The document you require is BEBS S12 (1964) which is available for purchase from the documentation section of the Energy Networks Association
. However, this document has now been superceded by the ENA specification TS 50-19 (2004). This new specification is also available for purchase from the Energy Networks Association.
The standard gives a basis for wire numbering by use. It consists of a letter, followed by a two or three digit number and possibly a suffix of a further letter. As an example, a rough guide to the initial letters we use is:
A, B, C for protection class CTs
D for metering class CTs
E for VTs,
G for voltages used for confirming synchronisation of supplies
H for mains (230v / 400v ac), odd numbers being live, evens neutral.
J for DC supplies, with odd numbers being positive
K for controls (Circuit Breaker opens, closes, trips), with odd numbers being positive
L for indications and alarms, with odd numbers being positive
S for analogue transducers, with odd numbers being positive
T for pilot cables