Introduced around 1970

3 Internal
Issue 3, Jul 76

Telephone SA 4252

This instruction describes the four types of telephone, and two ready mounted push button units, which have been developed to meet the telephone requirements of PABX's with DC Code "C" signalling.  The four instruments are based on Telephones No. 740 and 746 (see B1020). In all cases the dial has been replaced by a Push-Button Unit No. 3 ....

The design considerations to be observed for applications other than those shown on Diagram SA(L)4252 are stated in the paragraph Other Applications.


Telephone SA4252
This instrument is similar in appearance to the Telephone No. 746.  It is primarily intended as a basic direct PABX extension, but can house two add on press-buttons to provide Plan 1A.  Telephone SA4252 is also used at each extension of a Plan 1, and at the main, providing the total number of extensions does not exceed 2.

By replacing the instrument cord, Plan 4 facilities can be provided. Telephone SA4252 is used at the extensions of Plan 107/107A and Plan 105/105A.  The main of these arrangements is provided by associating a Telephone SA4252 with a Planset N625 and Power Unit No. 53A.  Each telephone is supplied complete with a 4 way instrument cord and Block Terminal No. 52A.

Telephone No. SA4252

Telephone 1/SA4252
Obsolescent and not to be used except for maintenance and growth of existing plan numbers.  Similar in appearance to the Telephone No. 740, this instrument is a fully equipped telephone having 4 press-buttons, a buzzer and two lamps.  It is supplied with a 20 way instrument cord and Block Terminal No. 37B. This telephone is used to provide Keyplans KDC2, KDC105/105A and 107/107A.  For these plans it is not necessary to fit additional parts to the telephone.  A 12 wire inter-connecting cable is used at all times, and the facilities vary, depending on which of the 25 terminals in the block terminal the 12 wire cable is terminated.  A Power Unit No. 53A is required for each extension plan.

On Keyplan KDC107A either station can be wired so that when the EXCH button is depressed the call is secret from the other station. (The AN should clearly indicate at which station the secrecy is required.)

On Keyplan KDC105A the secrecy wiring is sequential, i.e. the first station on the wiring route has, when the EXCH button is depressed, secrecy from the second and third stations.  The second station in the wiring sequence is secret from the third station but not the first. The third station is non-secret.  Because of this it is essential to clearly establish the order of precedence required by the customer for the various stations prior to the commencement of installation.

Telephone 2/SA4252
This is a basic instrument, similar electrically to Telephone SA4252.  The telephone case however, is fitted with four dummy buttons, two lamp opals, and two Lamp Fittings No. 16A wired to spare telephone terminals.  Lamps are not included. A suitable 12 volt, 0.02 amp lamp is the Lamp No. 48A.  This instrument is used for the main of a Plan 1 when more than 2 extensions are required and on Plan 2 and Plan 2A.  It is also used to provide the facilities of "Message Waiting" and "Absent Service".

Telephone 3/SA4252
This is a basic telephone for wall mounting with direct cabling.  It is electrically identical to Telephone SA4252, and can house two add on press-buttons.  The instrument is supplied complete with Bracket, Telephone No. 16.

Telephone No. 3/SA4252

Each of the 4 types of telephone contains a Push-Button Unit No. 3 .... This unit is designed for use with the DC Code "C" PABX signalling system.  The push buttons are arranged in 3 columns of 4. The 11th and 12th buttons are marked * and #-respectively.  The 12th button is used for enquiry and transfer.  The 2 variants of the Push-Button Unit No. 3 are as follows:-

PBU No. 3A used in Telephones SA4252, 1/SA4252 and 2/SA4252.
PBU No. 3B used in Telephones 3/SA4252.

The PBU No. 3A was originally called the Push Button Unit 1_SA4259.
The PBU No. 3B was originally called the Push Button Unit 2_SA4259.

PBU No. 3A PBU No. 3B

Telephones SA4252, 1/SA4252, 2/SA4252, and 3/SA4252 are shown in Diagram SA(L)4252. Diagram SA(L)4252 also shows the use of these telephones for providing extension plans.  For operating instructions see C3 A0065.

It is essential for the A and B conductors of the extension line to be connected to the telephone in accordance with the appropriate diagram. A reversal of the A and B connections will result in false operation of the Push-Button Unit No. 3.

Testers for DC Keyphones are to be held by Maintenance groups in Areas for the present, and should be borrowed by Installation groups if necessary.  Attempts should not be made to insert dial labels from the back of the telephone case, the correct procedure is to remove the Perspex cover at the front of the case.

For DC Code "C" Signalling applications not covered by Diagrams SA(L)4252 and 4259.  Telephones SA4252, 2/SA4252 and Press-Button Units 1 and 2/SA4259 are interchangeable with Telephones No. 746, 740, and separately mounted dials respectively, provided that the following design considerations are strictly observed:-

Terminal numbers on the appropriate sheet of Diagram SA(L)4252 for the keyphones and the corresponding terminal numbers on the appropriate N Diagram for Telephones No. 746 etc should be compatible.

Each telephone must be supplied with a signalling earth connection.

Correct line polarity must be observed.

Both wires of the line between the telephone terminals and the exchange must be kept free from shunt paths, such as bell circuits, when the handset is off-hook to ensure a clear line for dc keying signals.

Earthed-loop recall is provided by button # on the keypad.  A separate recall button is not therefore required.

Certain models supplied by Plessey - click here for more information.


Rate Book   Circuit diagram Specification Drawing No. Item Code
SA4252 Grey   SA/SAW 4252/1 S1120 93083 410289
1/SA4252 Grey   SA/SAW 4252/2 S1119 93084 410290
2/SA4252 Grey   SA/SAW 4252/3 S1120 93124 410302
3/SA4252 Grey     S1120 93136 410303

Obsolescent in 1982


Model Mark Grey Blue Black Green Red Yellow Ivory PBU Introduced Remarks
Tele SA4252 Mk 1 y y           3A 9/69 Original
  Mk 2 y y           3A/3 3/73 GEC  (TEL 71P)
  Mk 3 y y           3A/2 5/74  
  Mk 3 y y           3A/3 11/75  
Tele 1/SA4252 Mk 1 y y y y y y y 3A 9/69  
  Mk 2 y y y y y y y No. 6 2/74 GEC (TEL 88P)
  Mk 3 y y y y y y y 3A/2 5/74  
  Mk 3 y y y y y y y 3A/3 11/75  
Tele 2/SA4252 Mk 1 y y y y y y y 3A 9/69  
  Mk 2 y y y y y y y No. 6 2/74 GEC (TEL 88P)
  Mk 3 y y y y y y y 3A/2 5/74  
  Mk 3 y y y y y y y 3A/3 11/75  
Tele 3/SA4252 Mk 1 y             3A 6/69 GEC
  Mk 2 y             No 6 2/74 GEC
  Mk 3 y             3A/2 5/74  
  Mk 3 y             3A/3 11/75  


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Last revised: April 15, 2023