Click here to find out how the PABX 3 works

This page takes you on a walk around a typical PABX 3.  You have to remember that this is a very large room at least 10 foot high with equipment racks that are 7' 9" high and 4' 6" wide.  Included here is also some documentation that came with the installation to help you identify certain aspects of the equipment.  The documentation is actually the original, from a hospital site with capacity for 25 lines and 600 extensions.  The site also had 5 switchboards and a 4 digit extension numbering scheme.

Now switch on your pc speakers and hear them selectors go ........................

The floor plan

This is a plan view of the equipment racks.  The dotted lined areas are reserved for
additional racks.  Enter the room from the door at the bottom right.

RSR = Relay Set Rack
LU = Line Unit
MAR = Miscellaneous Apparatus rack
GS = Group Selector
MDF = Main Distribution Frame

The equipment racks
Enter the room from the door - lower right hand corner of the plan above and proceed to the Main Distribution Frame (MDF) which is on your right.  Cables from BT, premises wiring and extension wiring from the PABX 3 all terminate on the MDF.  These are cross connected together with jumper wire (the blue & yellow wires).



The whole MDF with GPO tester (left) used for testing the extension line wiring.

The strips at the top are where the PABX 3 extension and PABX 3 exchange line wiring terminates.

The strips at the bottom are where the site distribution wiring terminates.

GPO/BT incoming exchange line terminations are on the far right. 

Site distribution wiring termination - fuse side.
The red and grey fuses are dummies.  The glass fuses are real fuses.
Red fuses are for special circuits.


Wiring side of the same block.  Here you see the jumper wire (blue and yellow), which is routed to the PABX 3 extension terminating strips - pictured below.


Close up of the PABX 3 extension termination blocks with the exchange line termination strips nearest (not the grey boxes to the right!).

These termination strips are the old type with lightning  protectors fitted.

Opposite the MDF are the Extension Line Units.  These cater for up to 100 extensions per rack.

The equipment from the floor upwards are:-

Two rows of 1st Group Selectors.  These deal with the first digit dialled by an extension and are connected to a free 2nd selector.  A digit 9 will connect to a free exchange line relayset and 0 to the manual switchboard.

Two rows of Linefinders (50 extensions per row) - consisting of line relays (under the covers) and Uniselectors (the picture below shows a Uniselector with the mechanism removed, exposing the contact bank).  When an extension goes off hook a linefinder selects a free 1st group selector and then dial tone is returned.

Two top rows, of which the lower is used in this instance, of Final Selectors.  These deal with the last two digits of an extension number and then ring the phone. The two Selectors on the right hand side of the very top row are Enquiry finders, used when transferring calls.

The picture below shows a Uniselector with the mechanism removed.







This is a typical installation drawing of the line units, showing the installers how many selectors are to be fitted. The emboldened lines show population.

You will notice that not all the shelves are fitted with selectors.  A traffic study would have taken place before installation and this would have given an indication on how much equipment was needed.
Additional selectors could be fitted later.

LF = Line finder
G/S = Group Selector
F/S = Final Selector

Walk around the Line Units and there is the Group Selector rack.  This houses all the 2nd Selectors and these deal with the second digit dialled and connect the call to a free final selector.  These are shared around the Line Units.

The Final selector tests the line dialled and will either:-

  1. Ring the phone, giving ringing tone to the caller
  2. Give busy tone if the line is in use
  3. Give Number Unobtainable tone if the line is not used.





Behind the Group Selector rack you find the Relay Set Racks (RSR).  In general these are equipped with the ringing machines, Exchange Line relay sets and Enquiry Circuits.  This is a real life example of an RSR rack with the positions and uses of the relay sets clearly marked.
If ringers are fitted to the RSR then they are installed at the bottom of the rack.  In this instance we have two ringers, one is in reserve in case the other fails.  The relaysets above the ringers are associated with the ringers.


The Exchange Line Relaysets are fitted at the top of the RSR rack.  One line per Relayset.

One large sites the Exchange Line relay sets will have their own rack.

Close up of a ringing machine, actually called a Dynamotor.  They are quite weighty.

And finally - behind the RSR are the batteries and rectifier unit.  The batteries are provided to smooth out the power supply and ensure that there is no noise or hum.  Their other secondary function was to provide power in the event of a mains failure.


The switchboards are, of course, in another room, as the noise from the PABX equipment would have obliterated any conversation.

Later switchboards were covered in grey Formica and were of a similar appearance to the PMBX 4.

Thanks to Malcolm Saffin for the use of the PABX 3 pictures.

At this point the GPO engineer asks you to leave as the operators have just made him a cup of rosy lea.

Tour finishes


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Last revised: December 07, 2019