Introduced in the mid 1950's the PA 450 system was an
upgrade of the PA 350 system, which was used in
the London Metropolitan Police area. The new system was standardised
using mostly GPO equipment.
The PA 450 system superseded the PA 101 and PA 150 systems.
The PA 350 system used a Post P.A. No. 2 and this was
modified to reduce the cost and weight of the post. The Post was then
called the Post P.A. No. 3.
Different switchboards were used and these were based on GPO
stock switchboards thus no longer necessitating the need for a bespoke
The older PA 101 and PA 150 systems were extremely technical and all
components were special to the system. The PA 450 was technically much
simpler and therefore easier to maintain.
The system was also designed to operate lights on a location
map board if required.
As normal switchboards were used two special
terminating units were developed: the PA454 and 455. These would be
mounted on an apparatus rack.
Two other control units were also introduced, these being a
standby vibrating ringer and a ringing interrupter.
The ringing interrupter caused the remote post lamp to flash
on and off.
The system could also be installed in Police Kiosks.
Equipment on site would just be the lantern, telephone and relay-switch.
Typical arrangements in a Police Kiosk (Drawing EC2072).
A New Police Telephone and Signalling System
By W. Porrit
Taken from the POEE Journal Volume 49 - April 1956
new standard telephone and signalling system was developed for use in the provinces; it
includes only standard items of telephone equipment.
INTRODUCTION OF PA 450
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, was 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. Several systems were introduced and were used in use in the provinces, in
addition to the systems used by the London Metropolitan and City police authorities, which
were peculiar to the special requirements of the police authorities in the capital.
The various provincial systems (PA 150) had several features which had become outdated,
(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.
London Metropolitan Police variant showing clear lamp
THE NEW SYSTEM
For the new system, the loudspeaker type of communication (as used in PA 150
installations) has been abandoned and a single hand-microtelephone was provided at each
call point for use by both the police and the public. Each call point was connected by an
individual circuit to the switchboard at the police headquarters, which is a standard PMBX
No. 1A or PABX No. 2 or 3. 120 call points can be terminated on a PMBX No. 1A.
The facilities offered by the new system were:-
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
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 were 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 consisted of ringing current connected in pulses of 0.75s on and 0.75s off, and
remained locked in automatically until the call was answered at the call point, or was
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.
A "proving" circuit was 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
Call-point circuits were treated as normal external extensions and as such may have
been extended to any other extension, private circuit or exchange line circuit terminating
on the switchboard, but through dialling from the call points was 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 have been required to be
terminated at any one installation. The call-point circuits had been designed for
termination on lamp-calling switchboards, but so that smaller installations may be
provided more economically the call points may have 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 PMBX switchboards and, therefore, the 65-line Switchboard, AT 3796 had been
standardised for use where indicator calling circuits are required.
Switchboard Face Equipment
Associated with each call-point circuit in the switchboard face equipment was a line jack,
calling lamp, ringing supervisory lamp and non-locking "ring" key. On PMBX No. 1A
installations the face equipment was provided in units of ten circuits. When indicator
calling switchboards were 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
Special line equipment was necessary in each call-point circuit to enable the automatic
signalling, call cancellation, and circuit-proving facilities to be provided. At the
switch board end one or more apparatus racks were required to accommodate this auxiliary
Transmission and Signalling Limits
A central battery telephone was 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 had 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 was 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 PBX battery the signalling and transmission
limits from the PBX to the call point were 700-ohms loop and 600-ohms T.E.R. respectively,
and was expected that this would enable the majority of circuit requirements to be met. In
certain circumstances the limits may have been 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 (Post PA No. 2) has been adopted as the
new standard. This Post has been superseded by the Post PA No. 3 (see picture at top) which
bolts directly to the footway. This greatly reduced the weight of the Post as the 18 inch
foot was removed. The Post was of simple box-like construction in cast iron, and was
larger than the post used in the earlier provincial systems
PA No. 1). 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 police authorities 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 (PA 350) 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 of the
electricity supply, space for police use, and the telephone and signalling unit. The
signal lamp is mounted on top of the post. The Metropolitan area posts had a white lamp
cover whilst all others had an amber lamp cover.
The central compartment forms a writing shelf when in the open position. This compartment
and the lower compartment are fitted with "Yale" type locks (key, lock W) and
are normally only accessible to the police. The Mains relay set was installed at the back
of this compartment and this was called a Unit Signalling PA No. 2.
A mains operated bell (Bell No. 61A) could also be fitted in the telephone compartment. 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 have taken the form of a kiosk. If so, the kiosk was provided by the police
authorities, the Post Office being responsible only for the provision of the telephone,
calling lamp and associated signalling equipment, which is identical with that fitted in
the post (except the lamp fitting which was similar to that fitted to a Post PA No. 1).
By April 1956 the first stage of an installation had been completed and working
satisfactory. It was expected that by the end of 1957 some 21 installations would have
been completed, involving the provision of more than 750 call points, the majority of
which will terminate on the new type post.
Taken from the IPOEE Journal - April 1956
The end of an era!
With the introduction of Police mobile Radios in the early sixties, the Police pillar and
kiosk quickly became redundant. A few stayed in place for many years i.e. pillars in
Central London and Totnes, with kiosks surviving into the nineties in Scotland and
In the late 1980,s British Telecom started to recover the Police Posts in
London. About 9 were left in situ.
These Posts had been updated and were is varied states of repair.
Some had had the mains electrics updated and in general the mains relay set
had been replaced by a modern equivalent.
Click here for
information of the London Police Post restoration