plessey1.jpg (6468 bytes)PLESSEY
PAX 50

Automatic Switchboards
PAX 50 Type

pax50.jpg (23765 bytes)Picture shows a fully equipped system with cover removed

When the initial equipment required is somewhere about 30 lines and it is anticipated that the ultimate will never exceed 50 lines, the above switchboard is the one to adopt.

PAX 50 like PAX 25, is non-extensible and operates on 24 volts from batteries, or battery eliminator where a.c. supply is available. It is self-supporting but wall brackets which act as cable supports are provided.  The apparatus is of B.P.O. type and is totally enclosed.

Two registers are fitted, these receive the dialled impulses and transmit them to the switching apparatus to put the call through. A maximum of seven conversations may take place simultaneously.

The whole of the equipment is protected by a dust-proof cabinet.

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

24V DC working.

Dimensions:- 5 feet 9 inches high x 2 feet 7 inches wide x 12 inches deep.

30 lines + 4 connectors - 412 lbs.
50 lines + 7 connectors - 476 lbs.

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

N20365A - Equipped for 30 lines and 4 connectors.
N20365B - Equipped for 25 lines and 5 connectors.
N20365C - Equipped for 25 lines and 5 connectors.

P.A.X. 50

pax50n1.jpg (59515 bytes)

This equipment, similar in many respects to P.A.X. 25, is used when not more than 50 lines will ultimately be required. Excepting power equipment, and possibly some items required for special facilities, all apparatus is accommodated on a single unit.

The stock unit is equipped for 30 line and 4 connector circuits, but equipment can be supplied to order and extended as required to the capacity of 50 line and 7 connector circuits, the unit being fully wired to facilitate such extension.

Maximum extension line loop resistance, including the telephone, is 800 ohms, but long line equipment to extend this limit to 2000 ohms or more can be supplied.

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, extra equipment being on the unit or wall mounted, depending upon the quantity.

(1) Preference ('Automatic' or 'Optional'). Enables selected extensions to switch in on engaged lines. A resistor per preference line is required. With Optional Preference, access to an engaged line is obtained by dialling a code digit and then the number of the extension required.

(2) Tie Line. For intercommunication between two P.A.X.s. Up to three 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 and to a busy extension at the remote P.A.X. 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 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 Call. 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. Two numbers are allocated per two-party line.

(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. Visual code call systems, with or without an audible warning signal, for 7, 15 or 30 codes can be supplied. Details furnished on request. All signalling devices should be ordered separately.

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

(10) P.B.X. Hunting. Enables a number of lines (usually up to 4) to be called by dialling the first line directory number. If the first line is engaged, the call is automatically connected to the second line, and so on. If the directory number of the second or third line is dialled the call is not connected to the next line under engaged conditions.

(11) Emergency Lines. Where lines are used as reporting points for emergency calls such as fire alarm, ambulance, etc., normal line circuits would be terminated on telephones (usually red) not equipped with a dial, to prevent them being used for outgoing calls. If it is required that these extensions be called by dialling a number outside the normal numbering scheme, then additional equipment is necessary.


pax50n2.jpg (7563 bytes)This is a grey enamelled pressed steel dust-proof unit of small size, totally enclosed by removable doors at the front and rear, the supporting wall brackets at the top being 21 in. (533 mm) long, to allow access to the rear. Drillings for floor fixings if required are included.

The system operates on the 'register control' principle, the dialled impulses being received by either of two register circuits which drive the connector switches to the appropriate outlets.

The apparatus is arranged as shown in the illustration. All switches are Uniselectors of heavy-duty (B.P.O. No. 2) Type. N30000 (B.P.O. 3000) type relays are used in all except the line circuits, which employ twin relays Type N44700. The ringing and tones relay set incorporates a pole changer.

Three tie line circuits, or apparatus for other special facilities, can be accommodated in the space near the bottom of the unit.

The incoming wires enter through the top of the cabinet and are soldered to connection strips alongside the Uniselectors. A distribution box providing cross-connecting facilities for the lines can be supplied for wall mounting if specified.

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

Model No's
Equipped for 30 lines and 4 connectors - N20365A
Equipped for 40 lines and 5 connectors - N20365B
Equipped for 50 lines and 5 connectors - N20365C

Dimensions of Unit: Width 30.5 in. (775 mm), height 66 in. (1676 mm), depth 12.5 in. (324 mm) plus 2.5 in. (533 mm) for supporting brackets.

The working voltage is 20-26V (24V nominal).
If a reliable a.c. mains supply is available, a battery eliminator N22406B, may be used. It is designed for inputs of 100/125 or 200/250V, 40-60 c/s, has a 5.0A output, and mounts on a wall.  Alternatively a 20Ah battery and 3-amp auto charger can be supplied.   If the mains supply is d.c., duplicate 20Ah batteries and a charging resistance are necessary.

For ordinary extensions, Etelphones N1900 & N1065 are usually recommended. A special telephone for executives is available. Part No. N23868A.


Type No.   Equipment   Remarks
N44846TK   1-Line Circuit
N22451T   1-Connector Circuit   State Circuit No. (5, 6 or 7)
N22452T   Set of Spares
N22400AT Set of Maintenance Tools
N22422 Set of Installation Material
N3592A   Resistor for Pref. Line One per preference line
N22460T   1-Tie Line Circuit
N22462T   Direct Call without Conference   Serves up to 20 direct call extns
N22464AT Conf. and Direct Call 1-10   Not more than 10 direct call extns
N22464BT Conf. and Direct Call 11-20   can be connected for conference
N22469T   1-Two-Party Line   Does not include line circuits
N22466T   Staff Call (Audible System)   For a maximum of 15 persons, (Visual systems for 7, 15 or 30 codes 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