Ring Final

mapj1:

The assumpmtion is about bunching. The worse case is when all the load is near one end, say within the first 10% of one of the limbs.

By the miracle of resistance scaling with length of cable, and the voltage drop from origin to load being the same both the long and short way round, the current split is always in inverse proportion to the ratio of the lengths.


So if the load is equally far from each end, then the load is shared perfectly, and 16A goes each way.


But let us consider an example of the bad layout case..


It is possible to have say 3 sockets each pulling 10A  at 1m, 2m and  5m from the origin and then a 20m return leg with no load.


Now the first 10 amps is spit in the ratio  of 1/25 to 24/25, those being the tow path lengths to that socket.

The second 10A is split 2/25 to 23/25

the third 5/25 and 20/25


So as the common denominator is 25ths of 10A is 400mA 'units'

Going left we have (24+23+20)* 0.4A  = 26.8 amps

going right we have  (1+2+5)* 400mA = 3.2A

Quick sanity check total = 30A.

So that last metre of cable from CU to first socket  takes nearly 27A, and would be fine in plaster or clipped direct, but may age rather faster than we would like in thick insulation. It is the only stressed cable on the ring, and making it longer,  or moving one load to the return leg would restore order, as would making that first socket a spur off the MCB, so its current is not adding to the short leg current.

(This is often a quicker fix)

If you can avoid having more than one socket in the first 20% of the ring length on either end, the the problem vanishes even in quite thick insulation.

Mike.



 

At the risk of appearing to be a dissenter, I can not believe that most sparks installing a ring final even bother to consider the positioning of sockets regarding equal loading. Has unequal loading even been shown to cause cable thermal damage?


Having just re-read 433.1.204 I have concluded that the reg. is not about bunching of sockets or having them too close to the origin of the ring final circuit. The reg. is concerned with ring final circuits with or without unfused spurs. That is why the reg. requires that the load current in any part of the circuit (in ring or unfused spur) is "unlikely to exceed for long periods the current carrying capacity of the cable." This is especially important for unfused spurs. That is the main point being made.


I do seem to be a dissenter. Oh ek!


Z.




 

mapj1:

The assumpmtion is about bunching. The worse case is when all the load is near one end, say within the first 10% of one of the limbs.

By the miracle of resistance scaling with length of cable, and the voltage drop from origin to load being the same both the long and short way round, the current split is always in inverse proportion to the ratio of the lengths.


So if the load is equally far from each end, then the load is shared perfectly, and 16A goes each way.


But let us consider an example of the bad layout case..


It is possible to have say 3 sockets each pulling 10A  at 1m, 2m and  5m from the origin and then a 20m return leg with no load.


Now the first 10 amps is spit in the ratio  of 1/25 to 24/25, those being the tow path lengths to that socket.

The second 10A is split 2/25 to 23/25

the third 5/25 and 20/25


So as the common denominator is 25ths of 10A is 400mA 'units'

Going left we have (24+23+20)* 0.4A  = 26.8 amps

going right we have  (1+2+5)* 400mA = 3.2A

Quick sanity check total = 30A.

So that last metre of cable from CU to first socket  takes nearly 27A, and would be fine in plaster or clipped direct, but may age rather faster than we would like in thick insulation. It is the only stressed cable on the ring, and making it longer,  or moving one load to the return leg would restore order, as would making that first socket a spur off the MCB, so its current is not adding to the short leg current.

(This is often a quicker fix)

If you can avoid having more than one socket in the first 20% of the ring length on either end, the the problem vanishes even in quite thick insulation.

Mike.



 

At the risk of appearing to be a dissenter, I can not believe that most sparks installing a ring final even bother to consider the positioning of sockets regarding equal loading. Has unequal loading even been shown to cause cable thermal damage?


Having just re-read 433.1.204 I have concluded that the reg. is not about bunching of sockets or having them too close to the origin of the ring final circuit. The reg. is concerned with ring final circuits with or without unfused spurs. That is why the reg. requires that the load current in any part of the circuit (in ring or unfused spur) is "unlikely to exceed for long periods the current carrying capacity of the cable." This is especially important for unfused spurs. That is the main point being made.


I do seem to be a dissenter. Oh ek!


Z.