When ‘new-style’ wiring was introduced in the early eighties a domestic single line telephone installation would have a master socket which is provided by the service provider (B.T., cable company etc.). This was usually an LJU2/1A or a LJU3/1A type of socket. A master socket contains a 1.8µF capacitor to pass ringing current to the telephones, a surge arrestor 11A or 26A to suppress high voltage spikes etc. and a 470k ohm resistor for testing purposes. Additional secondary sockets are wired off the master as shown above, the colour codes shown are the standard, but may differ in some existing installations. The wiring diagram for a master LJU socket is shown here. Note that the pictorial view shown above only serves to indicate the two differing types of sockets and wiring connections. Contrary to how it might seem, C1 is connected to terminals 2 & 3, not 1 & 4

On older installations (pre-1980) cream or grey cables were used. They contained conductors coloured blue to pin 2, orange to pin 5, brown to pin 3 and green to pin 4 (if used), on some conversions green was used for pin 3 and brown for pin 4. On some installations of internal extension wiring (post-2012) BT/Openreach have now come full circle and have reverted back to using white four core cable containing solid colours: blue to pin 2, orange to pin 5, brown to pin 3 and green to pin 4 (if used).

Additional Bells are wired between pin 3 (orange/white) and pin 5 (white/blue).

External overhead telephone line colour code: orange / white = line 1; green / black = line 2.