PABX No. 5

Extension users guide

An article from the Ericsson Bulletin

An article from the GEC Telecommunications Journal

Descriptive Leaflet - 1962

Descriptive Leaflet - 1972

Descriptive Leaflet - 1976

Introduced in 1963.

Withdrawn by BT in 1993.

3 Internal

Issue 3
Aug 1976

Private Automatic Branch Exchange No. 5
(P.A.B.X. No. 5)


This Instruction describes the facilities afforded by and the equipment used for the PABX5.  Installation instructions are given in TI F1052.
The picture on the right shows a PABX5 with the door open.

The PABX5 was initially available in two sizes, namely for three exchange lines and ten extensions or five exchange lines and 20 extensions.  It is now only available in the larger size.  A Manual switchboard is not provided. All calls are answered by designated extensions and transferred as required.

The automatic equipment (Equipment PABX SA8450) comprises a single self contained unit which employs PO Type 2 Uniselectors and relays, mainly 3000-type, for all switching purposes.  The unit is floor-mounted and is supported at the top by two metal brackets which are bolted to a wall.  A detachable metal dust cover is fitted over the unit.  A power supply at a nominal 50V d.c. is obtained from a Power-unit No. 69A.  To aid installation a version has been introduced with more Uniselector mechanisms fitted.  This is coded Equipment PABX5A.

The layout of the Equipment's PABX 5A and SA8450 are shown in Drawings 93590 and 91839 respectively.  The approximate main dimensions are: height 1760mm (5ft 9in), width 760mm (2ft 6in), depth to wall 370mm (1ft 5in).

External view showing Power Unit at the bottom and the MDF to the left Internal view showing 3+10 variant

The following general facilities are provided as standard and a more detailed description is given in the relevant diagram notes.

Public Exchange Service
This is given over bothway working exchange line circuits terminated at the PABX with Relay-sets SA8455. The calling condition to the public exchange may be either a loop or an earth on the B wire followed by a loop.

Outgoing Exchange Calls
Access by an extension to a public exchange line, as with other standard PABX's, is obtained by dialling the digit 9. It is possible to prohibit this facility from any particular extension as desired.

Incoming Exchange Calls
Incoming exchange call bells are sounded and the calls are answered by extensions designated for that purpose. There may be a number of designated extensions (up to 10 or 20 according to the size of the equipment installed) any one of which may answer an incoming call on any exchange line merely by lifting the receiver. An incoming exchange call, having been answered by a designated extension, may be transferred by the designated extension to any other extension allowed connection to incoming calls by the use of the enquiry and automatic transfer facilities. If required any particular extension may be prohibited from receiving incoming exchange calls.

Extension-to-Extension Calls
These are dialled direct on connecting circuits. The connecting circuit is held for the duration of the call.

Enquiry calls
An extension engaged on an exchange call may make an enquiry call to any other extension or inter-PBX extension. An extension engaged on an inter-PBX call (either inter-PBX extension or inter-PBX private circuit) may make an enquiry call to any other extension or inter-PBX circuit.

The extension wishing to make the enquiry depresses the button on the extension instrument once to receive dial tone and then dials the required extension number. The public exchange or inter-PBX circuit connection is held for the duration of the enquiry call. When the enquiry call is ended the instrument button is again depressed and the extension is reconnected to the exchange line or inter-PBX circuit. Non receipt of dial tone after the first depression of the instrument button is an indication that all connecting circuits are engaged. The extension then should return to the exchange line in the normal manner, i.e. by a second depression of the button, and either abandon or attempt the enquiry call later.

Automatic transfer
An extension having made an enquiry to a second extension while holding an exchange line or inter-PBX line may also transfer the call to the second extension. This is done by asking the second extension to hold on while the handset of the originating extension is replaced. This operation may be repeated as often as required.

Exchange line transfer guard
Should transfer be attempted to an extension normally barred from receiving exchange calls, the incoming exchange call bells are again sounded after a short timing period and a designated extension would get access to the exchange line relay-set as for a normal incoming call. Similarly, if transfer is not successfully accomplished the incoming exchange call bells will be sounded.

Intrusion (or trunk offering)
A designated extension wishing to transfer an incoming exchange call to a second extension may obtain busy tone on dialling the required extension. To trunk offer, or intrude, the designated extension dials the digit 1 and obtains access to the busy extension to offer the exchange call. During the time that the designated extension is intruding in this way on an established call a 'warn' pulse is automatically applied to that call to indicate the intrusion.

Night service
The normal operating conditions are retained for night service but one extension arranged as designated and/or limited facility during the day can be converted to full facility and become designated when the night key is operated. Also additional or different call bells can be operated to indicate an incoming call. The night service key will be fitted on a selected extension telephone or the attendant's telephone if provided.

Inter-PBX circuits
Each inter-PBX circuit provided reduces by one the number of exchange line circuits available. The types of inter-PBX circuit available are as follows:-

Bothway dialling, loop signalling (Relay-sets SA8457) for working to other PABX's.

Bothway earth signalling with earth dialling inward (Relay-set SA8456) for working to PMBX's.

Bothway dialling for working to other PABX's or dialling in from PMBX's, 1 VF (SSAC13) signalling (Relay-set SA8460). This system is required when the signalling limits for 3.8.1 and 3.8.2 are exceeded or when HF plant is used. (It is necessary to provide Equipment, Signalling, No. 24/ ... when vVFsignalling is used.)

These circuits may be connected as inter-PBX extensions in which case incoming exchange calls may be extended over them, or as inter-PBX private circuits when access to public exchange lines is prohibited. Inter-PBX circuits may be provided over up to three different routes, direct access being obtained by a single routing digit (7, 8 or 0) in each case.

A fuse alarm lamp is provided on the night service control telephone or the attendant's telephone if provided.

Attendant's telephone
This consists of a modified Telephone No. 710 mounted on an Answering Unit No. 1A (grey) - see picture below. Two designated extensions are terminated on the Answering Unit No 1A which has SPEAK, RELEASE, and HOLD buttons associated with each extension. An exchange line calling lamp and fuse alarm lamp are provided on the telephone in addition to the night service key.

The extension numbering ranges for the two sizes are:-

Size   Number range
3 + 10   20-29
5 + 20   20-39

The single digits 7, 8, 9 and 0 are reserved for routing of calls to public exchange lines or inter-PBX circuits. A maximum of three routes for exchange and inter-PBX lines is available on the 3 + 10 and a maximum of four routes on the 5 + 20.

Diagram SA8450 shows the trunking scheme for a PABX5 and a brief description of the features of the circuits follows.

Extension line circuits, linefinders, connecting switch and exchange (EF) finders (SA8451). All finders and connecting switches are PO Type 2 25-point Uniselectors. The extension circuits are in two groups of ten and are multiplied to the banks of the line and EF finders and to the banks of the connecting switches. In addition each exchange line or inter-PBX circuit has an associated auxiliary extension line circuit and these circuits are also multipled to the banks of the-line and EF finders but not to the banks of the connecting switches.

Each linefinder and connecting switch is directly connected to a connecting circuit and the controlling relays for these switches are accommodated in the connecting circuit. The EF finders are correctly connected to the exchange line or inter-PBX circuits and the controlling relays for these finders are within the exchange line or inter-PBX circuits as appropriate. The connecting switches are arranged to 'home'; the linefinders and EF finders are not.

Connecting circuits (SA8454) Each connecting circuit is arranged as a strip-mounted set, a new feature being that the sets may be jacked in and out.

Up to two or four connecting circuits may be provided on the 3+10 and 5+20 sizes respectively. Access to the circuits is via a start chain, and to ensure that only one linefinder will search for the calling line the start circuits of the linefinders not in use are held open (or busy) until the calling line is found. Thus only one linefinder can hunt at any one time. When a calling extension has been found dial tone is returned from the connecting circuit and a period of approximately 15 seconds is allowed for dialling to take place. Failure to dial within this period causes the connecting circuit to be released and the extension made PG.

Local calls
If an extension number in the range 20-29 is dialled the CS Uniselector is stepped directly by the dialled pulses and the CS wipers are positioned accordingly. The connecting circuit then behaves in the same way as a final selector. If an extension in the range 30-39,is dialled, during the inter-train pause following the initial digit 3 the CS wipers are automatically driven past the contacts used for extensions 20-29 and a second. digit 1-0 will allow connection to extensions 30-39 as appropriate. On clear down at the end of a call the CS Uniselector homes.

Outgoing exchange line or inter-PBX calls
For an outgoing exchange line call the CS Uniselector steps according to the digit dialled and a start signal is applied to the start chain of the exchange line relay-sets via a CS are and wiper. The EF finder associated with the exchange line relay-set to be used rotates and finds the marking condition set up by the connecting circuit and its linefinder (this is possible since some arcs of the linefinders and EF finders are multipled together) so that both the linefinder and EF finder stand on the calling extension's position. The connecting circuit is then released, the calling extension is connected to the exchange line relay-set, and the CS Uniselector homes. A similar sequence of events takes place on outgoing inter- PBX calls.

Exchange line relay-sets (SA8455)
A maximum of three or five exchange line relay-sets may be fitted in the 3+10 and 5+20 sizes respectively. As with the connecting circuits these are strip-mounted sets and may be jacked in or out. It should be noted that the maximum number of exchange lines is reduced by one for each inter-PBX circuit fitted. An EF finder is associated with each relay-set.

Outgoing calls progress as described above, i.e. a start signal is received from the connecting circuit which causes the EF finder to rotate, find the extension, release the connecting circuit and connect the extension to the exchange. The calling signal to the public exchange will normally be earth on the B wire although the relay-set can be arranged to work 'loop calling' if required.

On incoming exchange calls ringing current operates a relay in the exchange line relay-set and the incoming call bells are sounded. A designated extension, to answer the incoming call, lifts his receiver and a start signal is applied to the calling exchange line relay-set. The EF finder of that relay-set rotates to find the answering extension, the incoming exchange line call is connected to this extension and the incoming call bells are disconnected.

Should two incoming calls arrive simultaneously the exchange line start chain is arranged so that only one EF finder can hunt at any one time. In these circumstances a designated extension would answer one of the incoming calls, the incoming call bells would continue to ring and a second designated extension would answer the remaining call. The incoming call bells would then be disconnected.

On an outgoing or incoming exchange call it is possible during the course of the call for the extension user to role an enquiry call to a second extension and to do this use is made of an auxiliary line circuit associated with the exchange line relay-set. Depression of the button on the extension telephone gives access to the auxiliary line circuit which is used in the same manner as an extension's line circuit to obtain connection to a connecting circuit. At the same time a loop is presented to the exchange line to hold the outgoing connection or maintain the correct answering condition. The enquiry call now proceeds in the same way as any other local call and when the enquiry has been made a second depression of the telephone button will re-connect the extension to the exchange line and release the connecting circuit.

If required it is possible for the exchange connection to be transferred from the enquiring extension to the second extension (except where this facility has been prohibited). During the enquiry call the connecting circuit is being used with its linefinder standing on the auxiliary line circuit of the line relay-set and its CS switch standing on the extension number dialled. Thus it is possible to mark this second extension via the linefinder and CS switch, and to effect transfer the calling extension's receiver is replaced. The following sequence of events then takes place. The connecting circuit begins clear down, the EF finder of the exchange line rotates until it finds the marked extension, the complete clear down on the connecting circuit is then permitted, the required extension is connected to the exchange line relay-set and then finally through to the exchange line. The holding loop is disconnected since the exchange line is now under the control of the extension. This process can be repeated as often as required.

Inter-PBX circuits (SA8456, SA8457, SA8460)
These can only be fitted where less than the maximum capacity of exchange lines is required, each inter- PBX circuit fitted replacing one exchange line circuit. An EF finder is associated with each relay-set and outgoing calls follow closely the pattern of outgoing exchange line calls, a single routing digit being necessary to gain access to a relay-set. If the inter-PBX circuit is to a PMBX no further dialling is required by the extension after the routing digit, but for circuits to another PABX a routing digit followed by the number of the required extension will be dialled.

On an incoming call seizure of the relay-set by the distant PABX or PMBX results in the seizure, via the auxiliary line circuit, of a connecting circuit. The required number is dialled into the connecting circuit and if it is free ringing is applied to the called extension's line. When the called extension answers, a reversal is repeated from the connecting circuit to the inter-PBX line relay-set and, at the same time. the position of the called extension is marked on an arc of the EF finders via the linefinder and CS switch of the connecting circuit. Receipt of the reversal from the connecting circuit causes the EF finder of the inter-PBX circuit to rotate and find the marked extension, the connecting circuit is released, and the extension connected through to the inter-PBX circuit.

It is not possible for the inter-PBX circuit to obtain access to exchange line routes.

Dial, busy, ring, number unobtainable and warn tones are obtained from a ringing, tone and miscellaneous circuits relay-set (SA8452) although the actual tone generators are detachable units within this relay-set. The diagram relating to the ringing and tone generators is SA8453.

If a fuse is blown an alarm lamp is lit on a particular extension telephone to give a fault indication.

In the event of a mains failure each exchange line is connected to a predetermined extension and incoming exchange calls are received directly on those extensions. The extensions are also able to make outgoing exchange calls. On the restoration of the mains supply the exchange lines are disconnected from the extension lines and reconnected to the exchange line relay-sets. However, should an extension be engaged on an exchange line call when the mains supply is restored the reconnection of the exchange line to the exchange line relay-set is delayed until the clear down of that call.

Diagram Title
SA8450 Trunking scheme
SA8451 Equipment PABX, 5A and SA8450 wiring (including extension line circuits and EF, LF, CS bank multiples)
SA8452  Relay-set SA8452 - ringing, tone and miscellaneous relay-set
SA8453 Ringing and tone generator units
SA8454 Relay-set SA8454 - connecting circuit
SA8455 Relay-set SA8455 - exchange line circuit
SA8456 Relay-set SA8456 - inter-PBX circuit (dialling in from PMBX)
SA8457 Relay-set SA8457 - inter-PBX circuit (bothway dialling)
SA8460 Relay-set SA8460 - inter-PBX terminating relay-set (bothway working)   1 VF (SSAC13 signalling)
SA8462 Telephone Unit SA8462


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Last revised: June 08, 2022