SIEMENS BROTHERS
S16 

An extract from the IPOEE Paper No. 142 (1932)
PABX Installations by R. Dennison

The Siemens No. 16 System employs the well-known types of Siemens 10 point uniselector, two-motion selector and similar circuits to those used in their No. 16 non-director exchanges.  A typical installation is shown in the picture below.  These items are mounted on open type racks, and so many of each assembled in this manner form a P.A.B.X. automatic unit. Systems working on the No. 16 principle are classified as "small" and "large" P.A.B.X. systems.

Small P.A.B.X's are worked on a two-digit basis, and large P.A.B.X's on a 3-digit basis. Both employ 10 point uniselectors and in three-digit systems 1st and 2nd pre-selectors are installed. The first preselector is a homing type, and the second a non-homing type. Both are stepped, by means of impulses delivered from an impulse machine, at 34 steps per second and the rotary motion of two-digit selectors is also affected in this manner Three-digit systems employ intermediate (or group) selectors to deal with the 100's digit.
100 outlet final selectors are installed in both large and small P.A.B.X's.

Calls to the manual board are obtained by dialling "0" which causes the final selector in two-digit systems and the group selector in three-digit systems to take ten vertical steps. Normal post springs are provided which are actuated on the tenth level, and bring about the operation of the manual board calling lamp associated with the calling extension. All the automatic equipment taken into use is then released. The caller hears ringing tone until the operator answers.

Direct access and tie line facilities may be provided on levels 9 or 8 respectively, by the addition of suitable relay sets wired to the bank contacts of these levels. Since the transmission bridge in the Siemens No. 16 system is located in the first selector in three-digit systems it is necessary to convert these selectors into repeaters or to cut them out of circuit when digits 8 or 9 are dialled.

P.G. and magnet alarm circuits are provided, but no provision is made for the forced release of automatic equipment taken into use under P.G. conditions.

Messrs. Siemens well known type of relay is employed throughout.

The systems operate on 60 volts, the range for satisfactory operation being from 54-66 volts.

Additional Information

The Southern Region railway company replaced their S15 at Waterloo, with the much larger (900 line) S16 in 1925.

This S16 PABX was associated with a CB10 switchboard, which was replaced later with a GEC/Reliance low level PABX switchboard of half the size.

The introduction of full internal dialling cut the use of manual trunks almost to nothing and the remaining load was I/C Exchange Lines, O/G Long distance and Internal Emergency 999 Calls.

The pictures below (Dated 1964) are of the CB10 switchboard and S16 PABX at BR Waterloo.

1st Pre-selector racks

The square units above each selector, contain the selector relays

Last revised: October 08, 2025

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