Ring circuit
A ring circuit (more formally a ring final circuit; informally a ring main or just a ring) is an electrical wiring technique that provides two paths for the live, neutral and earth lines by wiring a ring of cable (or sometimes three separate cores in conduit), starting from the consumer unit (also known as "fuse box" or "breaker box") in a single phase system or the distribution board in a three phase system (rings themselves are almost never 3 phase but it is normal to feed them from a single pole breaker in a three phase distribution board), going to each socket in turn, and then going back to the consumer unit.
The main advantage of using a ring circuit is that the required cross-sectional area of the cable used is reduced as compared to that which would be required for a radial circuit ("straight circuit") of the same current rating. The cable is therefore cheaper and easier to work with.
Ring circuits are a British idea and they are commonly used in the United Kingdom and Republic of Ireland. It is likely that they are also used in some other places especially where the British have had influence in the past.
British ring circuits
Ring circuits are commonly used in British wiring with the BS 1363 system of fused plugs.
They are generally wired with 2.5 mm² cable and protected by a 30 A fuse, an older 30 A breaker, or a European harmonised 32 A breaker. Sometimes 4 mm² cable is used if very long cable runs (causing volt drop issues) or derating factors such as thermal insulation are involved.
This system was devised in a time of copper shortage to allow for two 3 kW heaters to be connected to any two sockets and still allow enough spare capacity to support small appliances at the same time. It has proved to be a very practical installation method that has been the norm in the British Isles for a long time.
Rules for ring circuits say that the cable rating must be no less than two thirds of the rating of the protective device. This means that the risk of sustained overloading of the cable can be considered minimal. In practice, however, it is extremely uncommon to encounter a ring with a breaker other than 32 A and a cable size other than those mentioned above.
Many lay people in the UK refer to any circuit as a "ring" and the term "lighting ring" is often heard from novices. It is not unheard of to see lighting circuits wired as rings of cable (though usually still with a breaker below the cable rating) in DIY installations.
The British Wiring Regulations permit an unlimited number of socket outlets to be installed on a ring circuit, provided that the floor area served does not exceed 100 m2. In practice there is normally one ring circuit per storey in a residential installation. An installation designer may determine by experience and calculation whether additional circuits are required for areas of high demand such as a kitchen or utility room.
The ring was devised during a time of copper shortage to allow two 3 kW heaters to be used in any two locations and to allow some power to small appliances, and to keep total copper use low. It has stayed the most common circuit configuration in the UK although the 20 A radial (essentially breaking each ring in half and putting the halves on a separate breaker) is becoming more common. Splitting a ring into two 20 A radials can be a useful technique where one leg of the ring is damaged and cannot easily be replaced.
Fixed appliances with a power rating over 3 kW (for example, electric cookers and showers) or with a non-trivial power demand for long periods (for example, immersion heaters) are not normally connected to a ring circuit but instead are connected to their own dedicated circuit. Where fixed appliances are connected to a ring circuit a BS 1363 switched fused connection unit is used.
Spurs from a ring wired in the same cable as the ring are allowed to run one single or double socket (the use of two singles was previously allowed but was banned because of people replacing them with doubles). Spurs may either start from a socket or be joined to the ring cable with a junction box or other approved method of joining cables.
It is not permitted to have more spurs than sockets on the ring, and it is considered bad practice by most electricians to have spurs in a new installation (some think they are bad practice in all cases).
Substation ring bus
The ring circuit used for building distribution is not the same as the ring bus used in substations. In a ring-bus scheme each feeder or load shares two breakers, all connected in a ring with loads and sources tapped between pairs of breakers. This scheme is usually only used for up to 5 load/source line terminations; after that a breaker and a half scheme is more secure and economic.