Ring Circuit - History and Use

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 in the United Arab Emirates, Singapore, and Indonesia.

The ring circuit 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 devise a scheme that used less copper than would normally be the case. The scheme was specified to use 13 A fused socket outlets, and several designs for the plugs and sockets appeared. Only the square pin (BS 1363) system survives, but the round pin Dorman & Smith system was still in use in many locations well into the 1980s, and is still occasionally seen today. This latter plug had the distinctive feature that the fuse was also the live pin and unscrewed from the plug body, often unintentionally leaving the live fuse projecting from the socket.

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, but if the ring was wired with 1.5mm² wires, when it is split it could support only a 13 A current.

Another advantage of ring circuits was an economy of cable and labour, as one could connect a cable between two existing 15 A radially wired sockets to make one 30 A 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 two 15 A fuses, which worked well enough in practice, even if unconventional.

Many pre-war (round pin) installations used double pole fusing. When two 15 A radials were converted to a ring on these systems, the ring would then be supplied by no fewer than 4 fuses. Such circuits are rare today.