PMBX General Information

B 1001
Issue 2, 2.3.62

General Information on Standard Switchboards and Facilities

1. General
A private manual branch exchange (P.M.B.X.) is a manually operated exchange installed in a subscriber’s premises and comprises a switchboard upon which are terminated subscriber’s extension stations and exchange lines.

2. The switchboards provide for the termination of:-

  1. Exchange lines to any type of public exchange system.
  2. Extensions, internal and external.
  3. Private circuits.
  4. Inter-switchboard extensions.
  5. Inter-switchboard private circuits.

3. The switchboards provide for intercommunication between extensions, between extensions and private circuits, and also between the public exchange system and extensions. Private circuits and inter-switchboard private circuits are not permitted exchange facilities, and the terminations of these lines are arranged to prevent conversation over an exchange line in the event of such a connexion being set up. Exchange calls may be extended over an inter-switchboard extension, but such extensions must not be used for originating exchange calls.

4. Types of switchboard
Switchboards can be classified under three main headings:-

(a) Cordless non-multiple
Small switchboards designed for installation on a table or for mounting in a panel and where switching is effected by means of keys.

(b) Cord-type non-multiple
Usually floor pattern switchboards although the smaller versions are designed for mounting against or on a wall. The circuits terminating on the switchboards are jack-ended, and cords are provided for cross-connecting. If two operating positions are necessary to carry the traffic, two switchboards are provided side-by-side.

(c) Cord-type multiple
Used for large installations where more than two operating positions are required. They are formed by joining together a number of Sections, Switch side-by-side to form a suite of sections with the ends closed either by cable turning sections or by end panels.

5. Switchboard designation
The titles of non-multiple P.M.B.X. switchboards (except certain of the switchboards designed for magneto areas and the P.M.B.X. Nos. 2 ... and 3 ... series) include an indication of the quantity of equipment fitted in the switchboards and their capacity.

For example 'Switchboard AT 3796 10 + 30'
indicates that the switchboard is equipped for 10 exchange lines and 30 extensions, and has a total capacity of 65 lines;

and 'Switchboard LD 70 10'
indicates that the switchboard is equipped for 10 lines and has a capacity of 20 lines.

Because of the limitations of HTML any titles from this point onwards will be shown in the following format e.g. Switchboard AT 3796 10+30/65 or Switchboard LD 70 10/12.

6. The standard types and sizes of switchboards for use as P.M.B.X.s on the various public exchange systems are as follows:-

(a) Automatic and C.B. (manual) systems

* Switchboard P.M.B.X. No. 2/2A ( 2 + 6/8 ) - Cordless Switchboards
* Switchboard P.M.B.X. No. 2/3A (3 + 12/12) - Cordless Switchboards
* Switchboard P.M.B.X. No. 2/4A (4 + 18/22) - Cordless Switchboards
* P.M.B.X. No. 3/3A (3 + 12/15) - Cordless Switchboards
* P.M.B.X. No. 3/5A (5 + 25/30) - Cordless Switchboards
Switchboard CB 935 1 + 3/4, 2 + 4/6, 3 + 9/12 - Cordless Switchboards
Switchboard CB 873 5 +20/25 - Cord Type Non-Multiple Switchboard
Switchboard AT 3796 10 + 30/65, 10 + 50/65, 10 + 60/180 - Cord Type Non-Multiple Switchboard
Switchboard N 1070 10 + 30/65, 10 + 50/65, 10 + 60/180 - Cord Type Non-Multiple Switchboard
** Sections, Switch, B.E., C.B., Multiple, No. 9 - Cord Type Multiple Switchboards
** Sections, Switch, P.M.B.X., No. lA - Cord Type Multiple Switchboards
Sections, Switch, P.B.X., SA 7560 - Cord Type Multiple Switchboards
Sections, Switch, P.B.X., SA 7560 (1200 extensions version) - Cord Type Multiple Switchboards
Sections, Switch, B.E., C.B., No. 10 - Cord Type Multiple Switchboards

* Switchboards P.M.B.X. Nos. 2/3A and 2/4A are in course of introduction and, together with No. 2/2A, will supersede Switchboards CB 935 . . . and CB 873 . . . for the majority of new work. The quantity of equipment fitted and the capacity shown against each item of the No. 2... and No. 3. . . series are given for explanatory purposes, and are not part of the Rate Book title.

** These switch sections are superseded for new work by Sections, Switch, P.B.X., SA 7560 (see B 1010). (Installations comprising either Sections, Switch, P.M.B.X., No. lA or Sections, Switch, P.B.X., SA 7560 are classed as ‘P.M.B.X. No. 1A-type Installations.’ These have capacity for 800 extensions, but for large installations a 1200-line version of Section, Switch, P.B.X., SA 7560 is available. Such installations are designated ‘P.M.B.X. No. 1B-type’.)

(b) C.B.S. systems
Switchboards CBS N 934 1 + 3/4, 2 + 4/6, 3 + 9/12 - Cordless

(c) Magneto system
Switchboards, Wall Pattern, Single Cord, N.T. Nos. 1, 2, 3, 5, 6 and 7
Switchboards LD 70 10/10 and 20/20

7. If a switchboard with capacity for more than 12 lines is required in any C.B.S. area, an appropriate switchboard of the C.B. type is installed. In some circumstances (see B 3901), Switchboard CB 935 is fitted in lieu of Switchboard CBS N 934, even though the line capacity is not greater than 12. The method of terminating the exchange lines is dealt with in
B 3102.

8. The operating facilities afforded by the standard types of switchboard are as follows:-

(a) All types of switchboard listed in pars. 6(a) and 6(b).

  1. SPEAK keys are provided to enable the P.B.X. operator to set up and supervise any connexion.

  2. Provision is made on all switchboards for giving direct exchange service at night to any extension.

  3. Through-clearing is provided (see par. 10) except in Sections, Switch, B.E., C.B., No. 10.

  4. Press-button re-call facilities (see par. 11) are provided on P.M.B.X. Nos. 2... and 3 switchboards as standard, and may be provided on other types of switchboard if required.

  5. Means are provided to enable the operator to ‘hold’ an exchange line while attending to another circuit or making inquiries.

  6. Cord-testing facilities are provided on all cord-type switchboards.

  7. Ringing current to enable the P~B.X. operator to call the extensions is obtained from a hand-generator which is included in the equipment of the switchboard (except in P.M.B.X. Nos. 2. . . and 3... types) or, if the amount of traffic warrants it, a continuous ringing supply may be provided in accordance with POWER, General, S 3801. Continuous ringing supply is standard for P.M.B.X. Nos. 2 . . . and 3 types and is derived from a power unit.

  8. RING BACK keys are provided on cord type switchboards to enable the operator to ring on the answering cord.

  9. An audible alarm is provided with the supervisory signals of all switchboards having a capacity not exceeding five exchange lines and 20 extensions. Such facilities are only provided on larger switchboards at the request of the traffic staff.

  10. Subscribers’ private metering is provided by means of meters fitted on or adjacent to the switchboard. This is an optional facility and meters are only fitted when required (see B 3104).

(b) Switchboards listed in par. 6(c).

  1. Provision is made on all switchboards for giving direct exchange service at night to any extension.
  2. Through-clearing is provided (see par. 10).
  3. Press-button re-call facilities (see par. 11) can be provided if required.
  4. Ringing facility is. provided by hand generator.
  5. A jack for holding purposes is provided.

9. Line limits
The maximum permissible transmission and signalling resistance applicable to switchboards in C.B. and automatic areas is given in TRANSMISSION, Telephone, B 3531 - B 3549, B 3591; and for L.B. areas in TRANSMISSION, Telephone, B 3550.

10. Through-clearing
This is the term used to indicate the condition in which, at the termination of an exchange call, the extension user’s clearing signal controls the relative cord-circuit supervisory signal at a manual public exchange and, in the case of an originating call on an automatic public exchange, the automatic switches or the supervisory signal at an auto-manual position with ‘manual hold’ facilities, where the call passes through such a position.
11. Press-button re-call is a term used to indicate the facility (specially provided in certain circumstances) which enables an extension user, during the progress of an exchange call, to attract the P.B.X. operator’s attention without releasing the connexion at the public exchange.

12. Further information
Full details of the various switchboards are given in the series B 1010 onwards.

13. Instructions detailing the method of operating the switchboard are supplied to the subscriber with each switchboard. The stock-list numbers of the instruction cards and booklets for the various types of switchboards and systems are given in Stations, A 3201.

References:- B 1010, B 3102, B 3104, B 3901
Stations, A 3201
POWER, General, S 3801
TRANSMISSION, Telephone, B 3531 - 3549, 3550, 3591


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Last revised: November 28, 2010