Why are ring finals split up/down not side/side?

[EFLImpudence]

Yes, but one can't plug more into a 30A circuit than two 15A circuits.

I appreciate that one might be able to plug in more things in one room with the 30A circuit but that doesn't explain why no one else ever bothered to do it.

 

[JohnW2]

....Just multiple 15A sockets on one 15A radial wasn't going to cut it when faced with several multi-kW devices plugged in. So that could mean multiple 15A radials, or multiple sockets on one higher current circuit and taking advantage of demand diversity to plug in more devices than just adding their loads would suggest the circuit could support.

Indeed so - but in terms of that latter option (the one which was implemented) it would just as well be served by a 30A radial (and fused plugs) as by a 30A ring - and, in terms of new-builds or re-wires (when all cable would be 'new'), the radial would often/usually use little, if any, more 'new copper' than would the ring.

If there were any substantial 'new copper saving' due to the introduction of rings, it was presumably be in relation to turning existing 15A radials into 30 rings (thereby using some of the existing copper) - not relevant to new builds/re-wires.

Kind Regards, John

 

[JohnD]

Why, then, did only Britain think it necessary to have 30A socket circuits in domestic properties, the vast majority of which were very small?

having read some of the old documentation from the time, and met some old electricians in the past, it was envisaged that any homeowner might have a 3-bar electric fire as their biggest load. It was equivalent in heat output to a medium-sized coal fire, allowing for waste heat up the chimney. So 13A sockets. They might put it in any corner of any room of the house. So lots of outlets, any of them capable of supplying such a load. In the other sockets they might put an electric sewing machine, table lamp, radio, christmas tree etc. So fused plugs.

A very small number of people might have had a washing machine or an electric kettle, as well as their electric fire, and they might well, from time to time, run two of these appliances at the same time. The chances of anyone possessing three such appliances, and using them all at the same time, seemed improbable. Hence 30A circuits. Which could run two big appliances and as many small one as any reasonable person could hope to own.

Remember that previously, people had been connecting electric irons to light-bulb outlets, so this was a big step forward. Also there had been a variety of different size and shaped plugs for 2A, 5A, 10A and 15A use, which if by different makers might not have fitted the same sockets, and householders were unlikely to grasp the concept of matching appliance load, plug type, and circuit rating.

Many birds killed with a couple of stones with the new system.

The square-pin fused plug and shuttered sockets had numerous other advantages, of course.

 

[EFLImpudence]

Accepted - but why only in Britain?

Everywhere else, cold, must have had the same.

 

[SimonH2]

Yes, but one can't plug more into a 30A circuit than two 15A circuits.

Depending on semantics and how you view practicalities, yes you can. If you have two 15A circuits, you can only run one high power appliance on each of them at a time - so if you have three such appliances then you're back to deciding which to plug in together.
Plug all three appliances into a 30A circuit and then you only have to remember not to use all three at the same time - and in practice you are likely to get away with that anyway. Once you get to larger numbers of appliances, most of which won't be 3kW, then diversity means you can pretty well plug in what you like to the single 30A circuit - but would still be constrained by the 15A circuit.

Indeed so - but in terms of that latter option (the one which was implemented) it would just as well be served by a 30A radial (and fused plugs) as by a 30A ring - and, in terms of new-builds or re-wires (when all cable would be 'new'), the radial would often/usually use little, if any, more 'new copper' than would the ring.

Well one explanation given by someone in this forum is that the plan was you'd run the ring round the core of the house, and any sockets on the outside would be spurs. That way, the ring itself is relatively short for the area covered, and the spurs are no longer than branches off a radial would be.
Given this arrangement, I could well imagine the extra length of smaller cable needed to bring the other end of the ring back to the fuse would be more than offset by reducing the CSA of the "trunk" of the radial. There's also the factor, especially given some of the discussions regarding competence of some people, that the whole circuit can (for most installations) be done with one size of cable without any thought or maths needed on the part of the installer (who could just do the whole lot in 7/029 as long as he didn't exceed the guidelines for area covered).

 

[JohnW2]

... So lots of outlets, any of them capable of supplying such a load. In the other sockets they might put an electric sewing machine, table lamp, radio, christmas tree etc. So fused plugs. .... ....... Hence 30A circuits. Which could run two big appliances and as many small one as any reasonable person could hope to own. .... Many birds killed with a couple of stones with the new system. .... The square-pin fused plug and shuttered sockets had numerous other advantages, of course.

Yes, that all makes sense - as I've been saying, I personally see definite advantages in having 30/32A circuits and fused plugs (and am less happy with multi-socket 20A circuits). That answers EFLI's question as to why the UK was fairly unusual in introducing 30A circuits (and fused plugs) - but it doesn't answer the initial question as to why 30A rings, rather than radials.

Kind Regards, John

 

[JohnD]

rings are used in Distribution to reduce voltage drop at the end, while using thinner conductors. In a town street it can be a very sensible option.

Maybe Distribution experience was well-represented on the committees. Who knows? The possibility of a broken leg on a ring going un-noticed gets little attention.

 

[EFLImpudence]

rings are used in Distribution to reduce voltage drop at the end, while using thinner conductors. In a town street it can be a very sensible option.

Thinner conductors are not allowed in dwellings apart from the special ring final.

As I said, had we a 3mm² cable, the whole subject would be moot.

 

[SimonH2]

There's a very simple way to find out ...
It just needs someone to pop back with a time machine and sit in on the discussions - or at least ask those involved what the reasoning was. Problem solved
What do you mean, you don't have a time machine

 

[JohnW2]

rings are used in Distribution to reduce voltage drop at the end, while using thinner conductors. In a town street it can be a very sensible option.

If it's just a street with the cables going along it, then the total CSA of conductors (whether one as a radial, two in parallel as a radial or one as a ring) would surely be the same for a given VD at the end whether it was a ring or a radial?

Kind Regards, John

 

[JohnD]

in many respects, rings don't have ends.

In a street with two sides, the cable might go up one side and down the other.

If the street was originally cabled with just one, adding the second leg means the old cable can remain undisturbed and continues to earn its keep, rather than scrapping it and laying a new double-size cable.

Also saving on the cost of spades.

 

[JohnW2]

in many respects, rings don't have ends.

I only used the word "end" because you had Substitute "farthest point" instead!

In a street with two sides, the cable might go up one side and down the other. If the street was originally cabled with just one, adding the second leg means the old cable can remain undisturbed and continues to earn its keep, rather than scrapping it and laying a new double-size cable. ... Also saving on the cost of spades.

One could simply add the new cable, as another radial, without joining the ends of the radials to create a ring. Same amount of copper and spade work, and roughly the same VD at the "farthest point" - the only advantage I can see is a potentially small improvement in 'balance' of load between the old and new bits of cable if the loads themselves were fairly unbalanced (between two sides of road, or whatever).

Kind Regards, John

 

[JohnW2]

Thinner conductors are not allowed in dwellings apart from the special ring final. As I said, had we a 3mm² cable, the whole subject would be moot.

Do you mean because you think that a 30A radial would then have been possible - hence no need for a ring?

If so, I'm not so sure. Looking at today's CCC figures (Table 4D5), interpolation indicates that Method C 3mm² would probably have a CCC (assuming an MCB) of fractionally over 30A ...

However, in the days of BS3036 fuses, that would presumably have translated to around 21.75A - hence a 3mm² radial presumably could have been only 20A (or 21A !) ... or have I got that wrong?

Kind Regards, John

 

[SUNRAY]

Do you mean because you think that a 30A radial would then have been possible - hence no need for a ring?

If so, I'm not so sure. Looking at today's CCC figures (Table 4D5), interpolation indicates that Method C 3mm² would probably have a CCC (assuming an MCB) of fractionally over 30A ...

View attachment 170368

However, in the days of BS3036 fuses, that would presumably have translated to around 21.75A - hence a 3mm² radial presumably could have been only 20A (or 21A !) ... or have I got that wrong?

Kind Regards, John

7/0.029 is about 2.95mm² and all the charts indicated 23-25A IIRC.

 

[JohnW2]

7/0.029 is about 2.95mm² and all the charts indicated 23-25A IIRC.

Well,, that's not a million miles away from the 21.75A (with a BS 3036 fuse) that I arrived at (for 3.0mm²) on the basis of present-day figures and regs - and the point remains that such cable would not be adequate for a 30A radial.

[by the way, my calculation indicates that 7/0.029 is about 2.983 mm², so even closer to 3.0 mm² than you suggest]

Kind Regards, John