|Extension users guide
An article from the Ericsson
article from the GEC Telecommunications Journal
Descriptive Leaflet - 1962
Descriptive Leaflet - 1972
Descriptive Leaflet - 1976
Introduced in 1963.
Withdrawn by BT in 1993.
C MARKETING INSTALLATION
Automatic Branch Exchange No. 5
(P.A.B.X. No. 5)
SCOPE OF INSTRUCTION
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
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
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.
These are dialled direct on connecting circuits. The connecting circuit is held for the
duration of the call.
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
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.
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 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.
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
Bothway earth signalling with earth dialling inward (Relay-set SA8456) for working to
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
A fuse alarm lamp is provided on the night service control telephone or the attendant's
telephone if provided.
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:-
|3 + 10
|5 + 20
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.
DESCRIPTION OF CIRCUITS
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
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.
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
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
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
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
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.
FUSE ALARMS (SA8452)
If a fuse is blown an alarm lamp is lit on a particular extension telephone to give a
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.
||Equipment PABX, 5A and SA8450 wiring (including extension
line circuits and EF, LF, CS bank multiples)
||Relay-set SA8452 - ringing, tone and miscellaneous
||Ringing and tone generator units
||Relay-set SA8454 - connecting circuit
||Relay-set SA8455 - exchange line circuit
||Relay-set SA8456 - inter-PBX circuit (dialling in from
||Relay-set SA8457 - inter-PBX circuit (bothway dialling)
||Relay-set SA8460 - inter-PBX terminating relay-set (bothway
working) 1 VF (SSAC13 signalling)
||Telephone Unit SA8462