plessey1.jpg (6468 bytes)PLESSEY
PAX 25

Automatic Switchboards
PAX 25 Type
pax25.jpg (15322 bytes) pax25x.jpg (21402 bytes)
Front view with covers on Front view, covers removed
partially equipped for 15 lines and 
2 connecting circuits

The small automatic switchboard illustrated is designed for a maximum of 25 lines and is non-extensible.

The initial equipment may be as low as ten lines, so that it is ideal for small offices, etc. It is arranged for a maximum of four connecting circuits and wiring is provided for the full capacity of the board.

Facilities such as round call or person finder, preference, conference, loudspeaker, tie-line working, party lines, etc., can be readily incorporated.

Special care is taken to ensure a trouble-free system; all items of equipment are enclosed in dust-proof covers, all relays are of the 3000 type with twin contacts, and the switches are of British Post Office approved design.

The board operates from 24 volts, which may be from a small capacity battery, or a battery eliminator where a.c. supply is available.

Dimensions:- 4 feet 9 inches high x 2 feet 7 inches wide x 10 inches deep.
15 lines + 2 connectors - 204 lbs.
25 lines + 4 connectors - 234 lbs.
Further particulars and full description on application.

Taken from the Ericsson catalogue, edition No. 49 & 55

N20361A - Initially equipped with 15 lines and 2 connectors.

PAX 25

pax25n1.jpg (57747 bytes)

Long, reliable service with minimum maintenance is given by this P.A.X., which is designed for business houses, hotels, etc. requiring not more than 25 lines.

The single unit occupies very little floor space.

The stock equipment is 15 line and 2 connector circuits but equipment can be supplied to order and extended as required to the capacity of 25 line and 4 connector circuits, the unit being fully wired to facilitate such extension.

Maximum extension line loop resistance, including the telephone, is 800 ohms.

A 2-digit numbering scheme is used.

Full intercommunication by direct dialling, with complete secrecy except on party lines, is given.

The following special facilities are provided as ordered, the extra equipment listed in the Table being wall mounted unless otherwise stated below.

(1) Preference ('Automatic' or 'Optional'). Enables selected extensions to switch in on engaged lines. No extra apparatus required. Automatic Preference is given by means of a strap on the particular line circuit. For Optional Preference the telephone must have a push-button.

(2) Tie Line. For intercommunication between two P.A.X.s. Up to two circuits can be mounted on the unit; others may be wall mounted. Suitable terminating equipment is necessary at the remote P.A.X., therefore the type of exchange should be specified. Extra equipment is necessary for long lines. Preference extensions can be given access to a busy tie line if required. An extra rectifier per tie line is necessary if the remote P.A.X. is 50 volt.

(3) Loudspeaker with Direct Call. Executive's loudspeaker telephone (page 19) equipped with 10 keys for the direct calling of up to 20 selected extensions by a discriminated ringing signal. The discriminating ringing equipment is a separate item.

(4) Conference and Direct Colt. Direct call keys referred to in (3) above can be used to connect together the executive and up to 10 direct call extensions for conference when the Conference facility is specified. Direct Call facilities do not necessarily include conference.

(5) Secretarial. For the executive who may not wish to be always directly accessible by telephone. Various schemes available.

(6) Two-Party Line. Permits individual service to two extensions over one pair of wires, thus reducing line costs. The two can communicate with each other, and either can make or receive calls without disturbing the other. Three numbers and one line circuit equipment are allocated per two-party line. One additional M relay is required per connector.

(7) Loud-Ringing Bells. A loud ringing mains signalling bell, such as N3111E, can be operated via the contacts of a d.c. relay supplied in a case.

(8) Staff Call. Up to 15 persons may be called by dialling a code number to operate bells or buzzers throughout the premises. A person thus called dials 8 on the nearest telephone and is automatically connected to the caller. The equipment can be mounted on the unit unless a battery eliminator is to be accommodated. Visual code call systems, with or without an audible warning signal, for up to 15 codes can be supplied. Details furnished on request. An extra M relay per connector is required for either system. All signalling devices are ordered separately.

(9) Discriminating Ringing. For use by executives or other selected extensions to indicate when such persons are calling.


pax25n2.jpg (8153 bytes)This is a grey enamelled pressed steel dust-proof unit of small size which stands on the floor and is supported 3 in. (76 mm) from a wall by two brackets at the top. Drillings for floor fixings if required are included. The front cover is removable; the rear cover scaled.

The system operates on the 'line circuit control' principle. Heavy-duty uniselectors (B.P.O. No. 2) are used as linefinders and connectors. Relays are Type N30000 (B.P.O. 3000). The uniselectors are on a hinged shelf.

Line and miscellaneous relays are strip mounted. Jack-in mountings are used for connector and ringing and tones relay sets; the latter incorporates a polechanger.

The incoming wires are soldered to tags at the top of the unit, then cross-connected by jumpers to screw terminals below. A white painted strip on the top ironwork is used for wire designations.

The equipment has full tropical finish. Connecting wires are p.v.c. insulated.

Model No. 20361A

Dimensions of Unit, including supporting brackets:-
Width 30.5 in. (768 mm), height 69 in. (1753 mm), depth 9.5 in. (235 mm).

The working voltage is 20-26V (24V nominal).  With reliable a.c. mains, a 3.0 amp battery eliminator N22406A suitable for inputs of 100/125V or 200 /250V, 40-60 c,/s and which mounts on the unit may be used. Alternatively a 10Ah battery and 1-amp auto charger can be supplied.

If the mains supply is d.c., duplicate 10Ah batteries and a charging resistance are necessary.


Type No.   Equipment   Remarks
N22426T   1-Line Circuit
N22427T   1-Connector Circuit   State circuit No. (3 or 4)
N22429T   Set of Spares
N22400AT   Set of Maintenance Tools
N22422   Set of Installation Material
N22436T   1- Tie Line Circuit   Does not include long line equipment
N22438T   Direct Call without Conference   Serves up to 20 direct call extns
N22440AT   Conf. and Direct Call 1-10   Not more than 10 direct call extns
N22440BT   Conf. and Direct Call 11-20   can be connected for conference
N22444T   Discriminating Ringing   1 per 4 direct call lines
N22446T   1 - Two Party Line   Does not include line circuits
N22442T   Staff Call (audible system)   15 persons max. (Visual systems also supplied)
N22506C   D.C. Relay in Case   For operating loud ringing bell


Taken from the Ericsson Catalogue No. 60

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Last revised: April 18, 2022