No. 50 PAGE No. 2

A Modern House Exchange
G. STRONG - Circuit Development Engineering Department
D. W. ROBERTS - Apparatus Engineering Department

The House Exchange described has been designed to augment the existing range of key-telephone systems. It is particularly suitable for use in businesses where the calling rate between stations is high and where there is an immediate ~r future need for external stations or tie lines.

Key telephones are becoming almost universal in every-day communications and form the basis of a wide variety of systems developed to meet the different requirements of private telephone installations. As a useful addition to the available range, a modern House Exchange has been introduced primarily for use in businesses and large private residences where the demand exists for a reliable system offering a high internal/external traffic ratio in addition to a comprehensive range of facilities.
Designed in conjunction with the B.P.O., it employs press-key control throughout and basically provides access for a maximum of 11 telephone stations to 2 exchange lines, and communication between stations.

Although essentially an internal system with stations interconnected by multiple cable, it includes provision for 2-wire (non-multiple) stations and tie-lines in lieu of up to four internal (multiple) stations. This aspect of the system is of considerable significance to the expanding business with requirements for progressive and economical extension of telephone service to remote locations within the organization, or the integration of the House Exchange with an identical or similar system.

Figure 1
Multiple station telephone with associated wall mounting terminal block


The telephones employed are of two types; multiple and non-multiple, as illustrated in Figures 1 and 2. Both present a neat and pleasing appearance approved by the Council of Industrial Design and are equipped with identical high-performance transmission circuits. The light-weight handsets and the instrument covers are moulded in a tough ABS copolymer and either telephone can be produced in a wide range of light-fast colours compatible with modern furnishing schemes. Other main features include convenient press-key array, simple lamp signalling (exclusive to multiple-station telephones) and straightforward internal layout, with easy access to all parts.

Figure 2
Non-multiple station telephone
Figure 3
The exchange line relayset with cover removed

Relay Sets
A single Exchange Line Relay Set serves all stations. It accommodates answering and signalling apparatus and includes also a transistor device for current regulation of the station signalling lamps. With this device, maximum lamp life is ensured irrespective of the number of station telephones initially installed. The relay set (see Figure 3) has overall dimensions 12.1” long, 7.25” high and 8.75” deep (308 : 186 : 222 mm) and is suitable for table or wall mounting.

Relay sets of similar size are employed for tie-lines and non-multiple stations and are provided on a one-per-circuit basis.

Auxiliary Units
One of the multiple stations (selected as ‘The Main’) is equipped with a small desk-mounted auxiliary unit when non-multiple stations and/or tie lines are installed. This unit, equipped with lamps, buzzer and double-throw miniature switchboard keys, is used by the main-station attendant for the routing of certain calls via the tie-lines or to and from non-multiple stations. It is available in two sizes, catering for a single station or maximum of four (see Figure 4).

Figure 4
Auxiliary unit for up to 4 non-multiple stations and/or tie lines

In a mixed-station installation, any other multiple station can be equipped with an identical auxiliary unit to provide 2nd-choice main-station working. With this arrangement, the functions of the 1st main station can be transferred to its subsidiary by operation of a changeover key in the auxiliary unit at the 1st main station.

Operation of the system is from a 50V d.c. supply derived either from normal storage batteries or a mains-operated power unit.


Although the main station is usually responsible for answering duties and the routing of incoming calls to wanted stations, any multiple station can answer an incoming call if desired. This general-answering feature is of particular significance at installations where there are no non-multiple stations or tie-lines, since its adoption dispenses with the need of an attendant for centralized answering duties.

Outgoing exchange calls, on the other hand, can be established direct from both multiple and non-multiple stations, or any of these stations may be restricted access to the exchange via the main station during day service and allowed direct access at night. At these restricted stations the telephones are equipped with a ‘restrict access‘ miniature relay unit (one per exchange line) directly controlled from the main-station instrument.

All stations, restricted or not, can hold an exchange call for the purpose of making an enquiry call on other lines, and subsequently return to the exchange-line conversation. In addition, an exchange call can be held pending its transfer direct to any unrestricted station or indirectly to a restricted station via the attendant. Both incoming and outgoing calls can be repeatedly transferred among the stations.

Audible indication of incoming exchange calls is given by the instrument bell at any required number of multiple stations within the loading limits laid down by an Administration (normally four). For flexibility of arrangement, each telephone incorporates bell ON/OFF keys which can be made operative by a strapping adjustment. At one selected station the exchange bell is permanently in circuit.

Series-secrecy is provided on exchange calls. This means that the first station in the multiple, when originating or receiving calls automatically disconnects succeeding stations from the seized line. Similarly, the next station in the multiple excludes stations of a lower priority, and so on.

The heart of the system is the intercom circuit. It allows up to five two-way conversations to be held simultaneously, the maximum that can be provided in an equipment of this capacity. This is made possible by the provision of separate 2-wire transmission feeds for each station, the associated current-feed coil itself being contained within the instrument.
Calls between stations are non-secret. This feature, while presenting no inconvenience to most organizations, has the inherent advantage of simplicity which is exploited in the system design to provide a straightforward conference facility, enabling any multiple station to set up a call among a group of stations by sequential keying.

Intercom signalling at multiple stations is by the instrument d.c. buzzer and, at external stations, by the instrument a.c. bell which also serves to signal exchange calls under night-service conditions.

An interesting aspect of the multiple-station telephone is its use of printed wiring. Figure 5 shows the printed wiring of the basic transmission circuit on a synthetic resin panel. The components shown, i.e. the capacitor, resistors and induction coil, are soldered direct to this panel which also provides a convenient mounting for the jack-in line regulator and the ringer, both independently removable.

Figure 5
Synthetic resin panel carrying transmission components of multiple station telephone

An additional printed-wiring panel is used in the intercom-key assembly to provide an unusual contact operation when an intercom key is depressed to signal the distant station. The panel, secured at the rear of the main-chassis bracket by four screws, accommodates 10 pairs of contact springs (one pair per key), individually clamped at one end by metal eyelets. Mounted at the free end of each spring is a moulded buffer which is engaged by the key plunger when fully depressed, thus moving the springs into contact with the printed wiring. With this arrangement, the need of conventional pre-tensioned ‘make‘ springsets is avoided and no spring tensioning is required, since adequate contact pressure is exerted by the forward projection of the plungers.

At multiple stations, visual exchange-line signalling and supervision are provided by two pairs of ‘Exchange‘ and ’Seize‘ lamps. These are mounted beneath clear lenses integrated with a removable main key-switching panel (see Figure 6) in the sloping upper front of the instrument case. Each pair is associated with an exchange-line key and individually arranged in corresponding left and right-hand corners of the panel.

The seize lamp provides a steady-glow signal at the station occupying the related exchange line, whereas the particular exchange lamp emits three indications at all multiple stations:-

  1. A pulsating signal at ringing periodicity until the call is answered.
  2. A steady glow during an established exchange call.
  3. A ’wink‘ when a station holds an exchange line while making a call on another line. This signal becomes steady when the user returns to the held line.

The keys in the panel number l5. Ten of these are allocated for intercom lines and, to assist identification, their press-buttons contrast in colour with the remainder. The intercom keys are arranged in two horizontal groups of five between adjacent designation strips carrying double-sided slip-in labels. To the left of these keys are the exchange and associated exchange-release keys and, at the extreme right of the panel, is the single conference key.

Ancillary operations are provided by keys between the panel and the handset cradle. The bell ON/OFF keys at each station take up the two inner positions, with snap-in dummy buttons either side. These dummy buttons are removable and can be replaced by ’Bar 1’ and ’Bar 2‘ keys in the main station telephone for the control of stations restricted exchange-line access. At any other station these dummy positions may be occupied by ’Mon 1‘ and ’Mon 2‘ keys for monitoring.

There are only two keys associated with external station telephones; ‘Extension’ and ‘Exchange’. External stations have no lamp signalling because of their connection by 2-wire lines.


Between Multiple Stations

Calls between multiple stations are signalled by buzzer on momentary over-press of the intercom key beyond its normal locking position.

At the called station the lifting of the handset completes the through connection and the call proceeds in the normal manner, with the intercom key at the calling station remaining in the locked position until the handset is replaced at the termination of the call or any other key is operated.

If the wanted station is engaged on an exchange call, either of two conditions is given during the over-press of the calling intercom key, dependent upon strapping arrangements in the equipment:-
(a) Buzzer signal at the called station; usually employed at stations where internal calls have high priority.
(b) Buzzer signal at the calling station, serving as a busy indication.

From Multiple to Non-Multiple Station
A non-multiple station is called in the same way as for a multiple-to-multiple station call. In this instance, however, a static generator in the non-multiple station relay set is activated to extend ringing current to the distant station’s a.c. bell. Connection between the two stations is established when the called party lifts the handset and presses the locking extension key.

From Non-Multiple to Multiple Station
All internal calls from non-multiple stations are established via the main. As the extension key is pressed at the non-multiple station, a visual signal appears at the main-station auxiliary unit on a combined calling/supervisory lamp, particular to the calling line. At the same time the auxiliary-unit buzzer sounds continuously. Both indications continue to be given until the main station answers but, if the attendant is occupied with other calls, the audible signal may be disconnected by the ’Alarm off’ key.

After answering, the attendant proceeds to signal the required station. On reply, the called station is requested to operate the relevant key to establish connection with the waiting station.

Between Non-Multiple Stations
When more than one non-multiple station is installed, calls between these stations are also set up with main-station assistance. The attendant identifies the calling station by its glowing call-lamp and, after speaking to the caller, establishes connection with the wanted station from the main-station telephone. Subsequently, the attendant ’throws‘ the connect key associated with each station in the auxiliary unit to complete the talking path between the stations. On termination of the call, both lamps light, whereupon the connect keys are restored to clear down the connection.

Among Stations (Conference Call)
Calls for the purpose of joint consultation among station users, can be set up from any multiple station by first calling the stations individually. The conference key is then depressed, followed by all selected station keys. Replacement of the handset at the originating station restores all keys to normal.

Figure 6
The main key-switching panel removed from multiple-station telephone

Multiple Stations

An incoming exchange call is indicated by the appropriate visual signal at all stations and by the sounding of the instrument bell where connected. The call, normally answered by the main station, is accepted by lifting the handset and pressing the appropriate exchange key. At this stage, the audible signal is disconnected and all associated exchange lamps glow steadily. Simultaneously, an identical signal appears on the station-seize lamp in the answering-station telephone (see ’Transfer‘ multipleto-multiple station).

Outgoing calls from unrestricted stations are originated by employing the same key procedure as for an incoming call; corresponding lamp signals are given on seizure of a free line and during progress of the call.

If a call is to be allowed to a restricted station by the main station, the attendant informs the caller of the particular line to be seized, and momentarily presses the bar-key of the allocated line. The restricted station, in turn, presses the appropriate exchange key. Owing to operation of the bar-key, the add-on relay in the restricted station’s telephone actuates to give access to the exchange line. The act of seizing the exchange line causes the relay to lock in the operated position and remain held until the restricted station terminates the call.

Non-Multiple Stations
Calls from a non-multiple station are initiated in a similar manner to multiple-to-exchange calls. If the station is unrestricted, the momentary depression of the non-locking exchange key causes the adjacent extension key to operate and lock. Because of these combined key functions, relays are operated in the non-multiple station relay set to switch the calling station to line and connect the steady-glow signal to all multiple stations.

Incoming exchange calls during day and night service are described later, under respective headings ’Transfer’ and ’Night Service’.

If a station speaking over an exchange line desires to make an enquiry call on another line, either to a local station or via the other exchange line, the relevant intercom or exchange key is operated. Either action restores the exchange key originally operated to the hold position and applies a hold condition to the exchange line. Simultaneously, the wink signal appears at the corresponding exchange lamp. After completion of the enquiry call, the exchange conversation is resumed by a second depression of the restored exchange key.

When an enquiry call is made over an exchange line connected to a distant operator-attended telephone installation, the operator can be re-called by over-pressing the exchange key from its locking position.

Multiple-to-Multiple Station

After the wanted station operates the relevant exchange key, the lamp signal reverts from wink to steady, thus indicating to the transferring station to complete the transfer.

Should the transferring station be ahead of the wanted station in the exchange-line multiple and fail to replace the handset, the exchange line will remain in the held condition, so preventing transfer. As a reminder of this, the station-seize lamp at the leading station continues to glow until the handset is replaced to complete transfer of the call.

If the main or any other unrestricted station is talking on one line when a call comes in on the second line, the station may accept the call by depressing the second exchange key, thus restoring the first exchange key to the hold position. This call may then be transferred to another station, the called party being requested to pick up the second exchange line. When the usual lamp-signal change occurs, i.e. from ’wink’ to ‘steady’, the transferring station operates the release-exchange key associated with the second line to restore the relative exchange key and allow the called station to take over the call. This additional key operation by the transferring station is necessary in this instance, since normal completion of transfer by replacement of the handset would result in loss of the call on the first exchange line. Meanwhile, the first exchange key remains in the hold position and, by again fully depressing this key, the transferring station may resume conversation on the exchange line.

To and From Non-Multiple Stations
During day service, incoming calls for non-multiple stations, whether barred or not, are dealt with at the main station. The non-multiple station, after acknowledging the calling signal from the main, takes over the incoming call when the appropriate test and transfer keys are consecutively operated in the main-station auxiliary unit.

With a call established on an exchange line, a non-multiple station may re-press the non-locking exchange key to hold the line and simultaneously call the main station. On reply, the main station can be requested to signal the wanted station for the purpose of setting up an enquiry call or to take over the call. Alternatively, the external station can revert to the exchange line by re-pressing the exchange key a third time.

If an exchange call is accepted from a non-multiple station when the other exchange line is also in the held condition (i.e. both lamps winking) the attendant first operates the auxiliary-unit test key particular to the calling external station. This action causes the appropriate exchange-line ‘test‘ lamp to light in the unit, thus identifying the line to be picked up.

Outgoing tie-line calls from stations are established similarly to station-to-station calls. Incoming calls on the other hand are dealt with at the main station and signalled by lamp and buzzer in the auxiliary unit, call-transfer being accomplished in the same manner as for non-multiple/multiple station calls.

The possible absence of through clearing facilities at the distant installation is catered for by a clearing signal that appears on the auxiliary unit on replacement of the handset at the called station. On sight of this signal the attendant breaks down the connection by momentary operation of the tie-line release key.

Incoming exchange calls to the H.E. system can be extended over the tie-lines by a transfer key in the auxiliary unit. This same key is also used for the transfer of outgoing calls originated by the main station on behalf of the distant tie-line caller.

For connection of tie-lines to non-multiple station, non-multiple/non-multiple procedure applies.

Since all stations with full facilities have direct access to exchange lines and common signalling of exchange calls is provided, night service is automatic at these stations.

When non-multiple stations or certain tie-line circuits are installed, night service connection is by the ‘throwing’ of keys in the auxiliary unit. By their operation, incoming ringing is repeated to connected stations by the transistor generator in the appropriate relay set (i.e. non-multiple or tie-line), this signal being concurrent with the audible and visual signals given at all multiple stations.

Any or all of the non-multiple stations or tie-lines may be allowed incoming night-service calls on either or both exchange lines by simple re-arrangement of wire straps in the equipment.

When a monitoring station wishes to listen into an exchange call, the appropriate monitor key is operated to extend the station to the beginning of the exchange line multiple.

Figure 7
Typical installation catering for 2 public exchange lines, 8 multiple stations, 1 non-multiple station and 1 tie line


Connections between multiple stations are made at the cabling terminal block associated with each instrument, and require a total of 41 conductors. The cable employed is tapped in to all intermediate stations, terminating finally in the Exchange Line Relay Set.

At installations fully equipped for four non-multiple stations, an additional 39 wires are required. These are routed between the auxiliary unit and the corresponding relay set, seven of the wires being particular to each relay set.

Tie-lines are connected in a similar manner to external 2-wire lines.

Figure 7 shows a typical 2 + 10 installation.

The system is suitable for connection to exchange lines of up to 1000-ohm loop resistance. When a non-multiple station is installed, the combined resistance of the exchange and the 2-wire line must not exceed this figure.

For maximum local-signalling performance between multiple stations it is recommended that the total length of multiple should not exceed 440 yards using 6.5 lb/mile cable.

The new H.E. system will undoubtedly fulfil an urgent need in the medium-size organization when it is important to reduce operating attendance to the minimum. This is a significant aspect of the system, but perhaps its most striking is its ability to provide flexible communication services for businesses of diverse geographical layout.

Click here for the GPO House Exchange System No. 4


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Last revised: July 29, 2003