ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades Ring circuits UK style.

PaulUK

Could you please take the time to explain ring circuits to me?

Are the conductors protected at their ampacity or at some higher value because there are two paths to each power point?

Are your receptacle outlets provided with individual overcurrent protection.

Is the conductor continuous or is it spliced?
--
Tom

Hello Tom:

Your wish is my command....

A ring circuit is fed from a 30A fuse or C/B, but lower ampacity cable is used. The original ring specification used a cable known as 7/.029 (i.e. 7 strands each 0.029" diameter), which was rated at 20A maximum.

The equivalent size used since the change to metric cables is 2.5 sq. mm, which is just fractionally larger than your #14.

To be considered a ring circuit by the IEE Regs., the two live condutors from each cable must both be terminated into the fuse or C/B way at the panel, and both neutrals must similarly be connected directly to the neutral busbar.

Almost all domestic wiring is with "Twin & earth" (like Romex), and in this case the earth (ground) wires are also wired in a ring.

Cables may be run with or without splices. The IEE recommends not cutting into the wire if possible, but of course this is often just not practical. They specify that when spliced the joint must be made in such a way as to ensure the integrity of the ring. It's one of those rather vague phrases; after all, I like ALL my splices to ensure the integrity of the connection!

The most common is for cut wires to be twisted and clamped together in the terminal at each socket (recptacle).

In domestic wiring, each ring can serve any number of outlets within a floor area of, originally 1000 sq. ft, now 100 sq. metres (1076 sq. ft.).

The sockets and plugs are rated 13A maximum, chosen to allow for a load of up to 3kW (everything is 240V remember).

Each plug is fitted with a ceramic-bodied fuse, 1" long by 0.25" diameter. Several ratings are available, but 3, 5 and 13A are the most common nowadays. There are no two-prong versions of these plugs, by the way, so even a small lamp with no ground still has a 3-prong plug.

Besides sockets, it is also possible to use a "fused spur unit" on a ring. This is basically just a device to hard-wire some fixed appliance into the ring. It takes the same type of fuse as the 13A plug, and is available in switched or unswitched versions.

It's also possible to wire one or more "spurs" off a ring. A spur is simply a single cable running from the ring to a socket or fued spur unit. The spur may be tapped into the ring either at a socket or by using a junction box. The latter are designed in such a way that the ring cable can be stripped and laid into the terminals without cutting the conductors.

The ring circuit (introduced late 1940s) has been the most common arrangement for domestic 13A outlets, but other configurations are used as well.

P.S. Take a look at "Hello from the U.K." in the General area. I talked about rings there.

Excellent post, Paul!!! I printed the thread for reference!

Scott SET

Why, thank you, Sir. We aim to please!

Paul,
i guess someone's gotta ask sometime here, why a ring???

>why a ring???
That was covered in another thread, perhaps the one Scott found.

I've never been able to find a definitive explanation, but here are a few relevant points.

I guess the first thing to bear in mind is that the typical British home in the late 1940s was quite different to today. Central heating was rare, and most people had open coal/wood fires, gas fires, and portable electric heaters.

Up to then, there were outlets of three different ratings used: 2, 5, and 15A, with non-interchangable plugs. A 15A socket to run a large electric heater was often fitted next to a fireplace and wired back to its own 15A fuse. It was rare to have more than one such outlet in each main room, and sometimes none upstairs in the bedrooms.

Other "general purpose" outlets were 5A, and wired on various 5 or 15A branches. (The IEE Regs. still recognize these circuits with up to three 5A outlets permitted on a 15A branch.) Wiring was what the IEE now calls "radial" circuits - like American wiring.

The 2A outlets were intended mainly for plug-in table lamps etc. and were often wired on 5A branches with fixed lights.

To add to the confusion, both 2-pin ungrounded and 3-pin grounded versions of the 5A outlet were very common. Unlike American receptacles, however, the spacing of the hot & neutral is slightly different, so a 2-pin plug won't fit a 3-pin socket. 2A sockets also came in 2 and 3-pin types, although lighting circuits were rarely run with a ground wire at that time. 15A only came in 3-pin grounded versions by then, but a lot of 2-pin 15A sockets installed in the 1920s/1930s were still around (the old house I moved to as a kid in 1970 still had some!).

The mix of sockets meant that a plethora of adapters were often used. A typical type plugged into a 3-pin 5A outlet and provided a 3-pin 5A socket on the front plus a 2-pin 5A outlet on each side. Another type allowed 5A plugs to be connected to a 15A socket, but many different combinations were available.

It was this "electrical lottery" of plugs apparently, which led to the search for a universal connector and the design of the 13-amp plug with its built-in fuse.

That's the background. OK,back to those big 2 and 3kW portable heaters.....

I've heard it said that the IEE wanted to work out a simple circuit arrangement which would allow two 3kW heaters to be run simultaneously and still leave capacity for smaller appliances. The design was to be such that the heaters weren't restricted to just one or two outlets as before, but could use any sockets in the house.

Whether that's true or not, I'm not sure, but as a 30A circuit provides 7200 watts, it's certainly a possibility. And a massive 30A fuse at the panel, of course, necessitated the adoption of fuses in the plugs.

OK, that still doesn't answer "Why a ring?" From the practical standpoint, it allows the use of smaller cable; e.g. the IEE Regs. permit a 30A radial circuit, but this means the use of (in modern sizing) a 4 sq. mm cable - about 20% larger than #12 AWG.

I've also seen various claims (and counter-claims!) that in the design of homes being built at the time, the ring resulted in the use of the least material - important in post-WWII Britain with shortages and rationing.

I've never actually sat down & tried to work this out. It might prove interesting.

Other than those arguments, I've never been able to find out why for sure. I guess it seemed like a good idea at the time, and we've had rings ever since.

Oh boy, 2 a.m. already! Must've been talking too long!

What problems, if any, would the parallel paths of diffent lenghths cause?

Does this have the tendancy to naturally balance itself (as resistance rises with temp)?

Why does the NEC consider small parallel conductors bad (even at equal lenghts and with the same "path")?

Very interesting questions, Virgil. You've actually made me sit own with a pen & paper mand try out various scenarios.

As an example: Assume a ring with a total loop length of 120 ft. feeding standard twin (duplex) outlets. If both sockets of a twin were loaded to the full 13A, this would place a 26A load on the circuit. If this happened to be the first socket of the ring located, say, just 12 ft. from the panel, the long leg would be carrying just 2.6A while the short side would get 23.4 amps!

An extreme case, perhaps, and unlikely to happen, but it's possible, and of course is one reason why the cable used for a ring has to be rated more than half of the 30A protective device.

As for temperature stabilizing effects, I don't have the appropriate data to hand to work out what the changes in resistance would be. Obviously the short leg would heat up more than the long one, and in increasing its resistance would slightly affect the currents in favor of balance. I don't think the overall proportions would change by much though. Even doubling the resistance in the short leg would still leave it carrying over 21A of the load.

By the way, if you want to experiment with ring calculations, the IEE Wiring Tables specify that the voltage drop for 2-core 2.5 sq. mm cable (used for rings) is 18mV per amp per metre. So for a single conductor, the resistance is 0.009 ohm per metre, or approx.0.0082 ohm per yard.