TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Page 50
PART 6
LINE CONSTRUCTION AND LINE
VOLTAGE CURVES
Any form of construction suitable for ordinary telephone working is also suitable
for the Control System, but the ohmic resistance of the line is rather more
important than in ordinary telephone practice, as the voltage of the main sending
battery is dependent on this. The effect of line resistance on battery voltage can be
clearly seen from the curves on pages 51 and 52.
The use of hard drawn copper wire 150 lb per mile is recommended on short
lines, and 200 lb per mile copper wire where longer lines are required. Open wire
construction is usually the best because the static capacitance of wire to wire is small.
The insulation resistance should be maintained as high as possible.
Inductive interference from neighbouring telephone and telegraph lines and
power circuits must be kept to a minimum to ensure good transmission. This can be
best obtained by frequent transposition of the control line and, when practicable, of
the neighbouring circuits. Where specially designed telephone cable is used, only the
ordinary precautions which are usually taken in the case of cable circuits are necessary.
It should be noted that the Line Voltage curves consider the action of the
Selective Mechanism only and ignore Telephone Transmission.
In most cases, wires of sufficient size to give satisfactory Selective working at
a practical voltage also give lines which will be within the limits of good transmission.
In cases of very long lines with many instruments it is possible that, while Selective
requirements may be met, the Transmission would be below the standard.
The transmission efficiency between the Controller and the Way Station sets is
comparable with the high grade of speech obtainable between ordinary telephones.
Owing to improvements made by the arrangement of the high impedance reception
circuit, on sets using the speak listen key, it is possible to maintain this high grade of
transmission, even when as many as 40 of these sets, equally spaced across an open
line 200 miles long, made up of 1501b per mile copper wire, are all in the listening
position. This indicates that good reception will be obtained with the maximum
number of stations which would be listening in at any one time in practice.