P.O. ENGINEERING DEPT.
Issue 2, 1958
POLICE TELEPHONE AND SIGNAL
(USING "SWITCHBOARDS, P.A. 150MP")
A call-point may take the form of a street pillar or a police kiosk. In a
Street call-point party line there are three stations, the first (or A) station
and the second (or 13) station being termed side-stations. The third (or
last) station on the line is known as the terminal station. Each party line is
capable of serving a maximum of three call-points with full selective
signalling. Up to two additional side-stations may be provided, if
required, except on party lines which are teed at an exchange or cross-connexion
cabinet for signalling reasons (TRANSM., Tele., B 3572). They must be
connected as additional A and B stations in the manner indicated in P 3103.
An additional side-station is signalled at the same time as the normal
side-station and incoming calls to the switchboard are displayed on the same
lamp as the normal side-station.
These are generally situated in private residences, or other premises to which
the public have not access to the telephone. As the probability of two
sub-stations being required as side-stations on one party-line group are remote,
only two types of unit are available, i.e. for "A" and "terminal" call-points.
If necessary, however, a "Unit, Telephone, P.A., No. 8A" can be converted to
work as a "B" point by fitting the "B" point cam, which is supplied with and
carried in the case of the "Unit, Telephone, P.A., No. 8A" (see P 3202).
"Unit, Telephone, P.A., No. 7A, 7B or 7T "(see picture to the right), is
installed at the call-points. The unit is provided with a hand-set "Telephone
No. 164," suspended on a gravity-switch, and is used by both police and public.
4. "Mounting for Unit, Telephone, P.A., No. 3"
This base (see picture below), provides a jack-in mounting for the telephone
unit to which the permanent line-connexions are wired. In addition, the mounting
accommodates the fuses and double-pole switch for the local power supply for
lighting the signal lamp. When the unit is jacked in position the switch is held
operated, and when the unit is removed the switch is released; the object of
this arrangement is to cut off the power supply from the jack, whenever the unit
5. The line wires, earth to the instrument, and also the power
leads, are terminated on a special 5-point jack mounted on the base, and the
connexions of the telephone unit are similarly connected to a 5-point plug.
The telephone unit can be jacked-in to the connexions on the mounting, and is
held in position by a locking screw on the side of the mounting.
Mounting for Unit, Telephone, P.A., No. 3 to the left and
Unit, Telephone, P.A., No. 7 with case removed to the right
6. Pulse mechanism
The pulse mechanism consists of a modified automatic dial mechanism with metal
cover. The mechanism is normally held in the wound position, and is released on
the removal of the hand-set from the gravity switch. On release, a train of
earth pulses is connected via the pulsing springs to the B-wire of the line for
signalling the switchboard. The number of pulses transmitted varies with the
different call-points, to provide selective signalling. A second set of springs
completes the line circuit.
7. Flashing relays
These relays are provided with mercury-tube contacts and are of the A.C. type in
the "A" and " B " telephone units, whilst in
terminal " units polarized relays are fitted. Relays of the A.C. type respond to
ringing currents, and the polarized relays to battery reversals, and the signal
light is caused to flash at one-second intervals by the mercury-tube contacts
completing and disconnecting the circuit of the power supply to the lamp.
8. The "Units, Telephone, P.A., Nos. 8A and 8T" (see pictures
below) are provided with "Telephones No. 164." The "A" unit includes
a magneto bell, and is supplied with a "B" point cam for conversion to "B" point
working. Where necessary, arrangements should be made for this conversion
to be made locally. The equipment is assembled in a wooden box for wall
9. Pulse mechanism
This is similar to that provided on "Unit, Telephone, P.A., No. 7," already
10. Signalling relays
These relays complete the alarm-bell circuit, and in "A" units are of the A.C.
type, whilst in terminal units polarized relays are fitted. The A.C. type
of relay responds to ringing current, and the polarized relays to battery
11. Signalling bells
On "A" units the magneto bell responds to the ringing current. On
"terminal" units a trembler bell is provided, and its circuit is completed by
the signalling relay. Extension-bell facilities can be provided if necessary.
|Units, Telephone, P.A. Nos. 8A
||Units, Telephone, P.A. Nos. 8T
|Units, Telephone, P.A. Nos. 8A
|Units, Telephone, P.A. Nos. 8T