Ring circuit
This design enables the use of smaller-diameter wire than would be used in a radial circuit of equivalent total current. Ideally, the ring acts like two radial circuits proceeding in opposite directions around the ring, the dividing point between them dependent on the distribution of load in the ring. If the load is evenly split across the two directions the current in each direction is half of the total, allowing the use of wire with half the current-carrying capacity. In practice, the load does not always split evenly, so thicker wire is used.
Description
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.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.
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.
History and use
The ring circuit and the associated BS 1363 plug and socket system were developed in Britain during 1942–1947. They are commonly used in the United Kingdom and to a lesser extent in the Republic of Ireland. It is likely that they are also used in parts of the Commonwealth of Nations, where Britain had design 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 1980s. This latter plug had the distinctive feature that the fuse was also the live pin and unscrewed from the plug body.
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 20A radial (essentially breaking each ring in half and putting the halves on a separate breaker) is becoming more common. Splitting a ring into two 20A radials can be a useful technique where one leg of the ring is damaged and cannot easily be replaced.
Another advantage of ring circuits in their early days was an economy of cable and labour, due to the fact that one could simply connect a cable between two existing 15A radially wired sockets to make one 30A ring, then adding as many sockets as were desired. This was an important consideration in the austerity of the 1940s. This would leave the ring supplied by 2x 15A fuses, which worked well enough in practice, even if unconventional.
Many pre-war (round pin) installations used double pole fusing. When 2x 15A radials were converted to a ring on these systems, the ring would then be supplied by no fewer than 4 fuses! It is rare to find such circuits still in service today.
Installation rules
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 protective device other than a 30A fuse, 30A breaker or 32A breaker, and a cable size other than those mentioned above.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 most small and medium houses have one ring circuit per storey, with larger premises having more.
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 can cause minor overloading of one of the cables if near the end of the ring, so kitchens should not be wired at one end of a ring circuit.
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, showers and some electric cookers) or with a non-trivial power demand for long periods (for example, immersion heaters) are no longer recommended to be connected to a ring circuit, but instead are connected to their own dedicated circuit. There are however plenty of older installations with such loads on a ring circuit.
Criticism
The final ring-circuit concept has been criticized in a number of ways, and some of these disadvantages could explain the lack of widespread adoption outside the United Kingdom.
The only way to see the pros and cons of ring circuits is to compare them to the other option, radials.
Fault conditions are not apparent when in use
Ring circuits continue to operate without the user being aware of any problem if there are fault conditions or installation errors that make the circuit unsafe:- Part of the ring missing or loose connections result in 2.5 mm² cables running above rated current at times, resulting in reduced cable life.
- Radials with a loose connection will overheat severely and be an immediate fire risk
- Radials with a broken connection will not function (if L or N broken), or function with no safety earth connection (if E broken).
- Accidental cross connection between two 32 A rings means that the fault current protection reaches 64 A and the required fault disconnection times are violated grossly
- Testing at installation addresses this.
- Ring spur installations encourage using three connectors in one terminal, which can cause one to become loose and overheat
- The same situation occurs with both radial and ring circuits when branching off is used.
- Rings encourage the installation of too many spurs on a ring, leading to a risk of overheating, especially if spur cables are too long
Complexity of safety tests
Testing ring circuits takes 5–6 times longer than testing radial circuits. The installation tests required for the safe operation of a ring circuit are substantially more time consuming than those for a radial circuit, and DIY installers or electricians qualified in other countries may not be familiar with them.
Balancing requirement
Regulation 433-02-04 of BS 7671 requires that the installed load is distributed around the ring such that no part of the cable exceeds its capacity. This requirement is difficult to fulfill and may be largely ignored in practice, as loads are often co-located (washing machine, tumble dryer, dish washer all next to kitchen sink) and not necessarily near the centre of the ring.Electromagnetic interference
Ring circuits can generate strong unwanted magnetic fields. In a normal (non-ring, radial) circuit, the current flowing in the circuit must return through (almost exactly) the same path through which it came, especially if the live and neutral conductors are kept in close proximity of each other and form a twisted pair. This prevents the circuit forming a large magnetic coil (loop antenna), which would otherwise induce a magnetic field at the AC frequency (50 or 60 Hz). In a ring circuit, on the other hand, it is possible that the live and neutral currents are not equal on each side of the ring. Mains-frequency currents follow the path of least resistance, and it is possible, especially with aging oxidized contacts, that from a socket, the lowest-resistance live connection is along the left-hand side of the ring, and the lowest-resistance neutral connection is along the right-hand side. As a result, current is flowing around the ring and will therefore induce a magnetic field. In the extreme case of a defect ring circuit, the live connection could become completely interrupted on one side of the ring and the neutral connection on the other, and then the full current would supply the magnetic field. This can lead to substantial electromagnetic interference, such as mains hum in audio devices, accidental triggering of alarm and protection devices (burglar alarms, RCDs, etc.), malfunctions of consumer electronics and medical devices, ground loops, etc.Overcurrent protection
Ring circuits provide low protection against overcurrents. The purpose of ring circuits is to supply a large number of sockets, therefore they are protected only with high-rated overcurrent circuit breakers (typically 32 A). In comparison, the radial circuits used in other countries typically supply only a small number of sockets and are therefore protected with lower-rated circuit breakers (typically 10–20 A). As a result, countries using ring circuits find it necessary to add additional lower-rated fuses into the plugs of each appliance. This does create a possible improvement in safety in that an appliance with blown plug fuse will not be live when plugged in again (unless the fuse is first replaced), whereas with fuseless plugs a faulty appliance remains potentially dangerous to plug in, though in most cases it would trip a lower-rated circuit breaker if plugged in again.This incompatibility in the overcurrent protection of appliance leads between countries using ring and radial circuits has been a major stumbling block on the road to worldwide standardization of domestic AC power plugs and sockets. Although plug-fuses can, in principle, be better matched to the maximum current required by an appliance, in practice, some plugs in the UK are merely fitted with a fuse of the maximum permitted rating of 13 A, resulting in safety improvement with some appliances but not all. This is not a problem since all appliances are required to be safe with a 13A fuse, but it does mean the potential safety advantage is only partially realised and that the fused plug offers little advantage over an unfused plug used on radial circuit with a 13 A or lower circuit breaker. The introduction of regulations requiring new appliances to be sold with correctly fused pre-fitted plugs improves this situation further.
See also
References
This article is licensed under the GNU Free Documentation License.
Last updated on Saturday October 04, 2008 at 04:31:01 PDT (GMT -0700)
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