PA 450 POLICE SYSTEM
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The various provincial systems (PA 150) had several features which had become outdated, notably:-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 (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 1A or PABX 2 or 3. 120 call points can be terminated on a PMBX 1A.
The facilities offered by the new system were:-
(a) 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.
(b) 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.
(c) 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 the switchboard.
(d) 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 operation.
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 equipment.
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 (Post 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).
Taken from the IPOEE Journal - April 1956
The end of an era!
Last revised: July 08, 2017