereabouts on the ring the spur is connected. If you were simply asked to prove compliance, then provided it complies under the worst-case conditions you can answer that. But they do ask for a value for Zs (which may be highest at the spur) which IMHO makes it a daft question.

 

[static zap]

I take it they are checking you are cautious with "worst case" figures. (rather than being optimistic !)

 

[davesparks]

R1+R2 for the ring should be an easy calculation from the end to end values you have already worked out.
Then add the R1+R2 for 8m of cable to get the R1+R2 at the end of the spur, the location of the spur on the ring doesn't make a difference to this.

Without further information I would give the Zs of the ring and the Zs at the end of the spur assuming it is connected at the mid point.

 

[Deleted account]

From memory table I1 in onsite guide “I think” gives you the resistance values in milli ohms per meter of various size conductors.
once you have that multiply it by length of spur decided by 1000 to give the resistance of spur. Add that to the ring resistance, done.

As others have said not enough info on where the spur is taken from the ring