P.A. 150MP POLICE SYSTEM

POLICE TELEPHONE AND SIGNAL SYSTEM
(USING SWITCHBOARDS, P.A. 150MP)
Call Points

1. General
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.

POLICE CALL-POINTS

2. Sub-station call-points
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).

3. Telephones
"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 is removed.

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.

SUB-STATION EQUIPMENT

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 fixing.

9. Pulse mechanism
This is similar to that provided on "Unit, Telephone, P.A., No. 7," already described.

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 reversals.

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.

List of diagrams

Units, Telephone, P.A. Nos. 8A	Units, Telephone, P.A. Nos. 8T
Units, Telephone, P.A. Nos. 8A Case open	Units, Telephone, P.A. Nos. 8T Case open

Last revised: September 23, 2024