Ring circuit

Description

The ring starts at the consumer unit, also known as a Distribution board, visits each load in turn, returning to the consumer unit. The ring is fed from a fuse or circuit breaker in the consumer unit.

Ring circuits to BS 7671, 433.1.204 are commonly used in British wiring specifying only the use of socket-outlets on fused plugs to BS 1363. Because the protective device (breaker) rating is much higher than the cable's maximum amperage rating the ring final circuit can only be used with fused plugs on the socket-outlets, or 13 A fused connection units (FCU) for fixed appliances with flexible cords. 433.1.204 specifies 2.5mm² cable, or 1.5mm² mineral-insulated copper-clad cable, as the minimum size used and protected by a maximum of a 30A fuse or 32A circuit breaker.

433.1.204 ring final history

The ring circuit and the associated universal BS 1363 plug and socket-outlet system were developed in Britain between 1942 and 1947.

Pre-World War II practice was to use various sizes of plugs and sockets to suit the current requirement of the appliance, connected to suitably fused radial circuits. The ratings of the fuses were selected 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 that the ring final circuit offered a greater number of sockets delivering at least 2kW to each socket, and a more efficient and lower cost circuit. Several designs for the socket-outlet system for the ring design were considered, with the new BS1363 13 A flat pin and fused plugs selected, a socket-outlet system developed independently of the final ring circuit. Of the two British Standards socket-outlet systems, the BS546 and the universal BS1363 by the 1960s BS1363 became the single standard for new radial and ring final installations.

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

The ring circuit remains the most common socket-outlet power circuit configuration in the United Kingdom. Ring final circuits are commonly used to a lesser extent in the Republic of Ireland, and were also adopted, for example, in the United Arab Emirates, Singapore, Hong Kong, Beijing, Indonesia, Cyprus, and Uganda, as well as perforce in many countries of the British Empire cum British Commonwealth. It does, however, present more complicated safety concerns than do radial circuits for plug-in solar power.

Installation standards

Standards for ring final circuits that fall under BS 433.1.204 in the UK provide that the cable must be a minimum size of 2.5mm² 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, 433.1.204 permit an unlimited number of 13A socket outlets to be installed on a ring circuit, with a recommendation in Appendix 15 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. The source impedance and therefore voltage drop to the furthest point is lower using a ring. To deliver the same power to the same room using radial circuits would require more circuits or heavier cable.

High integrity CPC

Safety levels are increased as all fittings on the ring are connected to the circuit protective conductor (CPC) conductor from two sides. Two independent faults are needed, one each side of a fitting or appliance to create an 'off earth' fault.

Continuous continuity verification from any point

The continuity of each conductor at all the points on the ring can be verified from any point. If this needs to be performed as part of live installation monitoring, verification can by a current clamp with the system energised.

Disadvantages

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 433.1.204 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/2.5mm² cable ring, which means a ring has to be significantly out of balance to create 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 leg of the ring. However, an AFDD protecting the ring should detect a loose joint if arcing occurs.