PABX No. 2

See P.A.B.X. No. 1 for additional information.  The P.A.B.X. No. 2 is effectively a P.A.B.X. No. 1 with a cord switchboard.

Descriptive Leaflet

Introduced in 1950

C 1120
Oct 1954

Private Automatic Branch Exchange No. 2
(P.A.B.X. No. 2)


1. Scope of Instruction
This Instruction describes the facilities offered by, and the equipment used for, the P.A.B.X. No. 2.  The planning of a P.A.B.X. No. 2 installation is dealt with in C 3101 and installation instructions are given in C 3110.


2. Sizes
The P.A.B.X. No. 2 is available in four sizes, namely for 4, 5, 7 or 10 exchange lines with 15, 24, 35 or 49 automatic extensions respectively.  Capacity for up to 30 manual extensions is available on each of the four sizes. (See also par. 10.)  The P.A.B.X. consists of a cord-type manual switchboard ('Section, Switch, P.B.X. SA 7560') connected to a unit-type automatic equipment ('Equipment, P.A.B.X., SA 8100 . . .', see C 1115).  Power at a nominal 50v D.C. is supplied from power plant in accordance with POWER, General, S 1001.

3. Trunking
The trunking of P.A.B.X. No. 2 is shown on Fig. 2 of Diagram SA 8100. Extension-to-extension calls are set-up through connecting circuits, each consisting of a 25 or 50 point Uniselector and a 100-outlet, 2000-type selector, on the banks of which the automatic extensions are multipled. The automatic extension multiple is repeated on the banks of uniselectors in the exchange-line, inter-switchboard and ' 0' level (lamp-lighting) circuits, and connexion of automatic extensions to these circuits is established by positioning the uniselector under the control of marking conditions from the connecting circuit. The exchange-line and inter-switchboard circuits and the automatic and manual extensions have an appearance on the manual switchboard and connexions between them may also be made by cord circuits. Associated with the exchange-line and inter-switchboard circuits is an enquiry circuit which has an appearance on the banks of the connecting-circuit uniselectors. The function of this circuit is described more fully in pars. 16 and 23.

4. Numbering scheme
The allocation of levels on the two-motion selectors and the number ranges for the automatic extensions of the P.A.B.X. No. 2 are the same as for the P.A.B.X. No. 1 (see C 1110). Manual extensions on the P.A.B.X. No. 2 form a separate group. They are not included in the automatic extension numbering ranges because they cannot be obtained by direct dialling from the automatic extensions. They should be allotted 3-digit numbers, having the first digit ' 0' so that if an automatic extension user attempts to dial the number of a manual extension he will call the P.A.B.X. operator.

5. Automatic equipment
An 'Equipment, P.A.B.X., SA 8100' of the appropriate size and fitted with the required selectors and relay-sets is used. The quantities of the latter items necessary for any particular installation are given in C 3101. The capacity of the four sizes of equipment for various circuits is given in Table 1 and titles and diagram numbers of the relay-sets, etc., which may be needed are given in Table 3.

6. Manual switchboard
A single 'Section, Switch, P.B.X., SA 7560' is used without a cable-turning section. It is a steel-framed section introduced primarily for P.M.B.X. No. 1A but designed also for association with a P.A.B.X. No. 2 where it is used with 50v cord circuits instead of the 24v circuits of the P.M.B.X. A cable-turning section is omitted as being unnecessary on a single-section installation, where the few cables to be terminated can be brought into the switchboard via one end-panel. A view of the front of the switchboard is shown to the right and was taken in 1953.

7. Cord Circuits
The cord circuits of the 'Section, Switch, P.B.X., SA 7560' are on plug-in relay-sets. The 50v cord circuits for the P.A.B.X. No. 2 ('Relay-sets SA 8135') may be plugged into the shelf jacks in the section without alteration of the wiring of the section. Each 'Relay-set SA 8135' contains 3 cord-circuits. The switch section is wired for a maximum of 15 cord circuits (5 relay-sets) and is supplied with 12 cords fitted but without the  Relay-sets SA 8135 These must be obtained separately.

8. Switchboard Face
The face of the 'Section, Switch, P.B.X., SA7560' accommodates two panels for jack and lamp appearances. When used with a P.A.B.X. No. 2, the left-hand panel is normally used for exchange lines and automatic extensions and the right-hand panel for manual extensions and inter-switchboard circuits

As there is no extension multiple at a P.A.B.X. No. 2, a space is left free in the upper part of the board and this is used to accommodate the manual extension line circuits.

9. Manual extension line circuits
The apparatus for these circuits is strip-mounted on 'Relay-units SA 8139', which must be fitted locally when required. Each relay-unit provides for 10 manual extensions and accommodation is available in the upper part of the board for a maximum of 3 relay-units (30 extensions). 'Brackets, Mounting, DY' are added to the board locally to mount the relay-units (see C 3110).

10. Capacity
The capacity of the four sizes of P.A.B.X. No. 2 for various circuits is given in Table 1.


Circuit Sizes of P.A.B.X. No. 2 equipment
  4+15 5+24 7+35 10+49
(1) Exchange lines (Note 1) 4 5 7 10
(2) Automatic extensions (Note 2) 15 24 35 49
(3) Manual extensions 30 30 30 30
(4) Inter-switchboard circuits with automatic (dial 7) access, enquiry and transfer (Notes 1, 3 and 6)  2 2 3 3


(5) Inter-switchboard private circuits with manual access (Notes 1 and 3) 2 2 3 3
(6) Inter-switchboard auxiliary circuits with automatic (dial extension number) access (Note 4)  2 2 2 2


(7) Inter-switchboard auxiliary circuits with manual access (Note 4) 2 2 2 2


(8) Private circuits with manual access only (Note 5) 4 4 4 4


  1. On the 10+49 size, the number of exchange lines, plus inter-switchboard circuits [items (4) and (5) only, must not exceed 12 (i.e. 10 exchange lines and 2 inter-switchboard circuits or 9 exchange lines and 3 inter-switchboard circuits).
  2. The figures are reduced by one for each inter-switchboard circuit [items (4) and (6) only] fitted, except circuits using 'Relay-sets SA 8142'.
  3. The figures are the maximum for items (4) and (5) together. For possible mixtures of these circuits. Sec par. 30.
  4. The figures are the maximum for items (6) and (7) together. See also par. 33.
  5. The figure is reduced by 2 for each circuit to item (6) or (7) fitted. See also par. 33.
  6. The circuits must have a single destination. See par. 27.


11. Service between the P.A.B.X. and the public exchange is over both-way working exchange-line circuits.  Early P.A.B.X.s No. 2 use 'Relay-sets SA 8138' for the exchange-line terminations. Later P.A.B.X.s use 'Relay-sets SA 8137', which incorporate a somewhat simpler circuit.

12. Incoming calls from the public exchange are received during the day on the manual switchboard and are connected to extensions by means of cord-circuits.  Ringing is applied by operating the cord-circuit ringing key.  A 'click' test is provided for engaged extensions.  There is no ring-when-free facility. For incoming night-service, see pars. 22 to 24.

13. Outgoing calls to the public exchange by automatic extensions are normally obtained by direct access by dialling the initial digit '9'.  It is possible to withdraw this facility from any selected extensions, as desired by the subscriber.  Such extensions requiring a call to the public exchange must dial '0' and request the P.A.B.X. switchboard operator to set it up. For outgoing night-service, see par. 25.

14. Follow-on-call trap
The exchange-line circuits of P.A.B.X. No. 2 provide a trap for follow-on calls if, on a public exchange call connected via the switchboard, the P.A.B.X. operator does not remove the plug of the cord from the exchange-line jack until after the P.A.B.X. extension and the public exchange circuits have both released.  In these circumstances, while the cord for the released call is still connected, the exchange-line relay-set is maintained engaged so that it cannot be seized for an outgoing direct access call and the extension still connected by the cord cannot receive a ring if a second call arrives incoming from the public exchange.  At early P.A.B.X.s No. 2 using 'Relay-sets SA 8138' for the exchange-line circuits, a 'follow-on' incoming call does not give a signal on the switchboard while the cord for the released call is still connected but, when the cord is withdrawn, the exchange-line calling lamp will light in the normal manner.  At later P.A.B.X.s No. 2, using 'Relay-sets SA 8137', a 'follow-on' incoming call causes the cord circuit supervisory lamps to flash.

15. Release of directly-dialled calls to the public exchange
When the P.A.B.X. extension telephone is replaced at the end of a directly-dialled call to the public exchange, the P.A.B.X. exchange-line circuit remains held and tests engaged until a signal is received from the public exchange to indicate that the circuits at the latter have released This prevents the exchange line being re-seized from the P.A.B.X. until the circuit is free at both the P.A.B.X. and the public exchange. The terminating arrangements at the public exchange arc dealt with in General, M 3902.

16. Enquiry
One enquiry circuit, 'Relay-set SA 8103', is provided on each P.A.B.X. No. 2.  Over this circuit, the user of an automatic extension (or of a manual extension fitted with a dial) may obtain access to a connecting circuit and may call another automatic extension while holding an exchange call.  This facility is also available to an extension holding a directly-dialled inter-switchboard call over a circuit of the type indicated as item (4) in Table 1.  The extension user presses a button on the telephone once to pick up the enquiry circuit and to obtain dialling tone from a connecting circuit.  He may then dial the extension required.  After the enquiry, he may return to the held call, by pressing the telephone button once again.  This procedure may be repeated as often as desired.  If the call is an inter-switchboard call connected by cord circuit, or if the enquiry circuit is engaged when the telephone button is depressed, the P.A.B.X. operator is called in and may deal with the enquiry.

17. Transfer
During the day, when the manual switchboard is staffed, the transfer of calls is made by the P.A.B.X. operator, see par. 18 'Calling-in by press-button'.  Under night-service, when the manual switchboard is not staffed, an automatic transfer facility is available to automatic extensions as described in par. 23.  A summary of the conditions under which the enquiry and transfer facilities are available is given in Table 2.

18. Calling-in by press-button
A press-button, provided on each extension telephone and used for the enquiry facility as described in par. 16, is also used to call-in the P.A.B.X. operator on public exchange calls or inter-switchboard calls.  All manual extensions are also provided with a press-button and have the call-in facility.  The button is pressed twice. Dialling tone should be heard before the button is pressed the second time.  The operator will be called-in after the first depression if (a) the enquiry circuit is already engaged when the button is depressed, or (b) the call is an inter-switchboard call connected by cord-circuit on the P.A.B.X. manual switchboard, or (c) the call is a local one to a manual extension.  The lamp signals given on the manual switchboard for calling-in by press-button in various circumstances are summarised in Table 2.

19. '0' calls
The calling extension line lamp on the manual switchboard glows when '0' is dialled. It is possible for '0' calls from two extensions to appear on the switchboard at the same time.  Two '0' level lamp-lighting circuits are provided per installation on one 'Relay-set SA 8140'.  If an extension user dials '0' when both circuits are engaged, he will not receive tone but if he holds the line until a circuit is free his call will then be completed.



Extension connected to:- Type of call Day Service

Night Service

    Auto. enquiry Auto. transfer Press- button call-in Normal (Dial '8') Direct extn. night service
          Enquiry Transfer Enquiry Transfer
Public exchange Directly dialled Yes No Yes Line lamp flickers Yes Yes No  No
Public exchange Via cord circuit Yes No Yes Supy. lamps flicker - - No No
Inter-swbd. circuit with group hunting Directly dialled Yes No Yes Line lamp flickers Yes Yes -  -
Inter-swbd. circuit with group hunting Via cord circuit No No Yes Supy. lamps flicker - - -  -
Inter-swbd. circuit without group hunting Directly dialled No No No No No -  -
Inter-swbd. circuit without group hunting Via cord circuit No No Yes Supy. lamps flicker - - -  -
Manual extension Via cord circuit only No No Yes Supy. lamps flicker - - -  -

20. Manual extension calls
The manual extensions are terminated on the manual switchboard only and all calls to and from these extensions are connected by the P.A.B.X. operator.

21. Trunk offering is available but a ticking signal to warn the extension user that the operator has entered the circuit is not provided (a ticking signal is provided on P.A.B.X. No. 1.).

22. 'Dial 8' night service for incoming calls is available to all automatic extension users (but see also par. 24) in a similar manner to that provided on P.A.B.X, No. 1 (see C 1110).  The night-service key is fitted on a small alarm-indicating unit, fixed to the side of the manual switchboard.  Also, as described for P.A.B.X. No. 1, one automatic external extension may he given 'dial 8' night-service.

23. Enquiry and automatic transfer under night service
Automatic extensions having 'dial 8' night service also have the enquiry facility, as described in par. 16, and an automatic transfer facility, as described below.  When an enquiry call is made under night service, a signal is extended from the enquiry circuit, via the connecting circuit, to mark the extension to which the enquiry call has been made on the bank multiple contacts of the uniselectors of the exchange-line and inter-switchboard circuits so that, if required, the uniselector of the circuit held may be driven on to the marked extension so as to transfer the held call to it.  The transfer is affected by the calling extension miser replacing the telephone handset while the called extension user 'holds the line'.

24. 'Direct extension' night-service
is available to manual extension misers, and, only as an alternative to the 'dial 8' night-service, to automatic extension users.  Selected extensions are connected individually, via cord-circuits on the P.A.B.X. manual switchboard, to selected exchange lines (inter-switchboard circuits cannot be connected).  The cord-circuit keys must be in the 'normal' position when 'direct extension' night-service connexions are set up and they should then remain untouched.  If a key is accidentally operated the connexion must be taken down and re-established. At installations using exchange-line relay-sets 'Relay-sets SA 8138' a separate .night-service jack is required, connected in each exchange line, to cut out the exchange-line relay-set.  At installations using 'Relay-sets SA 8137', the normal exchange-line jacks are used for the night-service connexions. 'Dial 8' night-service may be operated to exchange lines not taken up for the 'direct extension' night-service but it will not be available to the directly-connected extensions.

25. Outgoing night-service
Automatic extensions having direct access during the day retain this facility at night and have automatic enquiry and transfer facilities on outgoing calls provided the extensions are not on 'direct extension' night-service. Automatic extensions on 'direct extension' night-service will have immediate access to the public exchange at night, including those extensions barred direct access during the day, but they will not have enquiry or transfer facilities. The prefixing digit '9' is also not required to be dialled.  If the public exchange is a manual one and the P.A.B.X. uses 'Relay-sets SA 8138', extension users on 'direct extension' night-service will press the telephone button to call the public exchange at night. The button must be held down until the operator answers; it may then be released. If the P.A.B.X. uses 'Relay-sets SA 8137', it is not necessary to press the telephone button.  Manual extensions on 'direct extension' night-service have outgoing service to a manual public exchange but, if they are required to have outgoing night-service to an automatic exchange, the telephones of those manual extensions will need to be provided with automatic dials.

26. Inter-switchboard and auxiliary circuits
Shelf jacks for these circuits arc provided on the automatic equipment in the positions indicated in C 1115.

27. Inter-switchboard circuits with 'dial 7' direct access and with group hunting
A limited number [see Table 1 item (4)] of inter-switchboard circuits in a single group to which automatic extensions have direct access by dialling initial digit '7' may be provided.  These circuits must all have the same destination because, for outgoing calls, they are taken into use, in turn, under the control of a start signal; they are sometimes known as inter-switchboard circuits 'in-group'.  These circuits also have a jack appearance on the manual switchboard so that, for example, calls to (or from) manual extensions may he connected.

28. Any one of the following classes of circuit having 'dial 7' access can he provided:-

(a) Both-way dialling, loop disconnect pulsing, using 'Relay-sets SA 8141"

or (b) both-way, 1-wire earth-signalling, using 'Relay-sets SA 8142'

or (c) generator-signalling outward, 1-wire earth-signalling inward, using 'Relay-set SA 8142'

or (d) generator-signalling outward, loop disconnect signalling inward, using 'Relay-set SA 8142'

or (e) both-way earth-signalling with earth-dialling inwards, using 'Relay-sets SA 8146'

The three relay-set types are arranged to serve either inter-switchboard extensions or inter-switchboard private circuits.  Enquiry and automatic transfer facilities as described in pars. 16 and 23 are available on these circuits. Each circuit using Relay-set SA 8141 ' or ' Relay-set SA 8146 reduces the number of available automatic extensions by one.  Circuits using 'Relay-set SA 8142' do not reduce the number of available automatic extensions.

29. Other direct access circuits
A maximum of two [see Table 1 item (6)] inter-switchboard or private circuits, not necessarily to one destination, may be provided using 'Relay-sets SA 8109' fitted in 'spare' jack positions on the automatic equipment.  Used in this way, these circuits are allocated automatic extension numbers which are dialled by other automatic-extension users to obtain access to the circuits.  Incoming service is by loop disconnect pulsing, either direct to the P.A.B.X. automatic extensions or by dialling '0' for connexion to the P.A.B.X. operator and to manual extensions via cord circuits.  Exceptionally, short inter-switchboard circuits may be connected direct to automatic-extension line equipments.  None of these circuits has enquiry or automatic transfer facilities.

30. Inter-switchboard circuits without direct access
A limited number [see Table 1 item (5)] of circuits connected to the manual switchboard only may be provided, using the normal inter-switchboard shelf jack positions on the automatic equipment.  These are bothway inter-switchboard private circuits only, using balanced-battery or generator signalling.

"Relay-set SA 8143' is used.  If desired, each circuit may have a different destination.  A mixture of these circuits and those described in pars. 27 and 28 may be fitted, e.g. one circuit using 'Relay-set SA 8143' with two using 'Relay-set SA 8141' could be provided on 7 + 35 and 10 + 49 sizes of equipment.

31. A maximum of two circuits [see Table 1 item (7)], not necessarily to one destination, may he provided using 'Relay-set SA 8109', fitted in the spare shelf jack positions on the automatic equipment.  Used in this way, the circuits are provided with a jack and lamp appearance on the manual switchboard only and an automatic extension line circuit is not taken up.  All connexions to the circuits are made by the P.A.B.X. switchboard operator via cord circuits.

32. Private circuits
maximum of four manual private circuits [see Table 1 item (8)] may be provided, using 'Relay-sets SA 8144' (2 circuits per relay-set), fitted in the 'spare' shelf-jack positions on the automatic equipment.  A jack and lamp appearance is provided on the manual switchboard only, and all connexions are made by the P.A.B.X. switchboard operator via cord circuits.

33. Long automatic extensions
Automatic extensions of over 600 ohms and up to 1000 ohms D.C. loop resistance require the use of an auxiliary circuit to apply a temporary local loop to the automatic extension line circuit to ensure its satisfactory pick-up.  'Relay-set SA 8104' is used.  It is fitted with three auxiliary circuits and may be extended locally to six circuits. A maximum of two 'Relay-sets SA 8104' may be fitted but, because they occupy the 'spare' jack positions on the automatic equipment, their use is alternative to other auxiliary circuits (Items Nos. 6, 7 and 8 of Table 1.)

34. Alarms
An alarm unit, 'Lamp, Alarm-indicating, No. 5' in 'Case No. 24', which is fixed at one end of the switchboard, gives separate lamp indications of failure of ringing (red opal), mains supply (red opal) arid automatic equipment (red opal), the latter glows as the result of a blown fuse on the automatic equipment or of failure of a selector to release.  A 'PG. pilot' lamp (white opal) glows when an automatic extension line becomes faulty.  The alarm unit also carries a NIGHT SERVICE/AUDIBLE ALARM key and an ALARM CUT-OFF/PG. TEST key.

35. Audible alarms
Whenever a fail lamp glows (except the P.G. pilot) an audible alarm is given by a buzzer in the switchboard and persists until the ALARM CUT-OFF key is momentarily operated or until the fault is removed.  An audible alarm of all calls to the switchboard is given when the AUDIBLE ALARM key is operated.  All audible alarms are suppressed by the NIGHT SERVICE key.

36. P.G. test
If the P.G. TEST key is operated, the line lamps of any faulty automatic extensions will glow.

37. In addition to the fail lamps on the alarm unit, there is a fuse fail lamp (red opal) in the face of the right-hand panel of the switchboard. This lamp glows when a fuse in the switchboard is blown and an audible alarm is given.  A pilot lamp (white opal) in the face of the left-hand panel of the switchboard glows when a call to the switchboard awaits attention.

38. Diagrams for P.A.B.X. No. 2 are listed in Table 3.


Diagram No's


SA 8100 Equipments, P.A.B.X. SA 8100. Trunking Scheme for P.A.B.X.s Nos. 1 and 2
SA 8101 Equipments, P.A.B.X. SA 8100. . . . Extension line circuit for P.A.B.X.s Nos. 1 and 2. (The apparatus for these circuits is strip-mounted in' the automatic equipment and is not available separately)
SA 8102 Selector SA 8102. Connecting circuit for P.A.B.X.s Nos. 1 and 2 Relay-set
SA 8103 SA 8103. Enquiry circuit for P.A.B.X.s Nos. 1 and 2
SA 8104 Relay-set SA 8104. 3 Auxiliary circuits for long extensions (600-1000 ohms loop) on P.A.B.X.s Nos. 1, 2 and 3
SA 8105 Equipments, P.A.B.X. SA 8100. Ringing-start and alarm circuits for P.A.B.X.s Nos. 1 and 2. (The apparatus for these circuits is strip-mounted in the automatic equipment and is not available separately)
SA 8106 Relay-set SA 8106. Ringing Vibrator and Tone circuit for P.A.B.X.s Nos. 1, 2 and 3 
SA 8107 Relay-set SA 8107. Ringing Pulse circuit for P.A.B.X.s Nos. 1, 2 and 3
SA 8108 Equipments, P.A.B.X. SA 8100. Night service external extension circuit for P.A.B.X.s Nos. 1 and 2. (The apparatus for this circuit is strip-mounted in the automatic equipment. and is not available separately)
SA 8109 Relay-set SA 8109. Auxiliary circuit on automatic extension for Long Private or Inter-switchboard circuits on P.A.B.X.s Nos. 1, 2 and 3
SA 8118 Tester SA 8118 for P.A.B.X.s Nos. 1 and 2
SA 8119 Common Services and Wiring on P.A.B.X.s Nos. 1 and 2
SA 8135 Relay-set SA 8135. 3 Cord circuits for Section, Switch, P.A.B.X., SA 7560. At P.A.B.X Nos. 2 and 3
SA 8137 Relay-set SA 8137. Exchange line circuit for P.A.B.X. No. 2
SA 8138 Relay-set SA 8138. Exchange line circuit for P.A.B.X. No. 2 (early P.A,B.X.s only)
SA 8139 Relay-unit SA 8139. 10 Manual extension circuits for P.A.B.X. No. 2
SA 8140 Relay-set SA 8140. 20 Level Lamp-lighting circuits for P.A.B.X. No. 2
SA 8141 Relay-set SA 8141. Inter-switchboard circuit (B/W. dialling) for P.A.B.X. No. 2
SA 8142 Relay-set SA 8142. Inter-switchboard circuit (D.C. signalling) for P.A.B.X.s Nos. 2 and 3
SA 8143 Relay-set SA 8143. Inter-switchboard private circuit (generator- and balanced-battery signalling) for P.A.B.X.s Nos. 2 and 3
SA 8144 Relay-set SA 8144. 2 Manual private circuits for P.A.B.X.s Nos. 2 and 3
SA 8145 Lamp, Alarm-indicating, No. 5. For P.A.B.X.s Nos. 2 and 3
SA 8146 Relay-set SA 8146. Inter-switchboard circuit (dialling in from P.M.B.X.)



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Last revised: April 27, 2023