P 1102
Issue 1, 1936

(Using Switchboards, PA 150 and 150TD)

1. General
This instruction describes the switchboards and associated equipment used in the police telephone and signal system.  Figure 1 illustrates the type of switchboard and shows the face equipment, cords and keys.  Figure 2 shows the equipment mounted in the rear of the switchboard.

Figure 1

Figure 2

2. Capacity
The switchboards are supplied in three sizes, i.e. 10-line, 20-line and 30-line, under the description "Switchboards, PA 150, N-line."  The capacity and equipment of each size is given in Table 1.


Equipment and circuits

10 Line 20 Line 30 Line
Wired Equipped Wired Equipped Wired Equipped
Party Lines 10 - 20 - 30 -
Exchange line circuits 10 4 10 5 10 5
Inter-switchboard private wires 10 3 10 5 10 5
Extension line circuits 20 10 30 10 30 10
Public call extension circuits 6 6 6 6 6 6
Cord circuits 12 6 16 10 20 12
Operators' circuits 2 2 2 2 2 2
Public call cord circuits 6 4 6 4 6 4
Ringing circuits 2 1 2 2 2 2
Vibrators, Ringing No. 4A * - 2 - 3 - 3
Dynamotors, No. 20 2 2 2 2 2 2
Units, Amplifying No. 13 6 2 6 2 6 2

* These figures include one spare vibrator in each case.
A "Clock No. 43 (was Clock No. 37), with bracket," is supplied with each switchboard.

3. Face equipment panels
Three panels are provided.  The centre one contains all the normal equipment of a private branch exchange, viz., for exchange lines, extensions, and inter-switchboard private wires.  In addition, the public call extensions are terminated on this panel.  The first and third panels accommodate the "Units, Switchboard, PA No. 2," which form the termination of the street call-point party-lines.  "Units, Switchboard, PA No. 1, Dummy" are fitted to fill up the unequipped positions in these panels.

4. Dial and hold exchange facilities
These facilities are provided by a 3-position key in each exchange line circuit.

5. Cord tests
A strip of five jacks is provided for the two cord-test circuits required in this system.

6. Lamps
A pilot lamp is fitted in each panel, and a FUSE ALARM lamp is also provided. A RING FAIL alarm lamp is associated with each "Vibrator Ringing No. 4A."

7. Alarms
The alarm bell is controlled by a 3-position key.  When operated in the downward (locking) position, it connects the alarm bell to the pilot lamp circuits. In the upward (non-locking) position, it disconnects the bell - if operated by the, ringer and fuse alarms - but the lamps remain glowing until the fault is cleared.

8. Multi-Phone working
A Key-mounting KY is fitted at the top of the centre panel to accommodate keys for multi-phone working when required.

9. Unit, Switchboard, PA No. 2
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, four keys and one jack, as shown in Fig. 3. The basis of provision of these units is detailed in P 3103, para. 4.

Figure 3

10. The arrangement and purpose of the six lamps is given in Table 2.


Designation Caps, Lamp Purpose
FAULT White with Black Ring Glows to indicate an earth, low-resistance loop, or disconnexion fault.
"A" CALL White Glows to indicate a call from the "A" street call-point.
"B" CALL White Glows to indicate a call from the "B" street call-point.
TERMINAL CALL White Glows to indicate a call from the terminal street call-point.
PUBLIC Red Glows in conjunction with a CALL lamp, indicating a call from the public side of a street call-point.
POLICE Green Glows in conjunction with a CALL lamp, indicating a call from the police side of a street call-point.

11. Of the four keys, three are 2-position locking keys, designated RING A, RING B, RING T, one being associated with each call-point of the group for ringing purposes.  The fourth key, which is also a 2-position locking key, is designated SPEAK and is used when speaking to the police side of the call-point.

12. A label is fitted adjacent to each RING key to indicate the particular street call-point with which it is associated.  The label is intended to bear a name, or code, and number, to be agreed upon locally.

13. A line jack is provided to allow of the various connexions referred to in P 1103, paragraph 4, being made.  A label is associated for use if required.

14. Keyshelf
This consists of three sections and is illustrated in figure 4.  The first and third sections accommodate the cord circuits for each operator's Position, together with the usual SPEAK-RING keys and also a RING-BACK key.

A COUPLING key is provided on each operator's Position, to enable the switchboard to be operated by one or more operators.  The number of cord circuits and operators' circuits provided on each size of switchboard is given in Table 1.  Positive lamp supervision is given on the cord circuits, and through-clearing is provided on all exchange calls.  The second section accommodates the public call cords, used in answering calls from the public side of street call-points, and the extension of these calls to special public call extensions.  Associated with each pair of cords is a RING key, a POSITION-SPEAK key, an alarm lamp and a supervisory lamp.  The POSITION-SPEAK key is used to connect the cord circuit with either operator's Position and the alarm lamp to indicate incorrect operation or failure of an amplifier.

Figure 4

15. Dials
A dial mounting is fitted in front of the key mountings of the public call cord circuits, to accommodate the "Dial, Automatic No. 10".

16. Units, Amplifying No. 13
These are single-valve amplifiers using "Valves, Thermionic No. 108," to provide amplified speech-currents for the loudspeakers at the street call-points. Each amplifier is made up as a complete unit, and is jacked-in to a jack mounted on top of the switchboard.  The low-tension voltage is supplied from a 4-volt battery common to all amplifiers.

17. Dynamotors No. 20
These provide the 180-volt high-tension supply for the anode circuits of the valves in the "Units, Amplifying No. 13" and are accommodated inside the switchboard.

18. Vibrators, Ringing No. 4A
These are supplied in accordance with the size of Switchboard and the standard provision is shown in Table 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.

19. Operators Instruments
The instruments used by operators may be either of the headgear and breast plate type or microphone type, in accordance with the desires of the Police Authorities.  Where a breastplate type of instrument is required a "Transmitter No. 23A" should be used with a headgear receiver.

20. Unit Auxiliary Apparatus PA266
This unit incorporates the equipment necessary to modify five exchange line terminations for "through dialling" on Switchboards, PA 150.  The relays are mounted on a frame similar to a Unit Switchboard, PA No. 2, except that it does not "jack in."  The unit is held in position on rails, which enable the unit to be withdrawn clear of the line units to facilitate relay adjustments (see P 1101, para. 2, and P 3103, para. 12).

List of diagrams

Switchboard in St Albans Police Station (1935)


BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: December 02, 2022