PA350 Police Telephone System

The PA350 Police Telephone System was used in the area covered by the Metropolitan Police in London, UK.

The system used centralised switchboard that covered a pre-defined area with Posts strategically placed.

The Posts were designated Post PA No. 2 by the General Post Office (GPO) and rented to the Police.  Kiosks were owned and installed by the Police force and the GPO would supply the telephone equipment.

This post is sunk into the ground and cemented in place to prevent movement.

After the second world war the Police Systems were simplified and the Post PA No. 3 introduced.  This post was produced with thinner cast iron sections and had no underground structure, as it was bolted to a concrete base.

Component Parts


The lamp was controlled by a Relay-Switch, Combination No. 2 which included the following components:-
18" Cord, Flex., E.L., 250V., Class D3, 0·001 sq. in.
1 x Relay-switch No. 3401NL 1-15 (modified: Tube, Mercury arranged for M. cont. action)
4 x Fuses No. 1/1·5
2 x Fuse-Mounting No. l0lA
1 x Relay-switch. No. 3502NL 1-15
1 x Plug for Socket-outlets No.8
1 x Resistor, Coil, No. 12, 10 ohms
1 x Rectifier-element No. 4/6AA
2 x Resistor, Coil No. 12, 15 ohms

Drawing - 63209.
Diagrams - PA 351 / 352.

Telephone No. 244, minus Cord

Door Closer
Door-closing, No. 7 used on the telephone compartment door.

A New Police Telephone and Signalling System
By W. Porrit
Taken from the POEE Journal Volume 49 - April 1956

A new standard telephone and signalling system has been developed for use in the provinces; it includes only standard items of telephone equipment.

Police alarm systems were introduced as an aid to both the police and the public for dealing with emergency calls and to assist the police in their duties.
They consist essentially of a number of telephones, suitably housed and sited in prominent places in public thorough-fares and directly connected to the police headquarters.

They are available for use by both the police and the public to establish contact with the police headquarters in an emergency with the minimum of delay. A signalling device, normally in the form of a lamp, is also provided at the telephone point to enable the switchboard operator to attract the attention of a patrolling policeman for relaying urgent messages.

The "emergency call" value of the police alarm system to the general public was lessened to a considerable extent by the introduction of the ”999” service, but it still remains an important part of the emergency services, apart from its special value to the police. To date, several systems have been introduced and are in use in the provinces, in addition to the systems used by the London Metropolitan and City police authorities, which are peculiar to the special requirements of the police authorities in the capital.

The various provincial systems in use have several features which have become outdated, notably:-

  1. Special switchboards are used, employing many non-standard components, and special stocks of spare parts must be held for maintenance.

  2. Party line working, which was adopted for reasons of line plant economy, has in actual practice not realised this saving and has sometimes given rise to noise trouble on the circuits. The present tendency to reduce the conductor gauge of distribution cables is also making it difficult to keep such circuits within signalling limits without resorting to bunched cable pairs.

  3. At the time when the systems were designed, people were not very ”telephone minded” and a loudspeaker method of conversation was therefore used for the public side of the call points. Nowadays, the conversations are considered to be too public and unwanted interest is attracted by the loudspeaker and lamp signal when the public side of the call point is in use.

It was therefore decided to introduce a new standard telephone and signalling system for the provincial police which would meet the Home Office requirements and at the same time use only standard telecommunications equipment.

The New System

For the new system, the loudspeaker type of communication has been abandoned and a single hand-microtelephone is provided at each call point for use by both the police and the public (as above). Each call point is connected by an individual circuit to the switchboard at the police head-quarters, which is a standard P.M.B.X., either of the lamp calling type for large installations, or with indicator calling where the installation is small.

The facilities offered by the new system are:-

  1. To call the police headquarters from a call point it is only necessary to lift the telephone handset. The call is answered by the operator as if it were an ordinary extension calling.

  2. For the switchboard operator to attract the attention of a policeman on the beat, a ring key is associated with each call-point circuit termination on the switchboard. These keys are non-locking and momentary operation of the ring key associated with the call point required causes the calling signal to be sent out to the call point. The calling signal consists of ringing current connected in pulses of 0.75 s on and 0.75 s off, and remains locked in automatically until the call is answered at the call point, or is cancelled by a second momentary operation of the ring key. The signal lamp at the call point flashes in response to these ringing pulses. Any number of call points may be called simultaneously. When the call is answered at the call point, the calling signal is tripped, the calling lamp or indicator for that call point on the switchboard is operated and the call proceeds as if it were an incoming call from the call point.

  3. A “proving” circuit is incorporated in the equipment at the call point, and while the signal lamp is flashing satisfactorily an interrupted earth signal is returned over the line to operate a supervisory signal associated with the call-point circuit on the switchboard.

  4. Call-point circuits are treated as normal external extensions and as such may be extended to any other extension, private circuit or exchange line circuit terminating on the switchboard, but through dialling from the call points is not provided.

Sizes of Installation
A survey of existing installations and outstanding requests for police alarm systems showed a wide variation in the number of call points that may be required to be terminated at any one installation. The call-point circuits have been designed for termination on lamp-calling switchboards, but so that smaller installations may be provided more economically, the call points may also be terminated on indicator calling switchboards. It was considered that the total number of circuits terminating at a police headquarters switchboard would usually exceed the capacity of both the cordless and 25-line P.M.B.X. switchboards and, therefore, the 65-line Switchboard, AT 3796 has been standardized for use where indicator-calling circuits are required.

Table 1 gives details of the maximum numbers of circuits that can be accommodated on these switchboards.

It will be seen that private circuits, which form an important part of the police telephone network, are not included in this table. They normally terminate in either the extension or exchange-line jack field and consequently their number must be included in the totals for these circuits.


Circuit Capacity of Switchboard

Type of
Maximum Circuit Capacity
Street Call Points Exchange Lines Extensions
Posn. 1 Posn. 2 Posn. 1 Posn. 2 Posn. 1 Posn. 2
1 Position Indicator Calling (A.T.3796) 20 - 30 - 30 -
2 Positions Indicator Calling (A.T.3796) 20
P.M.B.X. 1A Installation

Above 60

Arranged as required

Switchboard Face Equipment
Associated with each call-point circuit in the switchboard face equipment is a line jack, calling lamp, ringing-supervisory lamp and non-locking “ring” key. Fig. 2 shows the arrangement of these items in the extension multiple field at a P.M.B.X. No. 1A installation. The face equipment is provided in units of ten circuits, taking up 1.5 in. of multiple space.

When indicator-calling switchboards are used, one indicator serves the purpose of both the calling and ringing-supervisory lamps, the two types of signal being distinguished by the calling signal being a continuous operation of the indicator and the supervisory signal a flashing operation.


Equipment Racks
Special line equipment is necessary in each call-point circuit to enable the automatic signalling, call cancellation, and circuit-proving facilities to be provided. At the switchboard end one or more apparatus racks are required to accommodate this auxiliary equipment.

Transmission and Signalling Limits
A central battery telephone is fitted at the call points, the transmission feed being incorporated in the line equipment at the switchboard end of the circuits. The transmission limits for the call points have been assessed on the assumption that calls into the public network, and in particular over the trunk network, will be infrequent. On this basis it has been possible to allow a transmission limit for the call points which is independent of the length of the exchange line or the type of main exchange to which the system is connected. With the normal 24V P.B.X. battery the signalling and transmission limits from the P.B.X. to the call point are 700-ohms loop and 600-ohms T.E.R. respectively, and it is expected that this will enable the majority of circuit requirements to be met. In certain circumstances the limits may be increased to cater for the exceptionally long line, by increasing the voltage applied to line.

Street Call Points
The telephone post used by the Metropolitan Police has been adopted as the new standard. A typical example of the type of post in use in the London area is shown in Fig. 1, which gives an indication of the size of the post. It is of simple box-like construction in cast iron, and is larger than the post used in the earlier provincial systems.

The design of the post for earlier provincial systems was such as to make it unsuitable for use with the new system and, in addition, the policeauthorities were asking for a telephone post which provided them with more room for storing police equipment such as the constable's cape and first aid equipment. The Metropolitan Police post affords these facilities, and was also ideally suitable for accommodating the necessary telephone equipment required by the new system. In addition, it also had the advantage that very little development work was required before production could commence. Patterns for casting the post were in existence, and production could start without the delay that development of a completely new style of post would
have incurred.

The post contains three main compartments, which provide for the termination plunger type ring keys of the electricity supply, space for police use, and the telephone and signalling unit.  The signal lamp is mounted on top of the post. It will be seen from Fig. 1 that the door to the central compartment forms a writing shelf when in the open position. This compartment and the lower compartment are fitted with “Yale” type locks and are normally only accessible to the police.

The notices around the top of the post and in the telephone compartment door are translucent and at night time are illuminated by an internal light.

As an alternative to the post, the call point may take the form of a kiosk. If so, the kiosk is provided by the police authorities, the Post Office being responsible only for the provision of the telephone and associated signalling equipment, which is identical with that fitted in the post.

Fig. 3. Circuit of a Call Point and its Switchboard Termination

Circuit Operation
Fig. 3 shows the circuit for a call point connected through to the switchboard.

Operation of the ring key locks in relay's P and Q, which extend interrupted ringing over the A-wire to the call point.

The high-voltage relay A in the call-point equipment operates to this ringing current and causes the signal lamp to flash. The voltage drop across the 40-ohm resistor in the signal lamp circuit provides an operating voltage for relay B, another high-voltage relay, which returns an earth signal over the B-wire to operate relay LB and hence flash the supervisory lamp associated with the call-point circuit at the switchboard.

Relays P and Q remain operated until released either by the ringing being tripped when the call is answered or by the call being cancelled by a second operation of the ring key.

The incoming loop signal from a call point, when either an outgoing call is answered or an incoming call originated, lights the calling lamp via contact LAI, through clearing being provided by contact LA2.

At the time of writing, the first stage of an installation at Cardiff has been completed and is working satisfactorily, and a short account of this installation has already been published.  Installations are also in hand in three other towns, and it is expected that by the end of 1957 some 21 installations will have been completed, involving the provision of more than 750 call-point circuits, the majority of which will terminate on the new-type post.



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Last revised: March 02, 2022