Extension Plans off Automatic Exchanges

Various extension plan arrangements were available on all telephones prior to the introduction of Plug and Socket (PST).  This is the main reason for there being so many variations of each telephone style.  Extension plans stated in the era of the Candlestick, included the 200 and 300 type telephones eventually finishing with the 700 type range.

The introduction of the 700 type telephone reduced the amount of different telephones needed as this telephone was adaptable and could be fitted with all manner of additional items.  To this end there were effectively only three styles of 700 telephones:-
Single or two button (746 only) = Tele 706 or 746
Four button = Tele 710 or 740
Wall type (could take four buttons) = Tele 711 or 741

Not all extension plans can be fitted to every type of telephone exchange and the relevant BPO diagram must be referred to for further advise.

Plug and socket effectively killed special telephones and extension plans.  Small telephone systems took the place of most of the extension plans.

Extension Plan No. 1
This arrangement provides for one main telephone and from one to six internal extension telephones connected in parallel.  Each extension is provided with a DC trembler bell, the operation of which is controlled by a press button on the main telephone.  The DC to supply these bells is line powered.  The magneto bell is only fitted at the main telephone.

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Extension Plan No. 1A
This arrangement provides for upwards to a maximum of five internal extensions in parallel with the main telephone, with the facility for incoming calls to be received at any station.  Any station calls the exchange direct and no secrecy or inter-communication is provided between any station.  A bell cut off switch is provided on each extension (all stations have a magneto bell).

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Descriptive Leaflet - 1962

Descriptive Leaflet - 1980

Extension Plan No. 1B

This arrangement is the same as Plan No. 1 but without press buttons and extension bells. It is therefore only suitable where the main bell can be heard at all extensions. i.e. lift phones.

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Extension Plan No. 1C

As Plan 1A except that the extensions have no bell on/off facility.

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Extension Plan No. 2

This plan allows the connection of 2 exchange lines to many telephones.  Each telephone has a switch that allows connection to only one line at a time.  Two telephones are designated as main telephones and these have their magneto bells permanently connected to their respective exchange lines.

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Extension Plan 2A
This plan consists of 3 to 5 exchange lines terminating on a main telephone.

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Extension Plan No. 3
The main can call an extension via a DC bell and the extension can press a button marked SECRET that disconnects the main telephone from the line, thus affording secrecy.

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Extension Plan No. 4
One telephone line provides service at several different points of the house or office.  Up to six telephones, each connected to a plug, can be provided and can be moved from socket to socket, at will. Calls can be accepted at any point.  There was always one fixed bell on this plan.

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Descriptive Leaflet - 1973

Descriptive Leaflet - 1980

Extension Plan No. 5
Older version of the Plan No. 105. A switch unit or Bellset No's 20, 39 or 44 was used to switch the line to one of two extensions.

This scheme caters for two internal extensions where communication between the telephones is only occasionally required. Incoming calls are received normally at the main instrument; either extension can be called by pressing the appropriate button and turning the handle of the generator.  When the circuit is connected to a Magneto, Central Battery Signalling No. 2 or Central Battery Signalling No. 3 exchange, the switch shown dotted replaces the four-position switch, containing the bell, generator, and indicator.  The extensions call the main by a generator ring, which is received on the bell associated with the switch.  The purpose of the indicator is to show when an extension is using the exchange line.  For night service, calls can be received on one or other of the extensions, but not both; a switch is fitted at the selected extension and, when operated, the magneto bell at that extension is connected to the exchange line, the switch at the main instrument being left in the through position.

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Extension Plan No. 5A
The lay-out of the apparatus is similar to that for Plan No. 5.  Certain modifications to the wiring are made with the object of providing secrecy facilities.

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Extension Plan No. 7
Older version of the Plan No. 107.  A switch unit or Bellset No's 20, 39 or 44 were used to switch the line to one extension.

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Extension Plan No. 7A
This is similar to Plan No. 7, but certain modifications to the wiring are made to provide secrecy.

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Extension Plan No. 8

Two exchange lines are presented to two main telephones.  One extension off of these two telephones could speak to either line by means of a switch on the phone.  The main telephones have the ability to call the extension via trembler bells.

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Extension Plan No. 8A

This version of the Plan No. 8 affords secrecy to the extension telephone.  This is achieved by the use of a switch unit that also has line busy indicators.  In this instance the extension has a magneto bell and the main telephones call the extension, one via a DC bell and the other via a DC buzzer.  Available on Auto and CB systems only.

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Extension Plan No. 9

Connected to two extension of a PBX this plan allows one telephone access to two lines.

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Extension Plan No. 10

This is only applicable to a private branch exchange extension, and provides facilities whereby the extension can be made secret against the P.B.X. when connected to the exchange line.  Each exchange line is connected through a jack at the extension, and the extension line incoming from the switchboard also terminates upon a jack.  The extension telephone terminates upon a plug and, by inserting the plug into the jack of an exchange line, the switchboard is disconnected, and the extension telephone connected direct to the exchange line.

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Extension Plan No. 11

Similar to Plan No. 1A except that each telephone can call the next extension via a DC bell.  Only the main has a magneto bell.

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Extension Plan No. 12

Similar to plan Nos. 1A and 1C except that the main has a switch that affords secrecy from the extension.

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Extension Plan No. 12A

As plan No 12 except that main and extension telephones can call each other via DC bells.

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Extension Plan Nos. 105 & 105A
Consists of a main telephone and two extensions (internal or external) connected in parallel. A Plan set No. 625 is installed as a plinth to the main telephone.  This allows more complicated switching and signalling arrangements to give the following facilities.  Internal extensions can call each other directly without intervention of the main.  There is no direct signalling between external extensions, but inter-communication between them may be established with the assistance of the main. Incoming calls are usually received at the main, but can be received at either extension if desired.

Exchange calls to or from the main cannot be heard at the extensions.  Plan No. 105A provides for secrecy for extension to exchange calls from the main. A white lamp glows on the main (Plan Set 625) when the exchange line is being used by an extension.  The main line can hold the exchange line and speak to the extension without being heard on the exchange line (a red lamp glows during this condition).  Bothway press button signalling is provided between main and extensions.

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Descriptive Leaflet

Extension Plan No's 107 & 107A
Similar to Plan No's 105 & 105A but only one extension is provided.

Descriptive Leaflet

Extension Plan 108 & 108A

Consists of a main telephone and two additional telephones.  Two exchange lines can be answered by the main telephone and then extended to the additional telephone associated with that line.

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Extension Plans off Magneto, CBS and CB Exchanges

Magneto System, Plan No. 7
The three-position switch employed is known as an inter-through switch, and there are various forms of switch which will enable the required connections to be made, but it will here suffice to describe the Ericsson pattern (Switch N.T. 14). The four springs rest on the inner contacts, and in the positions shown the exchange line is connected through to the extension circuit, the magneto bell being bridged across the lines. When the switch lever is moved over to the right, the upper left spring makes contact with the top contact, whilst at the same time the lower right spring is brought into contact with the bottom contact. In this position the exchange line is connected to the main set instrument, and the magneto bell is connected across the extension line.

If the handle of the cam be moved to the left the bell is placed across the exchange line. In addition to the ordinary function of the magneto bell, an extension of the hammer is so arranged as to release a drop shutter. This shutter is restored to normal by a projection on the switch handle. If the main station has been left unattended, the fact that a call has been received will be obvious from the fact that the shutter is dropped. The connection shown dotted is made when the shutter falls and is required if automatic clearing to the exchange is effected by earthing the centre point of the magneto bell, a system now obsolescent.

It is sometimes necessary to prevent the extension circuit from being connected to the exchange line; for this purpose Switch N.T. 15 is employed, having only two positions. Two extra springs and contacts are added to render it electrically impossible to connect the exchange line to the extension. Similar arrangements to those of Switch N.T. 14 can be made by using three pairs of 5-point jacks (Jack No. 14) and two mechanically-connected three-way circular plugs (Plug 601).

Central Battery Signalling No. 1 System, Plan No. 1
All the telephones are connected in parallel to the exchange line and incoming calls are received at the magneto bell at the main instrument, but any one of the stations can call the exchange by lifting the receiver. A call having been received at the main telephone for one of the extensions, the press button at the main set in connection with that extension is pressed, thereby ringing the trembler bell at the extension and indicating that attention is required. For example, the main instrument may be placed in the general office of a firm of solicitors or merchants, and the extensions in the private offices of the partners or managers. Since the magneto bell is normally earthed, it is necessary that it shall be disconnected when any station lifts the receiver, and this is provided by taking the connection through contacts on each of the telephone gravity switches. The extension call bells are actuated by the main set speaking battery, together with the call extension speaking battery, which are connected in series when the relative press button at the main telephone is depressed. The trembler bell and the induction coil are contained in a separate wooden case (Bell Set No. 6) when table telephones (Telephone No. 4) are used.

Central Battery Signalling No. 1 System, Plan No. 7
The four-position switch used in this plan is known as Bell Set No. 7, and the various combinations of connections are determined by the four positions of the pointer of a rotary switch.

Position 1: The main set (Telephone No. 3) is connected to the exchange line and the extension to a magneto bell in the bell set. Thus the main set can call or be called by the exchange, and the extension can call the main station.

Position 2: The main instrument is connected to the extension line, which is called by the generator provided in the bell set. The exchange line is held by the magneto bell placed across the lines (i.e. the switching over of the main set to the extension does not disconnect the exchange line), and thus does not give a clearing signal while the extension is being called by the main station.

Position 3: The main instrument is connected to the extension line and the exchange line to an earthed magneto bell for the receipt of calls from the exchange during the progress of a conversation with the extension.

Position 4. The extension is put through to the exchange, and in order to give a clearing signal to the main set on conclusion of the conversation, a 4,000 ohm eyeball indicator is bridged across the lines in series with a magneto bell. When the extension restores the receiver, thus earthing the A-wire through the magneto bell, a clearing signal is given to the exchange, and on withdrawal of the plug the relay and battery are again joined up. The current flowing through the eyeball indicator is insufficient to operate the line relay (biased against 10 mA), but operates the indicator. The main station cannot overhear the conversation, but if secrecy against the main station is not desired, the main set is placed in bridge by joining the terminals marked A and A1, and B and B1 respectively. The night switch is for the purpose of disconnecting the eyeball signal, and, consequently, the local alarm circuit, when the extension is left connected to the exchange line during periods for which the main instrument is unattended.

Central Battery Signalling No. 1 System, Plan No. 4
The circuit arrangement is similar to that described for Central Battery circuits (see page 331), using Jacks No. 13, Plug No. 407, and Telephone No. 90 or Jacks No. 9, Plug 701, and Telephone No. 4.

Central Battery Signalling Nos. 2 and 3 Systems Plan No. 4
The arrangement of the plug and jacks is shown in (diagram) 235. The portable telephone is a Telephone No. 4 connected by means of a Cord Instrument No. 422 to a Plug No. 404. Jacks No. 21 are used at the extension positions and a Bell Set No. 5 is permanently connected to the first position. The magneto bell shown dotted may be omitted at any position from which the magneto bell in the Bell Set No. 5 is audible; when this is done, springs 1 and 4A on the jack are connected together. When the instrument plug is inserted in any jack, the connection between spring 4 and spring 4A is broken, so connecting the magneto bell at that position in circuit, in addition to the magneto bell of the bell set. (diagram) 236 illustrates the connection made when the plug is in a jack at an intermediate or last position.

Central Battery System, Plan No. 1
A Telephone No. 1 is used at the main set, and the extensions, which in (diagram) 237 are table instruments, use the induction coil and condenser of the main set. The bells for gaining the attention of the extensions are joined across the A and B wires, when the press buttons are operated and are rung by the exchange central battery.

If one of the extension telephones is of the wall type, the magneto bell is disconnected and it is connected in parallel across the exchange lines. In this case the extension uses its own induction coil and condenser and does not make use of that at the main set, as is the case of table instruments. The apparatus required is:

  • Main set: Telephone No. 1 or equivalent (two press buttons).
  • Extension 1: Telephone No. 1 or equivalent (magneto bell disconnected), Bell 13A or 56A.
  • Extension 2: Telephone No. 2 or equivalent (uses main set induction coil and condenser), Bell 13A or 56A.

Central Battery System, Plan No. 2
It is often desired that the extension station may speak without the possibility of the conversation being overheard at the main set located, say, in the general office. The principle of the arrangement consists in placing both telephones in parallel on the exchange line through a six, point, two-position switch, which, when moved to the secrecy position, cuts off the main instrument. Both instruments are provided with their own induction coil and condenser, and since failure to restore the secrecy switch prevents calls from being received at the main set, the switch in this position joins up the magneto bell in the extension bell set, so that a ring will be received on this instrument. The apparatus required is:

  • Main set: Telephone No. 1 (press button).
  • Extension: Telephone No. 2, Bell Set No. 1, Bell 13A or 56A, Switch six-point two position.

Central Battery System, Plan No. 4
To avoid the use of extension circuits and switches, a table telephone (No. 2) is connected to a Plug 407, and wall sockets are fixed in the various rooms where it is required to use the telephone (e.g. dining-room, study, and bedroom). A Bell Set No. 1 and a socket (Jack No. 13) is fixed in the dining-room, and in each of the other rooms a socket and a magneto bell. These bells are, however, short-circuited until the plug is inserted. Thus, the instrument may be carried to and connected in whichever of the rooms the convenience of the moment dictates. The arrangement will be readily appreciated from a consideration of (diagram) 239. A later arrangement consists in the use of Plugs 404 and Jacks No. 3 and 21.

Central Battery System, Plan No. 7
The arrangements previously described do not provide for intercommunication between the extensions and, where this facility is required, recourse must be had to switchboards. It is, however, possible to provide intercommunication between the main set and one extension by the aid of Bell Set No. 4, The conditions which the arrangement must fulfil are:

  • Main set connected to exchange. Extension connected on extension bell at main set.
  • Main set connected to extension. Exchange connected on extension bell.
  • Main set connected to extension. Exchange line held.
  • Exchange line connected to extension.

These conditions are effected in the four positions of the switch on Bell Set 4, and are shown in skeleton in (diagram)s. 241-244. The Bell Set No. 4 ((diagram) 240) comprises a 4-position switch, indicator 1,000A, generator No. 4, 1,000 ohm magneto bell, 1F condenser, and a 2-cell battery. The main instrument may be a Telephone No. 1, but at the extension set a generator is required for calling the main set, and therefore the apparatus fitted consists of a Telephone No. 1 or No. 2, with Bell Set No. 1, the line circuit passing through the back contacts of a generator No. 4, so that when the generator is used the extension telephone is disconnected. The switch pointer on the bell set normally remains at position 1, and thus a ring from the exchange is received on the bell of the main set. Raising the receiver extinguishes the supervisory lamp at the exchange in the ordinary way. The extension is connected to the extension bell, and thus provision is made at the main set for the receipt of a call from the extension. The extension is called by the generator fitted in the bell set.

In position 2, the positions of the main set telephone and the extension bell, with its condenser, are reversed; but, in addition, a 2-cell battery is placed in the circuit to the extension to furnish speaking current between the main and extension. Calling, in both directions, is by generator. A ring from the exchange is now received on the extension bell.

Position 3 holds the exchange line while speaking to the extension. The necessity for this condition may arise when the main set receives an inquiry which necessitates consultation with the extension before replying. The only change, as compared with Position 2, is that one coil of the bell set magneto bell is joined across the exchange line, thus preventing the lighting of the supervisory lamp at the exchange. Position 4 puts the extension through to the exchange, but inserts an eyeball indicator in the main circuit. At the conclusion of the conversation, when the extension replaces his receiver it disconnects the circuit, and thus the indicator, which has been operated while the call was in progress, falls back to normal. This is known as a negative clearing signal, to distinguish it from a positive clear, which may be defined as a signal which is operated to give the clearing indication. Failure to restore the switch to normal involves the receipt of a subsequent ring from the exchange on the bell set and on the bell at the extension set. On the Bell Set No. 4, links are provided so that the main set may, in Position 4, be connected in parallel across the circuit while the extension is speaking to the exchange, or, by their removal, secrecy of conversation between the extension and the exchange is secured.

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Last revised: January 19, 2022