Numbering and Labelling
Cable Colour Codes
Internal Telephone Cable colour codes are based on the five colours, blue(B), orange(O), green(G), brown(BN) and slate(S).
These colours can be expanded to twenty pairs :
These single wires are usually twisted with a second wire to form a pair. These second wires take the following colours and take the cable up to 100 pairs :
In an exchange, many circuits are three wire (eg neg, pos and P wires) and so the cable colours above can be further increased by the addition of a turquoise wire to make each pair into a triple.
Small cables are often made up with the following colours :
Current External Quality Cables use the following 30 pair base :
Railway Provided Cables are usually based upon the blue, orange, green , brown, slate sequence but have unpredictable variations. Some are not constructed with pairs but with quads. In a quad, each pair uses the wire opposite it in the quad to produce the pair and this reduces over hearing etc.
Line plant terminations in cabinets and DPs should be numbered "down". If the terminal strips are horizontal then they should be numbered from "left to right".
MDF & IDF
IDF tag blocks always number "up". However it should be noted that some blocks at Norchard exchange have been provided in the past which do not follow this convention. New work should number "up".
Relay Coil & Contact Numbering
It is suggested that the following U point allocation be reserved for new work at Parkend Exchange :
Unallocated U points can be used as required for other connections eg "line out", "prompt alarm", "deferred alarm" etc.