Sorry ricicle, but there is no "return leg" on a ring main, you have to calculate the cable size required for the length of the run, having more sockets in-line does not increase the load carrying capabilty of a cable in any particular direction.If your ring main were 50metres long with sockets every 2 metres it would be no different than having 1 socket at 5 metres, 1 at 7m, & the last 1 at 10m. It is hardly ever practical to equally split up sockets in lengths apart.ricicle said:jj4091 said:How can you balance the current in a ring main when you don't know what appliances are going to be connected to it, thats the whole idea of it surely.
True, but you need to make the balancing of load on each leg possible by ensuring that as close to the same number of sockets are on each leg rather than having all the sockets on one leg and then a long return leg from the furthest socket.
It is hard to gain an exact idea of loading but you have to make some assumptions.
Long live 4mm² radials!
Surely engineering in ring 1st / last sockets to have similar lengths back to the CU isn't required by virtue of the ring.
Mucky get.JohnD said:I just like rounded things.
(o)(o)
No it is not, the proportions of current flowing down each leg are proportional to the position on the ring.Chri5 said:Socket 1 demand will take the route of easiest resistance, but is balanced by virtue of the ring.
When I asked davy owen in my original post if this was a new theory, it was a serious question, not being flippant. This is a different electrical theory than I was taught almost 50 years ago, which was that current drawn on each leg of a ring main was equal. In other words if you connected an ammeter in each leg of a ring main at the consumer unit, you would measure the the same current being drawn on both meters. I can accept what you are saying may be true for a fraction of a milli-second at switch on, but after that then sorry I find it difficult to understand your theory. Which is likely to be the the leg with the higher resistance a 20m length of continuous cable, or a 10m length with several connections at sockets(in practice)?Adam_151 said:Surely engineering in ring 1st / last sockets to have similar lengths back to the CU isn't required by virtue of the ring.
Its not quite as simple as that, its not just about similar end leg lengths, its about complete balence of the rfc (which a radial with an extra return cable added is unlikely to have)
If you have a socket 1/10th of the way along the rfc, then the current will be divided up in a similar way (but 'flipped' around way), with 9/10ths taking the short leg and only 1/10th taking the long leg, you are always going to get this, but if the sockets are spaced as evenly as possible then it'll balance out in the end rougly enough, but if you have them all clustered towards the end of the rfc, then it can almost be as bad as a broken ring (using the idea of 1/10th again, three 10A loads, and we have maxed out one leg to 27A and only have 3A on the other), such a thing can happen if you put the kitchen on a ring shared with other rooms, and the kitchen is close to one of the 'ends' of the ring.
Have a read of this: http://www.iee.org/Publish/WireRegs...s_7671_amendment_1_2002_and_ring_circuits.pdf
The electrical theory of why the currents divide into a radio based on the reciprocal of leg lengths is simply that the resistance of each leg will be in proportion to its length, think about the fact that the voltage at the outlet will be a tad smaller than that at the DB by virtue of volt drop, stick ohms law on each leg and you get I=V/R
I hope I've answered the right question here...
JohnD said:I'd be very much happier to see an experimental measurement with a milliamp clamp meter.
then you were taught by someone who didn't understand basic electrical principles. Ohms law and Kirchhoff's circuit laws have been known for over 150 years.jj4091 said:When I asked davy owen in my original post if this was a new theory, it was a serious question, not being flippant. This is a different electrical theory than I was taught almost 50 years ago, which was that current drawn on each leg of a ring main was equal.
the cable by far! a contact with a resistance as high as a meter of cable would be smoking when run at full load. The fact that there aren't tables for connection stuff in the volt-drop section of the regs clearly shows that properly made connections have negligable resistance.Which is likely to be the the leg with the higher resistance a 20m length of continuous cable, or a 10m length with several connections at sockets(in practice)?
Well i'm actually studying electronic systems engineering but we have a common first year with the electrical engineering people and this a simple application of first year material.C&GStudent said:Isn't there someone here with an electrical engineering degree to clarify what goes on in a ring final circuit?
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