plessey1.jpg (6468 bytes)PLESSEY
PAX 50/400
Automatic Switchboards
PAX 50/400

pax50400.jpg (29079 bytes)Being of extensible type, this register-controlled PAX is ideal for progressive organizations where expansion from time to time may be anticipated. It employs single motion selectors throughout and  may be partially or fully equipped up to a maximum of 400 lines, the minimum number of lines being 50.

The exchange equipment is composed of 50-line units specially designed to facilitate extension, an open-type main distribution frame (accommodating fuses and arrestors, if required) and the power plant.

Maximum extension-line loop resistance, including the telephone, is 1000 ohms, but long-line equipment to extend this limit can be supplied.

Numbering and Facilities
A 3-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 is generally wall mounted.

Preference ('automatic' or 'optional')
Enables selected extensions to switch in on engaged lines.
One resistor per preference line is required for fitment on the unit. For automatic preference, one PF relay per register is also required. With optional preference, access to an engaged line is obtained by dialling a further digit.

PAX 50/400 stock units, with some apparatus covers and the front doors removed is shown above. Note cable support on top.

Tie Line
For intercommunication between two PAX's. Suitable terminating equipment is necessary at the remote PAX, therefore the type of exchange should be specified.

Extra equipment is necessary for long lines. Preference extensions can be given access to a busy tieline and to a busy extension at the remote PAX if required.

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.

Conference and Direct Call
Direct-call keys referred to in the previous paragraph 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.

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

Two-Party Lines
Permits individual service to two extensions over one pair of wires. The two can communicate with each other, and either can make or receive calls without disturbing the other. Three numbers are allocated per two-party line.

Loud Ringing Bells
A loud ringing mains signalling bell, such as N3137B, can be operated via the contacts of a d.c. relay supplied in a case.

Staff Call
Selected 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. There are various visual schemes for up to 64 codes, or audible schemes, or schemes giving a combination of both audible and visual signals. Details on request.
All signalling devices should be ordered separately.

Discriminating Ringing
For use of executives or other selected extensions to indicate when such persons are calling.

PBX 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.

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, additional equipment is necessary.

Switching Units
These are grey enamelled, pressed-steel, dustproof units, with removable front and rear doors. They are self-supporting and drilled for floor fixing.

Units may be type 'A' or 'B', equipped to order and are installed in suites in the sequence: A.B.B.A.B.B.A.B.

In order to simplify extension, all are fully wired and the maximum number of wire connections are completed in the factory. All switches are heavy-duty (BPO No. 2) uniselectors. Line relays are twin type N44700; other relays are N30000 (BPO 3000) type.

Inter-unit cables are run through the sides of the cabinets, whilst I.D.F. cables are on supports attached to the top. Both sets of cables are soldered to connection strips.

Stock Units
The equipment has full tropical finish. Connecting wires are p.v.c. insulated. Cables are sheathed in cream p.v.c.

Main Distribution Frame
This is a single-sided, open-type frame for wall and floor fixing. It is equipped with connection strips and jumper rings to form a cross-connecting field for the lines. Should any of the lines require fuses and arresters, these can be accommodated.

Power Equipment
The working voltage is 46-54V (50V nominal).  If the number of lines will never exceed 200 and a reliable a.c. mains supply is available, a battery eliminator N22405B, output 10A for up to 100 lines, or N22405C, 15A for up to 200 lines, may be used. Both are designed for inputs of 100/125 or 200/250V, 40-60Hz, and for wall mounting. A battery and auto charger are recommended for larger equipments, or in lieu of the eliminator for the smaller installations if preferred.

With a d.c. supply, duplicate batteries and a charging resistance are necessary.

Extension Telephones
Plessey phones (Catalogue Sheet 7159), specifically developed for PAX working, are recommended for general use. Alternatively, other types of standard automatic telephones such as ETELPHONES (Catalogue Sheet 7145) can be used.

These are equipped as indicated below, and wired to cater for ultimate quantities quoted in brackets.

'A' Unit   'B' Unit
N22475   N22476
*6 (7)   4 (7)   Group selector switches
*6 (7)   4 (7)   Linefinder switches
*5 (6)   3 (6)   Connector switches
2 (2) sets   1 (1) set   Register, 'units', 'tens' and 'hundreds' switches
50 (50)   25 (50)   Line relays, twin type Strip mounted
*6 (7) sets   4 (7) sets   Group selector relays
1 (1) set   1 (1) set   Miscellaneous relays
5 (5)   3 (5)   Connector relays (upper shelf)

1 (1) Connector relays (lower shelf) Jack-in
2 (2) Register relays (lower shelf)
1 (1) Tones set (lower shelf)
1 (1) Ringing pole changer (lower shelf)

Group selector and Linefinder No. 7 and Connector No. 6 are normally reserved for special requirements.


Equipment Item   Height in (mm)   Width in (mm)   Depth in (mm)
Both A & B Units   69 (1753)   30 (775)   18 (457)
10A battery eliminator   33 (838)   14.1 (375)   10.5 (267)
15A battery eliminator   33 (838)   18.1 (476)   10.5 (267)

The height stated does not include the overhead cable support


Equipment   Item Code No.   Net Weight lbs (kgs)
A unit   N22475   488 (222)
B unit   N22476   400 (182)
10A battery eliminator   N22405B   105 (47.7)
15A battery eliminator   N22405C   140 (63.6)

 Additional Equipment
Type No. & Equipment Remarks

N44847TK line circuit
N22477T line finder and group selector circuit State circuit No. (5, 6 or 7)
N22478T 1-connector circuit State circuit No. (4, 5 or 6)
N22479T Set of spares (for 100 lines)
N22400AT Set of maintenance tools
N22422 Set of installation material
N3554R Resistor for pref. line One per preference line
N129214TK Relay for auto pref. One per register (Auto pref. only)
N22480T 1 tie-line circuit Does not include long line equipment
N22483T Direct call without conference Serves up to 20 direct call extns.
N22485AT Conf. and direct call 1-10 Not more than 10 direct call extns.
N22485BT Conf. and direct call 11-20 can be connected for conference
N22490T 1 two-party line Does not include line circuits
N22487T Staff call, 15 persons max. (audible system) Visual systems for 7, 15 or 30
N22488T Staff call, 50 persons max. codes also supplied (audible system)
N22506C DC relay in case For operating loud-ringing bell

Taken from Plessey Publication 7117 (1971)

Also made by British Ericsson.
Part No's:-
A Unit - N20369 - Equipped for 50 lines, 6 linefinders & group selectors and 5 connectors.
B Unit - N20374 - Equipped for 25 lines, 4 linefinders & group selectors and 3 connectors.

Click here for an article in the Ericsson Bulletin


Home page BT/GPO Telephones Ericsson Telephones Ericsson Home Page Ericsson PABX All Telephone Systems

Last revised: April 18, 2022