20 +100
cord-type PMBX
indicator signalling
with auto exchange lines

n328.jpg (21542 bytes)This switchboard is arranged for positive supervision on all calls, and has capacity for 20 exchange and 100 extension lines with 17 cord circuits. Stock units are available wired and equipped for 10 exchange and 30 or 60 extensions and 12 or 15 cord circuits. The 10+30 unit has wiring for up to 70 extensions, the 10+60 unit has wiring for 100 extensions and for optional multiple facilities.

Both may be extended to full capacity as required. For installations of from 1 00 up to 200 extensions, two units may be bolted together to form a two position, non-multiple switchboard. When up to the maximum of 400 extensions are required, four units, with provision for multiple equipment, can be similarly installed. When two or more positions are installed en suite, each is equipped with longer connecting cords and double pulley weights.


  1. Through dialling and clearing.
  2. Separate positive supervision on local calls: double positive supervision on exchange calls.
  3. Visual ringing indication.
  4. Ring back.
  5. Visual and audible fuse alarm.
  6. Audible alarm.
  7. Exchange-line prohibition at selected extensions.
  8. Cord test and hold jack with called party answered supervision.
  9. Continued exchange line service in the event of local power failure.
  10. Multiple working with engaged test tone.
  11. Night-service working on cord circuits 1 to 1 2. Note. Extra plug-ended through-cords to connect additional circuits for night-service working can be supplied.

A welded, pressed-steel frame, with integral plinth, forms the main structure and component mounting, and is enclosed by laminated wood panels. These are surfaced with hard-wearing, buff-coloured plastic, edged and secured by metal trims finished in glossy elephant grey stove enamel. In contrast, the face panel is coloured black and the key shelf buff linette. When two or more units are installed ensuite, adjacent intermediate side panels are removed and the units bolted together. Similarly, both the front panel below the key shelf, and the rear door, can be removed for convenience of access.

The face panel has indicators arranged in two groups on either side of the jack strips, and includes provision for night-extension keys, fuse and night-alarm keys and cord-test jack. In addition, the jack panel has capacity for up to 100 multiple jacks for use when switchboards are multipled together.
The key shelf is arranged with supervisory indicators and switching keys for cord circuits and miscellaneous facilities.

Internal equipment is positioned and mounted for maximum accessibility. Incoming cables enter via covered apertures in the open constructed plinth, and terminate on soldered terminal fields positioned at the bottom of the unit.

For convenience of handling (e.g. during installation) the hinged key shelf can be raised and its apron lowered to reduce the depth by 8.5 in (21 6mm). The weight of the stock unit varies, depending upon the extension-line capacity.

Switching keys and signalling indicators are of miniature type. The wedge-shaped ivory-coloured key handles are tipped in contrasting colour, rendering operated keys instantly discernible. The indicators have twin shutters that restore automatically, and, when operated, display a black numeral on a back ground of the same colour as the associated jack label. Operating equipment, supplied as standard, includes a lightweight plug-in handset with rocking- armature receiver, a manually operated ringing-current generator and a dial adjusted to transmit 10 pps with 2:1 break/make pulse ratio and with standard numbering, i.e. 1 to 0.

Alternative equipment, available in lieu of, or in addition to, includes a lightweight plug-in headset, a transistor ringing-unit powered from a 24V source, and a dial with any desired numbering and adjustment.

Internal components, including relays and relay retards, are to British Post Office specification and are afforded tropical finish, i.e. with coils suitably impregnated and metal parts specially finished. Conductors are insulated with p.v.c., giving protection against moisture, fire, attack by insects and fungoid growth. Connecting cords are additionally protected by p.v.c. sleeving and nylon braiding. Additional relay equipment, for extensions requiring the exchange line prohibition facility, is mounted externally.

The switchboard operates on 24V d.c., the busy-hour current drain being approximately 2A. Power is normally derived from the local mains supply via a battery eliminator unit. Alternatively, batteries with charging equipment can be supplied if preferred. Exchange-line working is not affected by any local power failure.

Line Limits
Using Plesseyphones type N2020 (auto) or N1520 (c.b.), or Etelphones type N1900 (auto) or N1340 (c.b.) as extension instruments, the maximum permissible loop resistance between public exchange and extension is 1000 ohms.


Height: 48in (1220mm)
Width: 30in (722mm)
Depth: 32in (813mm)

Weights (approx.)
Between 266lb (121 kg) net and 300lb (136.2kg) net

Plessey Publication No. 7136/1 (1971)
Also in Catalogue 60, Section 2

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Last revised: October 12, 2019