P 1002
Issue 1, 1933

Switchboards P.A. 101

1. Capacity
The switchboards are supplied in three sizes, i.e. 10-line, 20-line and 30-line, under the description 'Switchboard P.A. No. 101', and the capacity of each size is as follows (a switchboard is pictured below):-

Police Lines Exch Lines Inter Swbd PW's Extns Cord Ccts Operators Ccts
10 Line Equipped - 2 5 6 5 1
10 Line Wired 10 3 10 20 8 2
20 Line Equipped - 2 6 10 10 1
20 Line Wired 20 4 10 20 12 2
30 Line Equipped - 3 6 10 12 2
30 Line Wired 30 4 10 20 15 2

All switchboards being equipped with:-

10-line 20-line 30-line
Vibrator Ringing No. 4A 2 3 3
Dynamotor No. 20 1 1 3
Units, Amplifying No. 10 2 2 2
Clocks, No. 37 1 1 1

2. 'Unit, Switchboard, P.A. No. 1' (see para. 8) will be requisitioned separately, in accordance with requirements. 

Face Equipment
3. Panels

Three panels are provided: the centre one contains all the normal equipment of a private branch exchange, for exchange lines, extensions and Inter-Switchboard Private Wires.

4. Keys
Dialling keys are provided for use on exchange lines connected to an automatic exchange.

5. Jacks
Three jacks are also provided, for the special cord test required in this system.

6. Lamps
A pilot lamp is supplied with each panel, and a fuse alarm lamp is also provided. A ringer-fail alarm lamp is associated with each 'Vibrator, Ringing No. 4A.'

7. A double-throw key is fitted.  When operated in the downward (locking) position, it connects an alarm hell for the pilot lamps.  In the upward (non-locking) position, it disconnects the alarm bell - when operated by the ringer and fuse alarms - but leaves the lamps glowing until the fault is cleared.

8. Unit, Switchboard P.A. No. 1
Each unit contains all the switchboard apparatus required for one group of street call points.  The units are self-contained and interchangeable, and are readily jacked into position from the back of the switchboard.  The face strip of the unit contains six lamps, three keys, and one jack. The picture below shows the arrangement.

9. The first three lamps from the top are white, and serve to give immediate indication when an earth or disconnexion fault develops on the line; thus ensuring, as far as possible, that immediate attention is given to faults as they occur.

  1. The top lamp glowing indicates an earth on the A -wire of the circuit.

  2. The second lamp glowing indicates an earth on the B-wire.

  3. The third lamp glowing indicates a disconnexion in the circuit. (A short-circuit on the line is indicated by the glowing of the police call (green) lamp when no police officer is on the line).

10. The fourth lamp, coloured red, is the PUBLIC call lamp and glows when a public call door is opened at a street call point (e.g. by a member of the public wishing to communicate with the police).

11. Arrangements are also provided whereby, if it happens that a police call is already in progress on a line when a member of the public sets out to communicate with the Police Headquarters at any other point on that line, the relative PUBLIC (red) lamp will still glow at the Police Station.  Many of the calls from the public must necessarily be of an urgent nature and this provision ensures that a public call will gain attention even when the line is already in use by the Police.

12. The fifth lamp, coloured green, is the POLICE call lamp and glows to indicate a call from the police side of a street call point.

13. The sixth lamp, which is white, is the REPLY lamp and glows to indicate to the operator that an officer whom he has signalled by means of the external signal light (Lantern P.A. No. 1) at the street call point, has come to the telephone in answer to the operator's signal.

14. The three keys are provided to enable the operator to operate the signal lights of the street call points by depressing the keys:-

  1. The two top keys are single-throw locking keys, and are depressed when signalling.

  2. The third key is double-throw, locking in each position.  It is depressed when signalling the point concerned, and raised when the operator wishes to speak to any point on the group.

15. Any one of the street call points connected to the switchboard can be signalled individually, by depressing the particular key associated with that point; or, a group of street call points can be signalled together, i.e., a mass call can be sent out to all street call points at the same time.

16. A label is fitted on each key to indicate the particular street call point with which it is associated.  The label will bear a name or code to be agreed upon locally.

17. A jack is provided, to connect the police side of any street call point to any line on the switchboard by means of the cord circuits.

18. Keyshelf
This accommodates the cord circuits for each operator's position, together with the usual SPEAK, RING and RING-BACK keys.  The number of cord circuits and operators' circuits, provided on each size of switchboard is given in paragraph 1.

19. Positive lamp supervision is given on the cord circuits, and through clearing is provided on all exchange calls..

20. A coupling key is provided at each operator's position, to enable the switchboard to be operated by one or more operators.

21. An AMPLIFIER key, and two lamps, are also fitted at each operator's position.  This is a double-throw locking key, which connects either No. 1 or No. 2 amplifier to the operator's circuit when the key is operated, and lights the lamp associated with the particular amplifier taken into use.  The lamps are multiplied at each operator's position, to indicate which amplifier is in use;

22. Units, Amplifier No. 10
Two of these units are provided in each switchboard, and are simple 2.valve amplifiers using 6-volt valves.  The valve in the first, or inter-valve position, is a 'Valve, Thermionic No. 76'; the second, or output valve, is a 'Valve, Thermionic No. 77.'

23. The low-tension voltage is supplied from a 6-volt battery; and, in order to avoid inter-action, a separate low-tension battery is provided for each amplifier.  No current is taken by the amplifiers until the AMPLIFIER key is thrown.

24. When an AMPLIFIER key is thrown, the operator's speech is diverted through the amplifier and amplified, before being passed to line to operate the loud speaker at the street call point.  Conversation then takes place between the operator and the public caller, with normal speech on the simplex principle from the public caller to the operator, and amplified speech in the reverse direction.

25. The circuit is arranged so that amplified speech can only be transmitted to street call points: it is impossible to speak on the cord circuits while the AMPLIFIER key is thrown.

26. Each amplifier is made up as a complete unit, and is jacked in to a jack on the base of the switchboard, thus facilitating replacement.

27. 'Dynamotor No. 20'
One of these is fitted per switchboard, and provides the 180-volt high tension supply required for the plate circuit of the valves in the 'Units, Amplifying No. 10.'

28. The dynamotor is operated from the main positive battery, and is started by an earth on its negative terminal via the contacts of the L.T. relay, which is operated when the AMPLIFIER key is thrown.  The machine then supplies 180 volts at its output terminals.

29. 'Ringing Vibrator No. 4A.'
These are supplied in accordance with the size of the switch. board, and the standard provision is shown in para. 1.  The wiring of the vibrators is terminated on a jack fitted on a metal wall frame, and the vibrator is jacked in, thus facilitating removal for maintenance purposes.

30. Operator's Instrument
The instruments used by operators may be either of the headgear and breastplate type, or the microtelephone type, in accordance with the desires of the Police authorities.

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Last revised: December 02, 2022