PA350 Police Telephone System
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, 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.
The lamp was controlled by a Relay-Switch.
RELAY-SWITCHES, COMBINATION, No. 2
Diagrams PA 351-2.
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.
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
Telephone No. 244, minus Cord
Door-closing, No. 7 For 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
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
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
(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:-
(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 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.
(c) 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.
(d) 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
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
|Maximum Circuit Capacity
|Street Call Points
|1 Position Indicator Calling (A.T.3796)
|2 Positions Indicator Calling
|P.M.B.X. 1A Installation
|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.
Fig. 2. ARRANGEMENT OF CALL-POINT CIRCUITS ON THE SWITCHBOARD
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
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
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.