This is something of an "old favorite" topic at ECN, but it's been a while since the last round of discussion. As we now have several more British members, and I've just replied to an e-mail query about rings, maybe we could go another round and get some fresh opinions.

ECN regulars will already know my opinion on this one...... [Linked Image]

Anyway, here's the brief reply I sent to the e-mail query, just for any new non-British members who are still not quite sure what we're talking about:

Quote
The ring circuit is a peculiarly British method of wiring, used only here in the U.K., in one or two ex-colonies, and to a limited extent in the Irish Republic. It is not an arrangement found elsewhere in Europe.

The British IEE introduced the concept of the ring circuit in the late 1940s, along with a new design of plug and receptacle ("socket outlet" in their terminology). The ring circuit soon came into widespread use and has been an accepted standard for wiring British homes ever since.

As the name implies, the wiring for a branch circuit starts from the distribution panel, runs to each outlet in turn, and then a final cable links the last outlet back to the panel. The hot wires from each end of the ring are terminated at the same fuse or circuit breaker terminal, and the two neutrals are similarly both connected to the neutral busbar. In the case of a domestic circuit where our equivalent of NM cable (Romex) is used, the grounding conductors are also paralleled at the appropriate busbar to form a ring.

In the original specifications, the cable size to be used was 7/.029, i.e. a conductor comprised of 7 strands, each 0.029 inch in diameter, and in domestic wiring a ring could serve any number of outlets within a floor area of up to 1000 sq. feet. When the IEE and manufacturing switched to metric in 1970, that became 2.5 sq. mm conductors and 100 sq. meters (1076 sq. ft.) area. A 2.5 sq. mm conductor is approximately 20% larger than #14 AWG.

The overcurrent protection for the ring is a 30A fuse, or a 30A (old) or 32A (new) circuit breaker. As an individual 2.5 sq. mm conductor is rated well below 30A, the integrity of the ring is essential for insuring that conductors are not overloaded.

Protection for individual appliances and cords is then provided by a tubular ceramic-bodied cartridge fuse fitted in each plug. The plugs and sockets are rated for 13A maximum, chosen to allow portable appliances of up to 3kW to be used. The plug fuses are 1" long by 1/4" diameter, and were originally available in ratings of 2, 5, 10, and 13A for use with different appliances. Some years later, the range was changed to 3, 5, and 13A, and in the latest specification, the IEE recommends the use of just 3 or 13A fuses, supposedly to simplify the choice for the layman. Other ratings are still available, however.

As well as receptacles, the ring can also incorporate a fused connection unit. This is basically just a device to allow a fixed appliance to be hard-wired into the ring, and has a removable carrier which takes the same type of cartridge fuse as used in the plugs. Fused connection units are typically used for a small wall-mounted heater, exhaust fan, and similar devices.

Specifications also allow a socket or fused connection outlet to be run as a "spur" off the ring, using a cable of the same size as those comprising the ring itself. Such a spur may be tapped at any existing outlet, or may be tapped at any other point on the ring with a junction box.

Note that in British wiring it is normal practice to wire lighting points on separate circuits to receptacles, so lights do not normally come into the ring-circuit picture, however it is permissible to use a suitably fused connection unit on a ring to feed fixed lighting if required. Such a method is normally found only where it is otherwise impractical to tap an existing lighting circuit, e.g. when wiring an extension.

The idea of the ring is that it provides a versatile arrangement capable of delivering a considerable amount of power to any outlet (over 7kW on a ring -- everything here runs on 240V remember). As the rules permit a
ring to serve an area up to 1000 sq. feet or thereabouts, in the early days, and indeed up until as recently as the late 1970s, it was quite normal to find a small or average-sized home with a single ring feeding all
receptacles. In more recent times particularly as kitchen loads have increased, the average house now has two ring circuits, sometimes three.

The arrangement certainly has drawbacks, however. Loads do not divide evenly around the ring, and a heavy load near to one end results in much higher current flowing through the short leg of the ring than the longer leg. Although the IEE considers it unlikely to happen in practice, it is certainly possible for heavy loads placed close to the end of a ring to overload the cables. There is also the danger that a broken connection somewhere can leave a heavy load on an undersized conductor. The problem is that a single break still leaves power at all outlets, but if the break is
near one end, the remaining leg of the ring can end up carrying the full current.

Most British electricians seem to accept the ring as "a good thing," although personally I do not like the arrangement and would like to see it abandoned.



[This message has been edited by pauluk (edited 05-25-2004).]