Engineering:Ring circuit

Short description: Electrical wiring technique

"Ring main" redirects here. For the London water infrastructure, see Thames Water Ring Main.

In electricity supply, a ring circuit is an electrical wiring topology in which power is distributed through a continuous ring. Each conductor of the ring is connected to the source of the supply at both of its ends. The topology contrasts with the typical radial circuit configuration, in which nodes and the distribution point at the protective device are connected in a line or a line with branches or legs off the main line.

Ring circuits are used in distribution, and at the final consumer current draw off point either for power or lighting. A ring circuit at the consumer draw off point is known as a ring final circuit. Ring final circuits are often termed by the historical name ring main in the United Kingdom.

Ring circuits are subject to typical cable protection relating for all types of circuit topologies, in which the cable is at a higher amperage loading than the protective device (breaker).

In the United Kingdom there is an anomalous ring final circuit with the cable at a lesser amperage loading than the protective device. It is under the concession, 433.1.204 in British Standards 7671.

433.1.204 ring final in the UK

A circuit designed with the cable at a lesser amperage loading than the protective device, 433.1.204 of BS 7671 is triggered. The standard states that such a circuit must only be a ring final, not a radial or star. The 433.1.204 implementation of the ring circuit is predominately used in the United Kingdom where it was developed, and to a lesser extent in countries that adopted the British electrical standards.

The 433.1.204 concession in BS 7671 simply says in layman's terms:

1. The circuit must be a ring final (the two cable legs into the protective device double the circuits amperage rating over the cable's amperage rating);
2. Protective device must be either 30A or 32A MCB/RCBO/AFDD/fuse;
3. Minimum 2.5mm cable;
4. Cable must be manufactured of copper;
5. Minimum current capacity of the cable is 20A, taking into account derating;
6. If socket-outlets are to be installed on the ring final they must only be the BS 1363 socket-outlet accessory family - the Type G 13A square pin plug and socket.

Further guidance for 433.1.204 is in Appendix 15 of BS 7671, recommending such as: a maximum coverage of 100 square metre floor area and that the load must be reasonably distributed around the ring final.

A ring final when designed to the 433.1.204 of BS 7671 enables the use of smaller diameter cable than would be used in a radial circuit of equivalent total current capacity, giving economic installation. The reduced diameter conductors in the flexible cords connecting an appliance to the plug intended for use with sockets on a ring final circuit are individually protected by a fuse in the plug. Its prime advantages over radial circuits are therefore reduced quantity of cable used, and greater flexibility of appliances and equipment that can be connected. The ring circuit 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 total current-carrying capacity. In practice, the load does not always split evenly, so thicker wire is used.

Description

Diagram of a possible configuration of ring final circuit. Consumer unit is at bottom left.

The ring starts at the consumer unit (also known as fuse box, distribution board, or breaker box), visits each socket in turn, and then returns to the consumer unit. The ring is fed from a fuse or circuit breaker in the consumer unit.

Ring circuits to BS 7671,433.1.40 are commonly used in British wiring with 433.1.204 specifying only the use of socket-outlets on fused plugs to BS 1363. Because the breaker rating is much higher than that of any one socket outlet, the system can only be used with fused plugs or fused appliance outlets. 433.1.204 specifies 2.5mm² cable, or 1.5mm² mineral-insulated copper-clad cable (known as pyro), as the minimum used and protected by a maximum of a 30A fuse or 32A circuit breaker.

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. They are also found, for example, in the United Arab Emirates, Singapore, Hong Kong, Beijing, Indonesia, Cyprus and Uganda, and many countries where British standards were adopted.

Pre-World War II practice was to use various sizes of plugs and sockets to suit the current requirement of the appliance, and these were connected to suitably fused radial circuits; the ratings of those fuses were appropriate to protect both the fixed wiring and the flexible cord attached to the plug.

The Electrical Installations Committee which was convened in 1942 as part of the Post War Building Studies programme determined, amongst other things, that the ring final circuit offered a more efficient and lower cost system which would safely support a greater number of sockets. The scheme was specified to use 13 A socket-outlets and fused plugs; several designs for the plugs and sockets were considered. The design chosen as the British Standard was the flat pin system now known as BS 1363. Other designs of 13 A fused plugs and socket-outlets, notably the Wylex and Dorman & Smith systems, which did not conform to the chosen standard, were used into the 1950s, but by the 1960s BS 1363 had become the single standard for new installations.

The committee mandated the ring circuit both to increase consumer safety and to combat the anticipated post-war copper shortage. The committee estimated that using ring-circuit and single-pole fusing would reduce raw materials requirements by approximately 25% compared with pre-war standards.

The ring circuit is still the most common socket-outlet circuit configuration in the UK, although radial circuits are also permitted by the installation standards.

Installation standards

Standards for ring final circuits that fall under BA 433.1.204 in the UK provide that the cable must be a minimum size of 2.5mm and the protective device being of 30A or 32A rating. This means that the risk of sustained overloading of the cable can be considered minimal.

British Standards 7671 permit an unlimited number of 13A socket outlets (at any point unfused single or double, or any number fused) 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 if additional circuits are required for areas of high demand. For example, it is common practice to have kitchens on their own ring circuit or sometimes a ring circuit shared with a utility room to avoid having heavy loads at one point on the main downstairs ring circuit. Since any load on a ring is fed by the ring conductors on either side of the load, it is desirable to avoid a concentrated load placed very near the consumer unit, since the shorter length of conductors will have less resistance carrying a disproportionate share of the load.

Unfused spurs from on a ring circuit wired in the same cable as the ring are allowed to run one socket (single or double) or one fused connection unit (FCU). Before 1970 the use of two single sockets on one spur was allowed, but has since been disallowed because of the popularity of double sockets. Spurs may either start from a socket on the ring or be joined to the ring cable with a junction box or other approved method of joining cables. BS 1363 compliant triple and larger sockets are always fused at 13A and therefore can also be placed on a spur. Since 1970 it is permitted to have more spurs than sockets on the ring, but it is considered poor practice by many electricians to have too many unfused spurs in a new installation.

Fused connection units (FCU) are in the BS 1363 accessories family conaining a 13A fuse. The FCU is design to supply fixed with or without flexible cables. The fuse is to protect the flex. They come in switched and unswitched versions.

Fixed appliances with a power rating of 3 kW or more (for example, water heaters and some electric cookers) or with a non-trivial power demand for long periods (for example, immersion heaters) may be connected to a ring circuit, but it is strongly recommended that instead they are connected to their own dedicated circuit. However, there are plenty of older installations with such loads on a ring circuit.

Advantages

Proponents of the ring circuit point out that, when correctly installed, there are also a number of advantages to be considered.

Area served

For rooms that are square or circular, a ring circuit can deliver more power per unit of floor area for a given cable size than a simple radial circuit, and the source impedance and therefore voltage drop to the furthest point is lower. Alternatively, to deliver the same power to the same building with radial circuits would require more final circuits or a heavier cable.

High integrity earthing

As all fittings on the ring are earthed from both sides, two independent faults are needed to create an 'off earth' fault.

Continuous continuity verification from any point

The continuity of each conductor right round all the points on the ring can be verified from any point, and if this needs to be done as part of live installation monitoring, it can be verified by current clamp injection with the system energised.

Criticism

The ring final circuit concept has been criticized compared to radials.

Fault conditions are not apparent when in use

Ring circuits may continue to operate without the user being aware of any problem if there are certain types of fault condition or installation errors. This gives both robustness against failure and a potential for danger.

Safety tests are complex

At least one author claims that testing ring circuits may take 5–6 times longer than testing radial circuits. The installation tests required for the safe operation of a ring circuit are more time-consuming than those for a radial circuit.

Load balance required

Standard 433-02-04 of BS 7671 requires that the installed load must be distributed around the ring such that no part of the cable exceeds its rated capacity. In some cases this requirement is difficult to guarantee, and may be largely ignored in practice, as loads are often clustered (e.g., washing machine, tumble dryer, dish washer all next to kitchen sink) at a point not necessarily near the centre of the ring. However, the cable rating is approximately 67% that of the circuit breaker in a typical 433.1.204 32A breaker/2.5mm cable ring, which means a ring has to be significantly out of balance to cause a problem.

In a ring circuit, if any poor joint causes a high resistance on one branch of the ring, current will be unevenly distributed, possibly overloading the conductor on the second branch of the ring. However an AFDD protecting the ring should detect a loose joint if arcing occurs.

See also

• Electrical wiring in the United Kingdom

External links

• Ring Circuit wiring guide

Fault condition	Observations
Part of the ring missing or disconnected can result in 2.5 mm2 cables running above rated current without this being obvious to the user.	Radials with a broken connection will not function (if L or N broken), or will function with no safety earth connection (if just E broken).