PABX No. 1

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Private Automatic Branch Exchange No. 1 (PABX 1)

The PABX 1 is a modern, automatic telephone system which effectively meets the communication needs of small or medium sized businesses and commercial organisations. The system accommodates up to 10 exchange lines and 49 automatic extensions and provides for automatic connection to the public telephone network and full internal dialling between extensions. Incoming exchange calls are received on a small, compact, key-operated switchboard on a table or desk, and are dealt with by the PABX operator. A small number of manual extensions and lines to other PBXs can be connected to the system. Note: PBX means either a Private Automatic Branch Exchange (PABX) or a Private Manual Branch Exchange (PMBX).
The automatic equipment is contained in a steel cabinet which has front and rear doors to let the maintenance engineer get at both sides of the racks. Power to operate the equipment is supplied from a secondary-cell battery linked with an automatically controlled charging unit connected to the mains. The PABX 1 is available in four sizes. Details are shown with the general information.

Extensions call each other simply by dialling the number.
Extensions make outgoing exchange calls by first dialling 9 for connection to an exchange line. Any extension can be connected in such a way that it cannot dial 9 for an exchange line but can get through to the public system only by way of the operator's switchboard. Incoming exchange calls are first received at the switchboard and then extended to the required extension by the operation of press-buttons.
A few manual extensions can be provided. Automatic extensions can call them direct but they receive incoming exchange calls and make all outgoing calls through the switchboard. Lines to other PBXs can be provided. Extensions dial 7 for connection to an inter-PBX line and in some instances it is possible over these lines to dial direct to extensions on a distant PABX. On an outgoing exchange call, timing of the call stops and the exchange line is released immediately the extension handset is put back. Extensions dial 0 to call the switchboard operator.

The PABX 1 could be fitted with one of two types of switchboard. The early, SA8120 version was made of wood, whilst the latter, SA 8133 type was of a modern design with the usual grey plastic cover.
Extensions are called from the switchboard by press-button.
Ringing is automatically applied to extensions on calls from the switchboard. Calls to or from extensions are automatically released when the extension handset is put back. If on an incoming call the called extension is engaged the caller can wait and be automatically connected immediately the extension is free. The PABX operator can interrupt an engaged extension to ask if another call can be accepted. A ticking sound indicates that the operator is on the line. An incoming or outgoing exchange call can be held on the switchboard by connecting it to a special holding number. After holding, the call can be connected to an extension by press-button.

Calls between extensions are released when either extension handset is put back. On calls to or from the exchange or another PBX, pressing the button on the telephone twice, calls in the PABX operator, who can then hold the call or transfer it as required. An automatic extension can, by pressing the button on the telephone once, hold an exchange call and make an enquiry of another extension. The original call can be returned to by pressing the button again; or if the handset is simply replaced the calls transferred to the other extension. This operation can be repeated as often as required on the call. Manual extensions do not have this arrangement. For night service there is a special key on the switchboard. When this is in the `night service' position incoming exchange calls cause suitably sited bells to ring continuously. Any automatic extension can then answer the call by dialling 8, and can, if necessary, transfer it to any other extension. A different kind of night service can be given by putting individual exchange lines through to selected extensions.

3 Internal
Issue 1, April 1973

PABX's 1 AND 2
Installation of Equipment

This Instruction deals with the installation of PABXs1 and 2 in subscribers' premises.

F0301 gives a short general description of the PABXS; F1010 gives a detailed description of the PABX 1 and F1014 of the PABX 2. F1011 and F1012 describe the cordless manual switchboards for the PABX 1 and F1013 describes the automatic equipment which is used for PABX's 1 and 2. Q0010 deals with the power plant required.

Equipment Cabinet Battery Cabinet


(a) Equipment's, PABX, SA 8100 and Equipment PABX 1B ... /SA 8100
The equipment's should be stood directly on a level floor. It does not require special fixing. If desired, the equipment may be made more manoeuvrable by lifting off the doors. For the equipment SA 8100.. a further reduction in weight and height may be obtained by temporarily removing the plinth. This may be done by removing the appropriate screws as shown on Drg 90000, Sheet 3 for the 4 + 15 and 5 + 24 sizes (10 screws), and on Drg 63959, Sheet 3 for the 7 + 35 and 10 + 49 sizes of equipment (12 screws). The plinth or doors must not be permanently removed.

The equipment's are partially equipped with Uniselector mechanisms in accordance with Table 1. Where the requirements of an installation exceed the quantities given in Table 1, the additional Uniselector mechanisms should be requisitioned separately and fitted locally.


		Exchange Lines	Conn Ccts	Inter-PBX	0-level		Uniselector
						ccts				mechanism

4 + 15 		3		3		-		2		2/55A-P
5 + 24		3		3		-		2 		2/55A-P
(1 B3)		4		4		-		2		2/10/2A-P
(1 B4)		5		5		-		2		2/10/2A-P 

(b) Jack-in equipment
The shelf-jack positions appropriate to the various relay-sets and selectors are given in F1013 and F1021. On each equipment, there are two spare jack positions which may be used as desired for the auxiliary Relay-sets, SA 8104, SA 8109 and SA 8144. Two relay-sets of any one of these types or one each of any two may be fitted in these jacks. The jacks are wired only with the common services wiring and additional wiring is necessary appropriate to the relay-set fitted.

(c) Modifications to the equipment on installation:-

(i) Certain modifications are required to be made on equipment's, relay-sets etc, on or after installation to cover retrospective changes agreed since the items were purchased. These modifications should be made in accordance with the current works specifications listed in D0020.

(ii) Modifications to the equipment to provide non-standard facilities on the PABX must not be made unless prior approval of THQ(TD2.5.2) is obtained.

(iii) When 1 VF signalling inter-PBX circuits are required the equipment must be modified and an Equipment, Signalling, No. 24/ ... provided. Installation details are given in F1020.

(d) Relay-set SA 8104 modified for up to six circuits
Normally, Relay-set, SA 8104 is equipped and wired for three long-extension circuits (600 to 1000 ohms line loop) but, if necessary, it may be equipped locally for up to six circuits, e.g. if there are, say, four long extensions and the second spare jack position is in use for another type of relay-set. The items required per circuit to extend the relayset are

1 Relay No. 9763 (Relay LS) and
1 Relay No. 9764 (Relay CO).


(a) PABX 1
Switchboards PABX SA 8120 or SA 8133 are used. Switchboard SA 8133,which is of a more modern design, supersedes SA 8120 for new work.

(i) The Switchboard, PABX, SA 8120 may stand on a desk or table. The base of the switchboard can be secured to the table if the subscriber permits or desires this but it should not usually be necessary. If the base is secured, access to the jack points for wiring may be obtained by loosening two round-head screws, one at each side of the base. Access to these screws is obtained by opening the sloping front panel of the switchboard. A dial or dummy dial, a telephone handset and lamps for the working circuits Should be fitted to the switchboard, as appropriate. A cleat for securing the end of the cable from the automatic equipment inside the switchboard should be made or provided locally. For convenience in packing, the switchboard will be received with the receiver hook and handle detached. The hook should be re-fixed on the end panel and the handle on the top of the sloping front panel of the switchboard (see Drg 63790/1).

(ii) The Switchboard, PABX, SA 8133 Mk 1 and Mk 2. Two switchboards are currently available Mk 1 (Dgm SA 8133/0) and Mk 2 (Dgm. SA 8133/1), although the former will gradually be phased out. The Mk 2 can be distinguished from the Mk 1 by the flexible cord (cord connecting No. 75/-1A) and lamps in the dial key speak extension and speak exchange key. The switchboard may stand on a desk or table. A dial, a telephone handset and lamps for the working circuits and alarms should be fitted to the switchboard. For convenience of packing the switchboard will be received with the handset cradle detached. This should be re-fixed on the left-hand side of the cover see Drg 92612). An additional, Jack No. 84C may be fitted if required. The wiring for this is tied back in the cable form. The jack is wired in parallel with the existing one. A connector 101/1A is required to terminate the cord connecting 75/1A to the 75 way cable.

(b) PABX 2
The Section, Switch, PBX, SA 7560 should be fitted, as required, with the miscellaneous items listed on Drg 90053. This drawing shows the positions of items which are special to the use of the section with PABX 2, such as the dial cords for use with Relay-set SA 8141, SA 8147 or SA 8148. it also shows the positions of the relay and resistor required if a Key-sender is used instead of a dial. Drawing 63791 shows the, method of fitting Brackets, Mounting EM. A Fuse Mounting No. 131B and Fuse No. 44A/1.5 for the 6v supply should be fitted as indicated on Dgm LD 143.

As no cable turning section will be fitted, the cables from the automatic equipment and the main frame will usually be taken into the switchboard through one end-panel which should be cut locally to admit them. The Lamp, Alarm-indicating No. 5 in Case No. 24 should be fitted near the top of one end-panel, preferably at the same side as the incoming cables so that only one end-panel need be cut. A Buzzer No. 33A in a Clip No. 91B for audible alarms should be fixed inside the switch section.

Label Dgms LD 141 and LD 143 should be obtained and pasted over the existing Dgms LD 131 and LD 71 which are supplied with the section and pasted on the inside of the rear panel.

Diagram LD 141 shows the cord circuit and associated wiring of the switch section when used for a PABX 2. If the maximum number of 15 cord circuits are authorised the necessary additional cords and pulley weights should be obtained and fitted locally.

Diagram LD 143 shows the cabling to be provided to jacks, lamps, terminal strips and busbars and to the alarm unit. The face equipment should generally be arranged in the following order:-

Left-hand panel
bottom - exchange lines
centre - automatic extensions
top - night service jacks

Right-hand panel
bottom - inter-PBX circuits
centre - manual extensions

Frames, Distribution No. 4 external circuits should be fused as indicated in Cl D1003. Internal circuits should be taken via connection strips.

(a) Circuits between the MDF and the automatic equipment The cable sizes and number of cables recommended for each size of equipment are as follows:-

4 + 15 size - one 41-wire cable

5 + 24 - one 41-wire cable and one 20-wire cable

1 B3 - two 41-wire cables and one 12-wire cable

1 B4 - three 41-wire cables

For the 4 + 15, 5 + 24 and 1 B4 sizes of equipment, these provide for the full capacity of the installation less one pair of wires. For an installation of one of these sizes requiring the full possible number of exchange lines, extensions and inter-PBX circuits it will be necessary to run an additional single-pair cable between the MDF and the automatic equipment. Generally, it will not be necessary to provide this additional pair of wires at the outset.

For the 1 B3 size of installation the number and sizes of cable shown above will be adequate for the full capacity and will provide also one spare pair of wires.

In addition, at each installation, a single-pair cable should be provided between the MDF and the automatic equipment for the battery and earth lead to the internal night bells, [see also (d)]. The terminations of the circuits on the automatic equipment for PABX 1 are shown on Dgm SA 8119, Sheet 2 and, for PABX 2, on Dgm SA 8119, Sheet 4.

(b) Circuits between the automatic equipment and the manual switchboard

(i) PABX 1
A 75-wire cable is recommended for all sizes of equipment. This will provide for the full capacity of a 1 B4 size installation plus some spare wires. The cable should be completely terminated irrespective of size of equipment. The terminations are shown on Dgm SA 8119, Sheet 2. These terminations provide connections in the cable for all circuits, thus allowing for growth of the installation up to its maximum capacity.

On the 1 B3 and 1 B4 sizes of equipment a third inter-PBX circuit may be connected. The wires for this ate included in the 75-wire cable in the terminations as shown on Dgm SA 8119, Sheet 2. On the 1 B4 size of equipment, .the th2rd inter-PBX circuit is only provided as an alternative to a 10th exchange line as its connections to the manual switchboard are over the same wires.

The colour scheme for terminating the 75-wire cable is shown in Table 2 for switchboards PABX SA 8120 and SA 8133 Mk 1 and in Table 3 for switchboards PABX SA 8133 Mk 2.

The battery and earth leads from the automatic equipment to the manual switchboard are included in the 75-wire cable, two wires being used for battery and a further two for earth. With this arrangement the switchboard may be up to 25 yards from the automatic equipment. Greater distances may be covered by making use of pairs from spare triples and wiring these in parallel with the existing wires.

(ii) PABX 2 The types and sizes of cables between the automatic equipment and the manual switchboard and their terminations on the latter are given on Dgm LD 143 for all standard circuits normally required. The terminations on the automatic equipment are given on Dgm SA 8119, Sheet 4.

(c) Circuits between the MDF and the manual switchboard (PABX 2 only) Types, sizes and terminations on the manual switchboard of cables for these circuits, e.g. manual extensions and night-service circuits, are given on Dgm LD 143.

(d) Circuits from MDF to extension telephones and night bells These should follow normal internal wiring practice. Cl D1100 deals with the distribution of earth to the extension telephones for the enquiry facility. Diagram N 806 deals with the connection of the earth at the telephone.

(e) Circuits from the power plant to the equipment busbars are dealt with in Q0071.

(f) Cable supports Elaborate cable racking is not necessary.

(g) Wire wrapping In accordance with Specification D2155.

Cross-connections etc on the automatic equipment must be made between tags of the connection strips (SCA - SCE) at the top of the automatic equipment as indicated on Dgm SA 8119, Sheet 2, for PABX 1, and Sheet 4 for PABX 2, and according to the facilities being provided at the PABX.

Marking wire cross-connections on PABX 1 only, must be made on early equipment's between the extension M-tags and the tags on strip connection SCA indicated on Dgm SA 8119 Sheet 2. These wires are already provided on later equipment's and must be removed when used for a PABX 2.

Other cross-connections, such as the provision or removal of straps on or from shelf jack points, must be made as appropriate to the facilities to be given and the type of public exchange to which the PABX works. Details of the connections, etc are given on the SAW diagram of the circuit concerned.

When using Relay-sets SA 8104, SA 8109 or SA 8144, all wiring to the shelf jacks, except the common services wiring, must be provided locally. Wire, Wire Equipment 1500 series should be used for connections within the equipment and should be tied in to the main cable forms.

(a) Relay-set SA 8104
Three wires per circuit are required from the shelf jack points indicated on the relay-set wiring diagram to tags A, B and H (on the equipment connection strips) of the auto extension line circuits for which the long line circuits are required.

(b) Relay-set SA 8109
On PABX 1, four wires per relay-set are required from the shelf jack points indicated on the relay-set wiring diagram to tags A, B, H and HL of the associated auto extension line circuit and one pair of wires from shelf jack points 21 and 22 to the MDF via tags on the equipment connection strip SCA. See Dgm SA 8119, Sheet 2.

On PABX 2 the connections required vary with the purpose of the relay-set and are indicated on the relay-set wiring diagram. The connections to the manual switchboard are given on Dgm LD 143.

(c) Relay-set SA 8144
Connections to the manual switchboard are given on Dgm LD 143. In addition, a pair of wires per circuit is required from jack points indicated on the relay-set wiring diagram to the MDF. These may be taken via spare tags on connection strip SCA in the equipment, if desired.

(a) Labels for the PABX 1 manual switchboard

Switchboard, PABX, SA 8120
The switchboard is fitted with engraved Labels No. 88H, White for regular services. For exchange lines, inter-PBX circuits and manual extensions, blank Labels No. 88H, White are fitted. These labels should be sign written temporarily, with the appropriate designation of the circuit concerned and arrangements should then be made for them to be replaced by engraved labels.

(ii) Switchboard PABX, SA 8133
The switchboard is fitted with printed Labels No. 438 for all common services and for the two 0-level circuits. Exchange lines inter-PBX circuits and manual extensions require Labels No. 438, Clear, Hot Stamped in Black with the required designation. These should be ordered as required from Factories Dept under the normal procedure. Up to three lines of seven characters per line may be accommodated on each label.

The clear Labels No. 438 supplied with the switchboard should be temporarily sign written and changed when the permanent labels are received.

Where an exchange name is longer than seven characters and cannot be abbreviated to seven or less characters, the local engineering code (or traffic code if preferred) should be used.

(b) Labels for the PABX 2 manual switchboard
For the stile casings, Labels No. 277 ... should be reversed and sign written locally with a general circuit description, e.g. exchange lines, against the appropriate groups of jacks. Engraved Labels No. 86A are included in the appropriate Labels, PABX, Set No. ... [see (c)] for the automatic and manual extensions. For exchange and inter-PBX lines, individual circuit numbers should be engraved on Labels No. 86A, White against each circuit appearance. (Temporary sign written labels should be provided until engraved labels can be fitted).

(c) Labels for relay-sets and selector These labels are supplied in sets as follows:-

Labels PABX Set No. 1A for PABX 1, 4 + 15 (Drawing 91053)
Labels PABX Set No. 1B for PABX 1, 1, 5 + 24 ( 91055)
Labels PABX Set No. 1C for PABX 1, 1, 1 B3 ( 91057)
Labels PABX Set No. 1D for PABX 1, 1, 1 B4 ( 91059)
Labels PABX Set No. 2A for PABX 2, 2, 4 + 15 ( 91054)
Labels PABX Set No. 2B for PABX 2, 2, 5 + 24 ( 91056)
Labels PABX Set No. 2C for PABX 2, 2, 1 B3 ( 91058)
Labels PABX Set No. 2D for PABX 2, 2, 1 B4 ( 91060)

The appropriate Labels PABX Set No. ... should be requisitioned and fitted in accordance with Table 4. Blank labels are included in each set for those relay-sets which have local circuit numbers, e.g. exchange lines. These labels should be temporarily sign written whilst arrangements are made for them to be replaced by engraved labels.

The location of the labels on the relay-set or selector is as follows:-

1 Label No. 379, White or Label No. 236, White on the test jack.

2 Labels No. 237, White on the cover

1 Label No.. 238, White on the cover

The fuses required on the automatic equipment for all the standard circuits are listed in Table 5. The position for these fuses may be seen from the sign writing provided on the equipment fuse-mountings. In addition, there are two spare fuse positions (see Drg 90000, Sheet 5 for the 4 + 15 and 5 + 24 sizes and Drg 93132 Sheet 6 for 1 B3 and 1 B4 or Drg 63959, Sheet 5 for the 7 + 35 and 10 + 49 sizes) which are not sign written but are wired to the two spare relay-set shelf-jacks (see C1115) which may be used for auxiliary circuits, i.e. Relay-sets, SA 8104, SA 8108 or SA 8144. When any of these are fitted the spare fuse positions should be approximately sign written locally and one Fuse No. 44A/1.5 inserted for each relay-set fitted (maximum two relay-sets).

The fuses required on the manual switchboard of PABX 2 are indicated on Dgms LD 141 and LD 143. The positions of the fuses are shown on the latter diagram which also shows a typical marking for the manual extension circuit fuse positions. These must be wired and sign written locally.

The equipment's are supplied fitted with Coils, Heat, A, Green in the release circuits of the connecting circuit selectors. If these coils are blown or removed during installation or testing they must be replaced by similar green coils. Coils, Heat, A, Brown must not be used because their higher current rating may involve a fire risk.

Before the installation is handed over for maintenance it should be tested in accordance with F1017 (PABX 1) or F1018 (PABX 2).

Each Equipment, PABX, SA 8100... or Equipment, PABX, 1B... /SA 8l00 is supplied with two keys for the locks of the cabinet doors and two keys for the locks of the plinth doors. All keys should be handed over to the maintenance staff.

From EI Telephones, PBXS, C3110

3 Internal
Issue 3, May 1973


Instruction describes a modified version of the Equipment PABX SA8100.

The equipment's are available in two sizes, 1B3(7 + 35) and 1B4(l0 + 49). The title in Equipment PABX 1B.../SA8l00.

Externally the Equipment PABX 1B... /SA8l00 can be distinguished from the earlier equipment as no plinth is provided and grey hinges replace the earlier chromium plated ones.

Internally, jack-in relay-sets replace the wired in relays on shelf  'A' for the extension line, start and alarm, enquiry, night service extension and engaged test circuits. A new heat-coil assembly is fitted, although the former arrangement may be encountered.

To increase manoeuvrability four holes are provided in the base to facilitate the fitting of castors, if required.

The facilities of the equipment remain unchanged (F1010).

This instruction should be read in conjunction with:-
F1010 - Description of the PABX 1
F1014 - Description of the PABX 2
F1013 - Description of the Equipment PABX SA8100
F1015 and F1016 - Planning and Installation of the equipment

See F1016 for details of the label sets. These should be requisitioned and fitted in accordance with table 2.


Equipment	Unequipped 	Fully equipped 		Dimensions
		weight 		weight 			H 		W		D	

1B3		300kg(661 lb.)	560 kg (1200 lb.)	5' 11.5" 	3' 9"		2'
1B4		565kg(1239 lb.) 565 kg (1239 lb.)	5' 11.5"	3' 9"		2'


Introduced in 1950

Additional Pictures

Earlier Switchboard SA8120 with the equipment cabinet in the background
Later Switchboard S8133
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Last revised: December 18, 2010