Private Manual Branch Exchanges
CB 20+100 cord-type (N327)
Indicator signalling
with automatic or C.B. exchange lines 

n327.jpg (22364 bytes)This switchboard, an indicator signalling type, has capacity for 20 exchange and 100 extension lines with 17 cord circuits. Stock units are available wired and equipped for 10 exchange lines, 50 or 70 extension lines and 15 cord circuits, and can subsequently be extended to full capacity as required.

The switchboard is specially suited for busy establishments where speed of operation is essential. The facility for automatic holding of exchange lines enables the operator to deal with other calls while waiting  for a called party to answer. For installations of from 100 to 200 extensions, two units can be bolted together to form a two-position, non-multiple switchboard. When two positions are installed en suite, each is equipped with longer connecting cords and double pulley weights.

Exchange lines are normally for connection to an automatic exchange, but alternatively can be C.B. Each cord circuit has a speak/ring key and a dial/ring-back key.

Through dialling and clearing.
Automatic exchange-line hold.
Separate negative supervision on local calls: double
negative supervision on exchange calls.
Visual ringing indication.
Ring back.
Visual and audible fuse alarm.
Audible alarm.
Cord test.
Continued exchange-line service in the event of local power failure.
Night-service working on cord circuits 1 to 11.
Note: Additional plug-ended through-cords to connect extra 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 hardwearing, 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 en suite, the adjacent intermediate side panels are removed and the frames bolted together. Similarly, the front panel below the key shelf, and the rear door are both removable 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.

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 8.5in (216mm). The total weight for stock units varies depending upon extension-line capacity

Switching keys and signalling indicators are of miniature type. The grey wedge-shaped key handles are tipped in a contrasting colour, rendering operated keys instantly discernible. Indicators are the twin-shutter type which restore automatically and, when operated, display a black numeral on a background 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 10pps with 2:1 break/make ratio and with standard numbering, i.e. 1 to 0.

Alternative equipment, supplied in addition to or in lieu of the above includes a lightweight plug in headset, a transistor ringing generator powered from a 24V source, a dial with any desired adjustment and numbering and a ringing-resistance lamp (where an external ringing supply is used).

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 PVC, giving protection against moisture, fire, attack by insects and fungoid growth. Connecting cords are additionally protected by PVC sleeving and nylon braiding.

The switchboard operates from a 24V d.c. supply, the busy-hour current drain being approximately 2A. Power is normally derived from the local mains supply via a battery eliminator unit. alternatively, a battery and float-charging rectifier can be supplied if preferred. Exchange-line service is not affected by any failure in the power supply

Line Limits
The switchboard is intended for use in areas within a medium distance from the public exchange to which it is connected, a factor that enables the facilities to be provided with inherent simplicity and minimal servicing requirements.

The loop resistance (exchange line via switchboard and extension telephone) is 700 ohms maximum. Exchange lines of lower loop resistance enable higher loop resistance lines to be used to the extension telephones.

The table shows the range of loop resistance values applicable to the PMBX, based on a 50V 200/200ohms public-exchange feed bridge and telephones of type N1900 or 617/1/02161/000 at the extensions.

Exch. to Extn max loop   Exch. line only max loop   Extn line loop
700   450   150
625   275   150-250
560   10   150 - 450

Note: 100 ohms in PMBX and 130 ohms in extension telephone

Height 48in (1218 mm)
Width 30in (772 mm)
Depth 32in (813 mm)

Weights (approx.)
272 lb (124kg) to 281 lb (128kg) net.

Also in Catalogue 60, Section 2

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Last revised: October 12, 2019