Circuit Design
UK electrical circuits are normally described as either radial or ring. A radial circuit is one where power is transmitted from point to point by a single length of cable linking each point to the next. It starts at the distribution board and simply terminates at the last connected device. It may branch at a connection point. Lighting circuits are normally wired in this way, but it may also be used for low power socket circuits.
In a ring circuit, a cable starts at the distribution board and goes to each device in the same way as a radial circuit, but the last device is connected back to the supply so that the whole circuit forms a continuous ring. This means that there are two independent paths from the supply to every device. 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. This practice was adopted in Britain to save on copper during the shortages after World War II. It is unknown in other national wiring codes.
Cables are most commonly a single outer sheath containing separately-insulated live and neutral wires, and a non-insulated protective earth to which sleeving is added when exposed. Standard sizes have a conductor cross sectional area of 1, 1.5, 2.5, 4, 6 and 10 mm2. Sizes of 1 or 1.5 mm2 are typically used for 6 or 10 ampere lighting circuits and 2.5 mm2 for socket circuits. The protective earth conductor in older cables was normally one standard size smaller than the main conductors but is now specified to be the same size.
The earthing conductor is supplied uninsulated since it is not intended to have any voltage difference to surrounding earthed articles. Additionally, if the insulation of a live or neutral wire becomes damaged, then the wire is more likely to earth itself on the bare earth conductor and in doing so either trip the RCD or burn the fuse out by drawing too much current.
Read more about this topic: Electrical Wiring In The United Kingdom
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