Introduced around 1970

3 Internal
Issue 3, Jul 76


tsa4252.jpg (11135 bytes)This instruction describes the 4 types of telephone, and 2 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 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 In all cases the dial has been replaced by a Push-Button Unit No. 3 ....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 746. It is primarily intended as a direct PABX extension, but can house 2 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 52A.

Telephone 1/SA4252
Obsolescent and not to be used except for maintenance and growth of existing plan numbers. Similar in appearance to the Telephone 740, this instrument is a fully equipped telephone having 6 press- buttons and 2 lamps. It is supplied with a 20 way instrument cord and Block Terminal 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 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 4 dummy buttons, 2 lamp opals, and 2 Lamp Fittings 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 2 add on press-buttons. The instrument is supplied complete with Bracket Telephone No. 15.

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 4 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

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 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 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 to see


Rate Book   Item Code
SA4252 Grey   410289
1/SA4252 Grey   410290
2/SA4252 Grey   410302
3/SA4252 Grey   410303


Model Mark Grey Blue PBU Introduced Remarks
Tele SA4252 Mk 1 y y   9/69 Original
  Mk 2 y y 3A/3 3/73  
  Mk 3 y y 3A/2 5/74  
  Mk 3 y y 3A/3 11/75  
Tele 1/SA4252 Mk 1 y   3A 9/69  
  Mk 2 y   No. 6 2/74 GEC
  Mk 3 y   3A/2 5/74  
  Mk 3 y   3A/3 11/75  
Tele 2/SA4252 Mk 1 y   3A 9/69  
  Mk 2 y   No. 6 2/74 GEC
  Mk 3 y   3A/2 5/74  
  Mk 3 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: September 21, 2010