Police Telephone Systems

A review of police telephone systems is given followed by a more detailed description of the present standard system


MODERN police problems have shown the necessity for police authorities to have a telephone system which will give rapid inter-communication and signalling between various points in any police area.

The speedy collection and distribution of information is an important factor in efficient organisation and in this connection extreme care is necessary in the selection of sites for street telephone points the positioning should be arranged to allow easy access by the general public and beat officers. The nature of the traffic carried varies from the extremely important emergency calls in respect of fire, police and ambulance services to the routine calls made at set periods by police officers.

It will be appreciated that the process of establishing a call should, at least in so far as the public are concerned, demand as simple an operation as possible, particularly as many people who use the police telephone in times of emergency are not normally telephone users.
The main requirements of an efficient police telephone system are:-

  1. Absolute reliability.
  2. Simplicity in operation.
  3. Instantaneous fault indication on all lines terminating on street units.
  4. Facility for operator to set up a mass call to street points in the shortest possible time.
  5. Visible and audible signals to street telephones.

The purpose of this article is to deal briefly with one or two of the early types of system and, in more detail, with the present-day British Post Office standard system.

Simple Visible and Audible Signal System
The most simple arrangement comprises a simple P.B.X. extension with an associated visual calling signal ; this system was, until quite recently, used extensively in the Metropolis. A relay, which is of the mechanically locking type, is connected in series with the bell and operates, at normal ringing frequency (16.6 cycles per second), to complete a mains circuit for the signal lamp. The lamp is mounted on top of a kiosk, or on a wall bracket, situated preferably at a street corner in order that it may be readily observed from as many points as possible by beat officers.

Although attractive in its simplicity, the system is subject to the failing that the mechanically locking relay must be hand released, and this often necessitates a special visit to the street point. In addition, the signals cannot be readily distinguished during hours of darkness from street lamps, etc ; in this respect the use of coloured lamps is not permitted in view of the possible confusion by road users with traffic signals. The method of signalling does, however, possess the feature that ringing current only is necessary for the operation of the signalling relay.

Relay Scheme
A system was developed by the Post Office in 1934 to meet the requirements of the Metropolitan Police and became known as the Post Office relay system. The telephone was housed in a recessed compartment of Police kiosks and was therefore readily accessible to members of the public from outside the kiosk and to police officers from inside. The particular facilities given by this scheme were:-

  1. A signal was given to the switchboard operator to denote that the street point signal lamp had definitely lit when signalling current had been applied to line, and
  2. the operator was able to clear down the street point signal at will.

A feature of the signalling system in this circuit is that only a momentary application of ringing is necessary to operate the street signal, the circuit being so arranged that retention of the relay is effected by the mains power lighting the lamp.
The equipment designed to give these facilities was arranged for association with a normal P.B.X; it was found necessary, however, to employ a ringing and signalling unit at the Police Station. As it was desired to dispense with the audible signal at the street point in certain instances, it was deemed necessary to provide a signal lamp inside the kiosk in addition to the external lamp in order that a police officer, while in the kiosk, could be signalled.

Metropolitan Police System
Another system, similar to that just described, has recently been designed for use in certain Divisions of the Metropolitan Police Area. The method of use and the facilities provided are almost identical with those of the relay scheme but a departure was necessary in circuit design in view of the restricted housing accommodation at the street point.

As in the relay scheme the special circuits are terminated on a standard P.B.X. and access to the public network is therefore provided. Unlike the relay scheme, however, it is necessary to apply ringing to line throughout the period during which it is required to flash on the street point signal lamp.

The anti-side tone induction coil is employed in this circuit in order that the street noises reproduced in the street point receiver could be reduced to a reasonable level.

A feature of this circuit is the use of an electrolytic condenser in shunt across the coils of relays X and Y in order that a prolonged operation and release of these relays may be effected. The operation of this circuit element is as follows The relay coils are connected in a non-inductive manner, i.e. equal currents to each coil do not operate the relay. When contact C3 closes, the current in the condenser leg opposes the current in the resistor leg until the condenser is charged, thus retarding the operation of the relay. On opening the contact C3, the condenser commences to discharge through the resistor, and the two relay coils in series aiding. During this time the current is producing a magnetic flux in both coils tending to hold the relay operated. This arrangement prevents welding at the contact C3 which would arise if a normal condenser shunt were employed on a single coil relay.

To call a street point the operator flicks the non-locking ring key to the ”On” position to send pulses of 50 c.p.s. A.C. when available, or alternatively 17 c.p.s. A.C. over the B line. During the time the signal lamp is lit an earth is extended to the switchboard termination over the A line to give an indication that the lamp is functioning. Application to line of the telephone loop causes the calling indicator and switchboard lamp to glow until the operator answers. Where the operator has called the street point and a reply is not forthcoming release of the signal may be effected by a momentary depression of the ring key to the “OFF” position.

City of London Police
To meet the requirements of this body a special switchboard was designed in 1933. The facility required was the termination of 80 street point circuits, 20 of which were to be arranged for the setting up of broadcast calls with ease and rapidity; a mass call can be set up by the operation of one key per group of ten circuits. The special switchboard was fitted adjacent to a standard P.B.X. and calls from street points may therefore be extended to normal extension circuits.

Signalling is effected by the reversal of direct current to line and the use of a ringing supply is avoided. Unlike the two systems previously described, the arrangement does not give positive indication at the switchboard of the correct functioning of the street point signal lamp.

The street call points take the form of a microtelephone housed in police kiosks or pillars, the calling signal being a lamp arranged to flash at periods of approximately one second on and one second off.

Switchboard P.A.101
With the exception of the City of London switchboard, the systems previously described are such that termination of the special police circuits on standard Private Branch Exchanges could be arranged; in addition all circuits have been worked on a direct line basis.

In 1932 the Switchboard P.A. 101 was introduced to meet the particular demands of police authorities and, in order that requirements might be provided on an economical basis, the street point telephone circuits were designed on party line principles. This board was arranged in three sizes each having three panels. Panels one and three housed the street point party line terminations and the normal P.B.X. circuits were located on the centre panel. The street point telephone unit was arranged with a lock-up microtelephone for the use of police officers, and, for public use, a loudspeaker and inset transmitter were located behind an unlocked spring-controlled door.

A modified form of this arrangement was necessary for use in the Metropolitan Police Area as the Metropolitan Police, while realising the advantages to be gained by party line working, did not favour the use of separate instruments for police and public use. A modified street unit consisting of a single microtelephone for joint use was therefore employed in conjunction with a modified form of Switchboard P.A.101.

The experiences gained by the use of this particular switchboard served to show that the facilities provided did not fully meet the needs of all Chief Constables. As previously stated the call points were arranged for party line working but discrimination between the particular points on each circuit was not provided; in addition, it was considered that calls from the public side of street points would not need extending to extension points on the installation. Representations from certain bodies necessitated an investigation which resulted in major modifications in so far as the extension of public calls were concerned and, in order that the scheme might be arranged to meet all known requirements, a superseding item - coded Switchboard P.A.150 - was introduced in 1934. The Switchboard P.A. 101, however, adequately provided the facilities required in most areas and many are still working and giving every satisfaction.


The British Post Office Standard for Police Work
This board is arranged in three sizes, each size providing for the termination of 10, 20 and 30 groups of street points respectively. The early issues of this two position, lamp signalling switchboard were not arranged for direct dialling from extensions to the public exchanges; arrangements were made, however, to provide this facility where required by the introduction of units. In view of the rapid conversions to automatic working and the resultant increasing demand for through-dialling it has been decided to add this facility to installations at the outset and the switchboards so arranged have been coded with the suffixed title Switchboard P.A. 150T.D. This switchboard has been adopted by the British Post Office as standard for police work. In addition to accommodating the street point party line circuits it also meets all the facilities provided on a standard P.B.X.

The complete range of terminations is:-

  1. Public exchange lines, to any type of exchange system.
  2. Inter-switchboard private wires. These terminations are readily converted to inter-switchboard extensions when required.
  3. Internal and external extensions.
  4. Public call extensions.
  5. Street call-points.

The facilities provided are as follows:-

  1. Connection of any street call-point (police side) to the public exchange, an internal or external extension, or an inter-switchboard extension line if required.
  2. Connection of street call-points (public side) to the special public call extension (if provided).
  3. Speaking from street call-points (police side), with secrecy against public, to other street call-points (police side) on the same, or any other, party line.
  4. Signalling (flashing) and speaking to street call-points (police side) either separately or simultaneously, in groups of any number.
  5. Distinctive calling signals at the switchboard to indicate whether a call is from a member of the public or from a police officer and from which particular street call-point the call originates.
  6. Visual indication, by means of a lamp on the switchboard unit, that the public side of a street call-point has been operated while a police call is in progress at another street callpoint on the same party line.
  7. Visual indication, by means of steady signal lamps (either internal and/or external at a police kiosk, or external on a pillar), which can be lit, when desired, during the period that the public call door is open, thus attracting the attention of police officers.
  8. Automatic fault indication on all lines terminated on switchboard units, i.e. street call-point lines, whereby any fault is visually brought to notice immediately it occurs.
  9. In order that the apparatus fault-duration period may be reduced to a minimum, the switchboard units, amplifiers and ringing vibrators are of the unit type and are jacked-in.
  10. Through-clearing on exchange calls.
  11. Divided clearing on extension to extension calls.
  12. Supervision on police calls to police on the same or any other party-line group.
  13. Supervision on extended public calls.
  14. Through-dialling by extensions on originating calls.

As has been previously stated Switchboard P.A. 150T.D. has been provided in three sizes, the capacity and equipment of each size being:-

Equipment and Circuits 10 Line Wired 10 Line Equipd 20 Line Wired 20 Line Equipd 30 Line Wired 30 Line Equipd
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
Ringing vibrators - 2 - 3 - 3
Dynamotors, (to supply amplifier H.T.) 2 2 2 2 2 2

Switchboard Description
Of the three panels provided the centre one contains all the normal terminations of a private branch exchange and, in addition, the special extensions arranged to accept public calls. The first and third panels accommodate, in unit form, the street call-point party line terminations ; these units contain all the switchboard apparatus required and are arranged on jack-in principles. The face strip of the unit contains six lamps, four keys and one jack, the purpose of each lamp being

Fault Glows to indicate an earth, low-resistance loop, or disconnection fault.
"A" Call Glows to indicate a call from the "A" street call-point.
"B" Call Glows to indicate a call from the "B" street call-point.
Terminal Glows to indicate a call from the terminal Call street call-point.
Public Glows in conjunction with a CALL lamp, indicating a call from the public side of a street call-point.
Police Glows in conjunction with a CALL lamp, indicating a call from the police side of a street call-point.

Of the four keys, three are ringing keys and serve to ring the individual party line points; the fourth key is operated to speak to the police side of street points. The jack serves to extend calls from the public side to the special extensions; in addition, this jack is used to provide, from a normal cord circuit, the transmission feed necessary to enable a police officer to talk from one street point to another street point on the same party line.

The keyshelf consists of three sections, the first and third accommodating the normal cord circuits and the second the amplifier cord circuits necessary for use in connection with the public side of street call points.

Special Equipment
In addition to the normal P.B.X. equipment and party line units each switchboard accommodates. ringing vibrators, amplifiers for use with the special cord circuits and dynamotors to supply the high tension necessary for anode circuits of the amplifier valves.

Power Equipment
The switchboard battery consists of four 24-volt batteries, forming two sets of 48-volt batteries, the junction of the two 24-volt batteries being connected to earth. Provision is made for 24-volt positive and 24-volt negative potentials for the continuous fault test feature and for other circuits according to the circuit conditions. In addition, a 6-volt battery is provided for the valve filaments and is trickle charged. through a suitable variable resistance.

Two dynamotors (or rectifiers) are necessary, one for charging the positive batteries and one for the negative batteries; the associated charging panel provides facilities for charging the batteries either separately or two simultaneously, this being necessary on account of the unequal drain on the 24-volt batteries under working conditions.

Call Points
A call-point may take the form of a street pillar or a police kiosk. On the street call-point party line there are three stations, the first (or A) station and the second (or B) station being termed. side stations, and the third (or last) station on the line being termed the terminal station. Each party line is capable of serving a maximum of three callpoints with full selective signalling. One or two additional stations may be provided, if required, and. these must be connected. one to each of the side stations. The additional stations are signalled at the same time as the side stations to which they are connected, and incoming calls to the switchboard from an additional station are displayed on the same lamp as the side station to which it is connected.

A specially designed telephone unit is installed at these points from which dual telephone facilities are provided, i.e.

  1. By loudspeaker and transmitter available to the public upon simply holding open a door, which is self-closing but non-locking.
  2. By a microtelephone available only to the police, or other authorised persons in possession of a key.

Calling from street points is effected by an impulse mechanism which is released on removal of the microtelephone by a police officer or, alternatively, by the opening of the door by a member of the public. The mechanism is a suitably modified Dial Auto No. 10, the cam fitted being in accordance with the position (A, B or T) of the point on the party line. Separate mechanisms are necessary for public and police, the public mechanism being fitted with an auxiliary cam and spring set to enable a follow-on earth to be provided for discrimination purposes. Signalling to the street points is effected by the operation of A.C. relays on the A and B points and by operation of a polarised relay at the terminal points. These relays carry mercury tube contacts which serve to close the mains circuit to the signal lamps.

Circuit Operation
Continuous Line Test
A small current (approximately 9 mA) is normally flowing in the line circuit via the terminal point relay to operate relay LA; due to the higher operate current required by relay LB this relay is unable to operate under fault-free conditions: in addition relay V, being wound differentially, does not operate.
Under fault conditions:

  1. Relay LA releases to a disconnection,
  2. Relay V operates to an earth. (Relay LB also operates to “B” line earth and LA remains operated to an “A“ line earth).
  3. Relay LB operates to a Loop.

In each case the fault lamp associated with the line is lit.

Call from a Street Point
When the public door is opened or the police microtelephone is removed the impulse mechanism is released and a train of earth impulses transmitted over the B line from contact PD3 or HS3 to operate relay V. A circuit is completed for the A, B or Terminal lamp in the line relay set according to the impulse cam fitted. Discrimination between public and police calls is given by an additional delayed earth transmitted from contact PD4 of the public impulsing mechanism.

The operator answers a public call by means of a public call cord circuit and, if required, the call may be extended to a public call extension.

Police calls are answered by operation of the line unit “Speak” key.

Call to a Street Point
Calling to street points is effected by operation of the ring keys in the line circuit. Relay K operates and relay IP pulses from a common alarm circuit in order to flash the street point lamp.
The ring A key extends 17 c.p.s. A.C. to the A line.
The ring B key extends 17 c.p.s. A.C. to the B line.

The ring Terminal key extends negative battery to line to operate the polarised relay at the terminal street point.
The remaining circuits terminating on the Switchboard P.A.150 follow the general principles employed on standard private branch exchanges; the exchange line and cord circuits, however, are arranged so that a battery is always present on a party line when the police side of a street point is extended to another point on the installation. The battery makes possible the intrusion of a public call on the police call.

Switchboards P.A. 150 MP
As previously stated the standard police telephone and signal system did not meet the needs of the Metropolitan Police Authorities and when intimation was received (in 1935) that an extension of the police telephone system - on party line principles - was required in the metropolis the design of a new switchboard was considered desirable. The resultant item - coded Switchboard P.A. 150 MP - included both physical and circuit improvements which, from experience gained with switchboards of this type, proved to be necessary.

As with the Switchboard P.A. 101, which was modified for the Metropolitan Police, this body desired to continue the policy of employing a common telephone for the joint use of the public and police officers. This method of use has resulted in a decrease in the equipment required and, therefore, a simplification of circuit design. The circuits employed follow the same principles as in Switchboard P.A. 150 but from the facilities required it will be appreciated that the telephone unit is considerably simplified as a result of the single telephone required at street points; the party line terminating unit also requires less apparatus as discrimination between the different points on the party line only is required. In addition it will be realised that the use of public call cord circuits, amplifiers, etc. is unnecessary. The retention of intrusion facilities, however, is essential and to make this possible the principle of connecting the battery feed from cord circuits and public line terminations is continued. A feature in this respect is that the use of identical circuits on the P.B.X. panel of Switchboard P.A. 150 and Switchboard P.A. 150 MP is made possible.

The existing police telephone arrangements available appear in general to meet all the requirements of police authorities in this country. The conditions to be met in the Metropolitan Police area are, it is thought, exceptional and the departure from standard would, therefore, appear to be inevitable. With regard to the provinces, however, all demands with the exception of Bradford and Glasgow have been met by standard switchboards. The number of street points required at Bradford necessitated the provision of extension wings to enable an addition of 50 party line terminating units to be housed, while at Glasgow is was considered that, in view of the large number of private branch exchange extensions and exchange line terminations, the practice of team working over both the normal P.B.X. circuits and the street points circuits was, for traffic reasons, undesirable. A special 3 position switchboard (Switchboard P.A. 190) was accordingly designed, positions 1 and 2 being arranged exclusively for the normal P.B.X. terminations while position 3 housed the street point party line units.

The Switchboards P.A. . . . referred to have been designed in conjunction with Messrs. Ericsson Telephones, Ltd.


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Last revised: June 04, 2019