What is electrically wrong with this design to extend a ring circuit?

Question

I want to add an outdoor socket directly through a wall behind two internal sockets, so only a minor extension in terms of cabling distance. However, I also want to run a spur off that outdoor socket, so the first socket cannot itself be a spur.

My understanding is that, in the United Kingdom, almost all domestic mains electrical circuits are ring circuits. Ideally, every socket would be part of the ring but it is acceptable to add a single or double socket via a radial spur. It is not permitted for more than one socket to be powered by a spur, unless protected by a fuse control unit.

But if we are wiring a spur with a single 3-core cable, why not create a mini ring using two 3-core cables? Alternatively, why not add an entire secondary ring off the main ring?

(I haven't drawn it, but you could even question whether it's acceptable to use a 'grid' type design, connecting the socket in the middle directly to the one above it?)

As I have not seen this design anywhere, I'm assuming there are good reasons not to implement it. But what are those reasons? I find it odd that you could reduce direct connections between two sockets from two to one to ensure that there was only one ring, and that this would somehow be 'better'.

From the perspective of the cable capacity, you might argue that the points which connect the main ring to the mini or secondary ring create a bottleneck, but surely they are no worse than those at the distribution unit? From a regulatory perspective, perhaps adding cabling in this manner is likely to increase the total length beyond the permitted maximum - or make it difficult to determine whether a circuit will breach that maximum?

What is wrong with this circuit design:

• electrically (is it unsafe)?
• regulatorily (does it breach code / why)?

Answer 1

Perhaps that will explain the fallacy of the "double ring".

By removing cable one becomes code conformant.

Ok, about the reason why:

Since the ring-regulation allows you to overload an single conductor, verifying a correct ring is essential for fire safety.

Verifying a one-ring-only circuit is easy: Just consume power on one outlet and measure the current somewhere in the ring. An intact ring yields roughly half the current. The tester only needs to find out if the tested conductor is a "spur" or part of the ring. The exact location isn't important as the current will be split half across the two paths, and is exactly the same in each path.

Verifying a maze with random placed interconnections is practically impossible, as one needs to map the structure, calculate the current and verify for every single conductor.

Answer 2

The answer to this lies in how ring circuits are protected at the consumer unit.

A radial socket circuit using 2.5mm2 wire must be protected by a 20A circuit breaker. In general 2.5mm2 wire can carry a 20A load without overheating. (These aren't exact calculations which require lots of things taken into account, but pretty reliable rule of thumb)

A radial circuit protected by a 30A breaker must use (at least) 4mm2 wire, which is a lot more expensive

Ring circuits can use 2.5mm2 wire with a 30A breaker because (in theory) half of the current from each point flows around each leg of the ring, so the 2.5mm2 wire never sees more than 15A.

A double-socket spur is allowed because it is unlikely that 2 loads each drawing the full 13A will be used at the same time (and the 2.5mm2 cable may be able to take 26A non-continuous)

More than 2 sockets on a spur runs the risk of overloading the cable between the ring and the first socket on the spur. It used to be allowed to have 2 single sockets on a spur, but this was removed from the regs, presumably because of the risk of people upgrading single sockets to double without realising they were spurred)

As Martin says in his answer one of the reasons circuits with multiple rings aren't used is because the ring continuity test no longer works correctly.

The other main reason for not creating multiple mini-rings and/or grids is because the main reason for allowing ring circuits is to save copper, and adding extra connections would defeat that object

Answer 3

I love the accepted answer, but I'm going to answer the actual question, "What is wrong ..."

There is nothing wrong with the picture in the Question, as long as it remains a picture. :) The rules are not engineering standards on paper, they are meant to be implementable and enforceable in practice in millions of homes by electricians with specific training and procedures. They are backed up by observation and experience of dumb things people do and are proven to be safe.

A complex, poorly understood and untestable cable configuration is unsafe because the accepted standards of installation, testing and usage that have been proven to be safe cannot be assumed. And without that, everything is dangerous.

Answer 4

This sounds to me that the simplest solution would be to go to one of your indoor sockets, remove one of the two ring cables from it and re-route this cable into an additional new socket nearby indoors. You now have two indoor sockets next to each other with only one cable in each, so you now have an extra socket indoors which has created a break in the ring. So now fit your outdoor socket, and run two cables from it through the wall with one cable connected into the original socket and the other cable connected into the additional socket. You now have an outdoor socket which completes the ring by going out to the outdoor socket from the original indoor one and coming back through the wall and connecting the additional socket. Hey presto you now have an extra socket indoors and a new outdoor socket. This arrangement is still a pure ring main without any spurs. Next Job? You can now simply connect a single spur to the outside socket, which is permissible !