Ring circuit
From Wikipedia, the free encyclopedia
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. In a single-phase system, the ring starts at the consumer unit (also known as "fuse box" or "breaker box"), visits each socket in turn, and then returns to the consumer unit. In a three-phase system, the ring (which is almost always single-phase) is fed from a single-pole breaker in the distribution board.
The main advantage of a ring circuit is that a cable can be used that has a smaller cross-sectional area than that 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.
The ring main came about because Britain had to embark on a massive rebuilding programme following World War II. There was an acute shortage of copper, and it was necessary to come up with a scheme that used far less copper than would normally be the case. The scheme was specified to use 13 Amp fused socket outlets and several designs for the plugs and sockets appeared. Only the square pin (BS1363) system survives, but the round pin D&S system was still in use in many locations well into the 1980's. This latter plug had the distinctive feature that the fuse was also the live pin and unscrewed from the plug body.
[edit] British ring circuits
Ring circuits are commonly used in British wiring with fused 13 A plugs to BS 1363. They are generally wired with 2.5 mm² cable and protected by a 30 A fuse, an older 30 A circuit breaker, or a European harmonised 32 A circuit 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. 1.5 mm² Mineral Insulated Copper Clad cable ('pyro') may also be used (as mineral insulated cable can withstand heat more effectively than normal PVC) though obviously more care must be taken with regard to voltage drop on longer runs.
The ring circuit 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.
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 IEE Wiring Regulations (BS 7671) 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 - for example it is common practice to put kitchens on their own ring circuit or sometimes a ring circuit shared with a utility room to avoid putting a heavy load at one point on the main downstairs ring circuit. A heavy concentration of load close together on a ring circuit is likely to cause overloading of one of the cables unless it is near the middle of the ring.
Unfused 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) or one fused connection unit (FCU). 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. Triple and larger sockets are generally fused and therefore can also be placed on a spur.
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).
Where loads other than BS 1363 sockets are connected to a ring circuit or it is desired to place more than one socket for low power equipment on a spur a BS 1363 fused connection unit (FCU) is used. In the case of fixed appliances this will be a switched fused connection unit (SFCU) to provide a point of isolation for the appliance but in other cases such as feeding multiple lighting points (putting lighting on a ring through is generally considered bad practice in new installation but is often done when adding lights to an existing property) or multiple sockets an unswitched one is often preferable.
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.
One disadvantage of the ring circuit is that it can lead to higher levels of magnetic fields within the rooms served by the ring which can lead to problems with some types of electromagnetic interference such as mains hum and ground loops. Some people claim that these magnetic fields can have undesirable health affects although this is not generally accepted by the scientific community.
[edit] 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.
