HES No. 3

Introduced around 1963

HES 3 diagram (ZIP)

HES 3 circuit diagrams (PDF)

Descriptive Leaflet - 1965

Descriptive Leaflet - 1980

3 Internal
Issue 2, Aug 1973


This Instruction describes the facilities provided in a House Exchange System No. 3 (HES No. 3) which has been given the sales name of 'Keymaster'. This system provides intercommunication between a maximum of five stations with direct access from these stations to an exchange line.


  • Multiple station A multiple station is one equipped with a Telephone, Intercom, No. 3/1 (F or CB) which is a 700 type instrument. Multiple stations are linked by multiple cable and are normally confined to one building but the multiple may be extended to link stations in a nearby building if signalling limits are not exceeded.

  • Non-multiple extension This is an extension station which is equipped with a Telephone No. 710 (F or CB); it is connected to the multiple by a two-wire line and a relay-unit.

  • Main station This is the multiple station where incoming exchange calls always give an audible signal, and which assists the non-multiple extension with calls to other stations. The main station also controls soon of the additional facilities which may be added to the system.

A HES No. 3 comprises one exchange line and up to five stations. One non-multiple extension may be connected in place of the last multiple station, and stations may also be replaced by private circuits to another house exchange system or a PBX. The system also includes one or two relay-units and a Power-unit No. 51A; all these units are wall-mounted.

The HES No. 3 works on CB principles but may be connected to any type of public exchange. In LB areas an auxiliary unit must be fitted at the exchange to provide the required CB conditions.

The HES No. 3 is unsuitable for shared service.

Exchange service is available from any station including the non-multiple extension. Access to the exchange line is by a locking press-button with gravity switch restoration. Exchange calls are secret. Calls may be held by any station and transferred to any other station.

Incoming exchange calls normally ring a standard magneto bell mounted inside the Telephone, Intercom, No. 3/1 at each multiple station but they do not ring a bell at the non-multiple extension except for night service. Each Telephone, Intercom, No. 3/1 is fitted with a bell cut-off key. This key is made inoperative at the main station to ensure that at least one bell of the installation can be rung by incoming exchange calls. If the exchange ringing supply is sub-standard, e.g. from a vibrator, it may not be possible to ring all bells simultaneously and one or two may have to be permanently short-circuited.

A multiple station can call any other station by means of a non-locking press-button. The non-multiple extension cannot call multiple stations directly and relies on the main station to signal the required station.

Buzzer tone is returned to the non-multiple extension as an indication that the main station is being called. Intercom calls are signalled at multiple stations by buzzer and at the non-multiple extension by magneto bell.

The intercom speech circuit is common to all stations, thus intercom calls are not secret. At multiple stations automatic connection to the intercom circuit is made when the handset is lifted but at the non-multiple extension the EXTN button must also be pressed.

All stations can be called in to a local conference by signalling each in turn. Conference facilities are not available on exchange calls.

Telephones, Intercom, No. 3/1 are fitted with two MES bulbs. A red lamp signal indicates that the exchange line is engaged and warns other stations not to operate the exchange button as this interrupts the exchange line circuit. This lamp glows in rhythm with the ringing current on incoming exchange calls but is not intended to be a general calling signal. This lamp also flickers when an exchange call is held, e.g. while arranging for the transfer of the call to another station.

A white lamp signal is given when the intercom circuit is in use to warn stations wishing to use this circuit not to interrupt a call in progress unnecessarily.

A multiple station which attempts to call an engaged station operates its own buzzer.

The non-multiple extension may be given night service on incoming exchange calls by means of switching provided at the main station. Under this condition incoming exchange calls will ring the non-multiple extension bell as well as the bells at multiple stations but, to ensure that the number of bells does not exceed that which can be rung by the exchange, it is desirable to cut off bells which are not required.

Outgoing exchange service continues to be available at all stations and intercom calls are not affected by night service conditions.

A HES No. 3 installation may be associated with a PBX in several ways.

  • An extension from a PBX may be terminated on the exchange line connection at the HES No. 3 installation. All the facilities available to PBX extensions, including operator recall, are also available to HES No. 3 stations.

  • Subject to satisfactory transmission, the HES No. 3 non-multiple extension may be terminated as an exchange line or private circuit on the distant switchboard. Exchange calls cannot be extended in either direction.

  • A Relay-unit Q415, fitted in place of a multiple station, will usually provide all normal inter-switchboard facilities between a HES No. 3 and a PBX.

Individual PBX extensions may be-terminated on the HES No. 3 so that each station there has an exclusive line to the PBX in place of direct exchange line access. Local intercom, calls can be made between stations , but access to exchange service or other PBX extensions must be obtained via the PBX. Transfer of exchange calls must be made via the PBX.


  • The non-multiple extension of the HES No. 3 may be connected to an exchange line position of HES Nos. 1, 2 or 4. Exchange calls cannot be extended in either direction.

  • The non-multiple extension of HES Nos. 1 or 2 may be connected as the exchange line of the HES No. 3. Subject to satisfactory transmission and signalling, exchange calls can be extended to the HES No. 3.

  • Using a suitable Relay Unit in place of a multiple station the HES No. 3 may be connected to an exchange line or non-multiple position of an HES No. 1 or 2; use of the latter position allows exchange calls to be extended to the HES No. 3.

  • Two HES No. 3 systems can be connected together by a private circuit using Relay-.units Q415 at each installation in place of a multiple station. Exchange calls can be extended in one direction only.

  • An HES No. 3 installation may be connected using suitable relay units in place of multiple stations to an HES No. 4 installation. Exchange calls can be extended in one direction only.

Plan arrangements similar to Extension Plans 1... and 107 can be provided at the non-multiple extension. Extension plans cannot be provided at multiple stations.

Extension a.c. bells may be connected in addition to or in place of. the existing magneto bell at all stations. Extension d.c. bells or buzzers may be connected to the intercom signalling buzzer at multiple stations.

Loud-sounding bells may also be provided.

A Meter No. 19 can be fitted to the exchange line but if the HES No. 3 is a subsidiary to a PBX equipped with operator recall, a Filter, Frequency, No. 144A must also be fitted in series with the signalling earth in the HES No. 3.

Optional trunk barring by means of Lock No. 29A can be provided at any  station. Permanent trunk barring can be fitted provided that access to the STD network is always available from one station.

Exchange service may be permanently barred to certain stations if required. Exchange service may also be restricted at certain stations so that either incoming or outgoing calls can only be connected at the discretion of the main station.

Although exchange calls are normally secret, one multiple station may be given the facility of monitoring exchange calls; this facility degrades transmission on the exchange line and its use is restricted to one station only.

Handset No. 4 may be provided, when required at multiple and non-multiple stations.

The HES No. 3 derives its 50V operating supply from Power Unit 51A which is mains operated; secondary cells are not required. If mains power fails, or is switched off, bothway exchange service is available at all stations but intercom service, lamp signals and the monitoring facility are not. Under these circumstances restricted service stations cannot be given access to the exchange line.

The following are applicable:-

C3 C3032 - Description
C3 C3033 - Circuit Operation C3 C3034 - Installation

Formerly EI Telephones, Stations , Q1011

3 Internal
Issue 2, Sept 1973

Description of Equipment

This Instruction describes the various items of equipment used on the House Exchange Systems (HES) Nos. 3 and 4. The definitions of terms and facilities for the HES No. 3 are described in C3030 and for the HES No. 4 in C3031. Installation of the HES No. 3 is described in C3034 and the HES No. 4 in C3036.

2 CONTENTS Paragraph

This telephone is used at a HES No. 3 installation which caters for one exchange line and up to five stations. The telephone is basically a Telephone No. 710 and has a cover (Part 6/DCO/627)which is provided with two lenses; it is fitted with a self-locking BELL OFF button in position A, an EXCH button in position B and two split buttons in each of positions C and D.

The moulded base (a standard item) is fitted with a special gravity switch and has been coded Part 4/DBA/107.

Fitted between the gravity-switch pillars in a sub-assembly known as a Switch Composite, No. 1A, which consists of a spring contact unit wired to a Strip, Connection, No. 155B, the assembly having seven flexible spade-ended leads for connection to the telephone circuit. The contact unit is held in position between the gravity-switch pillars by four screws and the connection strip is held at the rear of the pillars by two locating pins and a spring clip.

The plungers and spring contacts located in positions C and D of the latch- plate assembly form the intercom signalling circuits. The spring contacts and plunger in position B are contacts KX of the exchange line circuit, and these may be released either by replacing the handset or by pressing any one of the four buttons in positions C and D. The spring contacts in position A which are adjacent to position B, are contacts KH and these also operate when contacts KX are operated, but they can only be released by replacing the handset; these form the hold circuit of the exchange line. The remaining spring contacts in position A control the magneto bell cut-off or additional facilities. These springs are operated by a special plunger which is fixed to the latch plate by a bracket. The spring assembly is illustrated in Diagram Q 422 and the button arrangement is shown in Diagram Q 424.

Fixed on the sides of the gravity-switch pillars are two Lamp-fittings, No. 16 (incomplete); viewed from the front of the telephone, the fitting on the left-hand side contains a Lamp No. 26H which provides the red lamp signals for the exchange line, whilst in the fitting on the right-hand side is a Lamp No. 26i which provides the engaged signal for the intercom circuit.

The d.c. buzzer for intercom calling is fitted onto the cord grommet frame and the complete assembly has been coded Buzzer No. 2B-2.

The telephone is fitted with a Cord, Inst, No. 20/03AJ, ..., 72 in which is terminated with the multiple cables onto a Block, Terminal, No. 37.

The telephone is normally fitted with a Handset No. 3. The CB telephone has a Dial, Auto, Dummy, No. 6A, The complete circuit of the telephone is shown in Diagram Q 422 and the wiring of the Switch, Composite, No. 1A only in Diagram Q 423. The instrument cord and cable connections for the telephone are shown in Diagram Q 424.

The telephone is available in black, grey and ivory.


This telephone is used at a HES No. 4 installation which caters for one or two exchange lines and up to ten stations (exceptionally eleven stations).

The telephone mechanism is mounted on a metal base and is enclosed by a moulded plastic cover (Part 1/DCO/683).

The telephone when issued is fitted with a Handset No. 3 and a Connector No. 1046A. The overall dimensions of the telephone are approximately 11.25 in (285 mm) x 6.75 in (170 mm) x 4.75 in (120 mm) high.

Above the front sloping face of the cover containing the dial aperture, is an almost horizontal face in which is a rectangular aperture for the press-buttons of the key unit. Over the rectangular aperture and surrounding the press-buttons is a separate escutcheon which is sprung into place and clips onto the metal face of the key unit. This escutcheon contains the station identification labels with their clear plastic covers and the lenses for the exchange line lamps. The labels and plastic covers are removed by pressing at the end of the plastic cover with the blunt end of a pencil or similar object; this causes the middle to bow upwards where the cover and label may be gripped and pulled gently upwards. If the label does not rise with the cover it may be eased up by inserting a pin at the middle by the side of the label and levering gently upwards. The escutcheon is removed by inserting a thin blunt edged blade, or finger tip under the chamfered edge at the back and springing off the rear clip, then lifting upwards to release the front spring clip. In front of the handset rest there is provision for four press-buttons which are used to control the bell cut-off facility and any additional facilities that may be provided.

The telephone cover is fixed by two screws in the handset rest and by a screw in the front of the dial mounting.

The press-buttons are in two rows, from left to right there are the two ivory exchange line buttons engraved 1 or 2, two smaller ivory exchange line release buttons engraved R; ten grey press-buttons for signalling and speaking

to other stations over the intercom circuits, and the single ivory conference button engraved C.

At the front of the base are two bell gongs, over which Relay-units Q 527 and Q 528 (see para 18 and 19) may be mounted as required. The magneto bell is a Bell No. 59A with long flexible leads and has been coded Bell No. 59A-2.

Behind the bell and across the base 'is a printed circuit board with the components of a standard 700-type telephone transmission circuit.

The key unit is in two parts; a metal frame carrying spring-sets for the exchange lines and intercom circuits, and a press-button assembly.

The metal frame screwed to the base of the telephone, carried on each side, the spring assembly of an exchange line and a printed board with spring contacts riveted to it, for intercom signalling and speaking.

At the end of the frame near the exchange line spring contacts is a plunger assembly which operates on second and subsequent overpresses of an exchange press-button. The plunger operates a spring-set to provide operator-recall facility to either exchange line circuits. A second spring assembly inside the frame operates when the station press-buttons are overpressed and provides the intercom signalling condition.

At the end of the frame near the conference button are a number of `polytags' carrying resistors associated with lamp circuits etc.

On the top, near each of the four corners of the frame, is a lug which carries a Lamp-fitting No. 19 and a Lamp No. 41E. The press-button mechanism is held in the frame by a countersunk screw at each corner of the top plate.

On the left-hand side of the base, beneath the dial mounting, is a metal bracket which carries a d.c. buzzer (Buzzer No. 2B-3) for intercom calling.

Behind the key unit frame is a 72-way double-sided terminal block. On the top side is the telephone wiring; both soldered and screwed terminations. The instrument cord, a Connector No. 1046A, and any straps that may be required are terminated on the underside and these terminals are exposed by removing a cover plate from the base.

Over the cord entries at the rear of the base is a metal bracket which carries the intercom transmission feed coil and its associated resistors and capacitor.

The telephone is connected to the multiple cabling via a Box, Connection, No. 6A. The circuit and connector connections of the telephone are shown in Diagram Q 540.

The telephone is available in black, grey and ivory.

The weight of the complete instrument is 81 lb.

5 BLOCK, TERMINAL, No. 37...
The Block, Terminal, No. 37... is used at HES No. 3 installations to terminate the multiple cables and Telephone, Intercom, No.3/1 at multiple stations. It is also used with a Control-unit Q 535 (see par 21) at a HES No. 4 installation.

The Block Terminal, No. 37... consists of a moulded plastic base (Part 1/DBA/99) on which is mounted a terminal block (Part 1/DBL/91), containing 25 double-sided screw terminations the tablet and base being enclosed by a moulded plastic cover (Part 1/DCO/62.5). At one end of the cover is a cut-out for cord entry and at the other end are three knockouts for cable entry.

The base has been modified to include a hole for back entry of cable and fixing holes for mounting the Block, Terminal, No. 37... over square type conduit outlet boxes, i.e. standard outlet boxes with 2.75 in (70 mm) fixing centres.

The Box, Connection, No. 6A has a base area of approximately 6.25 in (160 mm) x 5.75 in (145 mm) and is nearly 2 in (50 mm) high.

Mounted on the metal base is a Connector No. 203B55B,, which is a 55-pin plug. Also fixed to the base on raised pillars are two 28-way terminal blocks (Part 1/DST/861). The inserts of the terminal blocks have screw terminals for multiple cabling on the front side, and solder terminals on the rear side which are wired to the pins of the Connector No. 203B55B. At the side of the connector is a bracket to retain the grommet of the telephone instrument cord (Connector No. 1046A). Also in the base is a hole for back entry of cables.

The grey moulded plastic cover is held on by two fixing screws, and the sides contain knockouts for cable entry and a cut-out for the cord entry. Details of the wiring between the connector and terminal tablets are given in Diagram Q(L) 510.

The Box, Connection, No. 7A is identical in size and shape to the Box, Connection, No. 6A. The metal base of the Box, Connection, No. 7A provides mounting on raised pillars for three 28-way double-sided terminal tablets (Part 1/DST/861). These tablets have screw terminals on front and rear sides. There is no provision in the base for back entry of cables. The sides of the moulded plastic cover contain knockouts for cable entry. Typical uses of the Box, Connection No. 7A are shown in Diagram Q 508.

Two sizes of metal case have been introduced for use with HES No. 3 and No. 4.

Relay-unit Q 405 is contained in a Case No. 131... which has a mounting surface of 12 ins (305 m) x 4.5 ins (110 mm). All other wall-mounted units are contained in Case No. 133... which has a mounting surface of 12 in (305 mm) x 8.75 in (220 m). Both cases extend 7.25 in (185 mm) from the mounting surface.

Circuit components are mounted on a metal plate which is hinged to the baseplate and the wiring form from the components is terminated on a connection strip, which is fixed to the baseplate. Cable entry is via a hole fitted with a grommet in the baseplate.

A clear space of 6 ins (152 mm) must be allowed on the left side of the units when fixing to the mounting surface. This space allows full movement of the hinged plate, with access to the wiring of the relays etc.

A description of each wall-mounted relay-unit is given in para's 9 to 14.

Relay-unit Q 405 is provided at HES No. 3 installations which have multiple stations only. The unit contains the circuits which control the lamp signals in the Telephone, Intercom, No. 3/1, and the transmission feed relay for the intercom circuit. The wiring of the unit and the multiple cable terminate on a connection strip (Part 1/SST/71) the cable being connected to the screw terminals.

Wiring details are shown in Diagram. Q 405 and in Diagram ID 174, which is pasted inside the cover.

.Cable connections are shown in Diagrams Q 403 and Q 404.

When a HES No. 3 is working subsidiary to a switchboard with a low-power ringing source, it may be necessary to provide a local ringing supply, the output of which is controlled by relay contact RG2. Diagram Q 417 shows details of alternative ringing arrangements when working a HES No. 3 subsidiary to a switchboard or when more than 'five bells are to be rung.

The complete unit weighs approximately 51 lb. (2.5 kg).

Relay-unit Q 410 replaces Relay-unit Q 405 at HES No. 3 installations when a non-multiple extension is required. In addition to the components and circuits of the Relay-unit Q 405, the Relay-unit Q 410 also contains the components and circuits which connect the two-wire line of a non-multiple extension to the multiple wiring of the system.

In this unit there is no spare contact on relay RG, and when a HES No. 3 with a non-multiple extension is worked subsidiary to a switchboard which has a low-power ringing source, it will be necessary to change relay RG. The relay should be changed for a Relay No. 9871, and the additional contact wired as shown in Diagram Q 417, which gives details of alternative ringing arrangements. The relay RG must also be changed and wired to Diagram Q 417, when more than five magneto bells are to be rung at any installation.

The unit wiring and the multiple cable are terminated on a connection strip (Part 1/SST/71), with the cable connected to the screw terminals.

Wiring details of the Relay-unit Q 410 are shown in Diagram Q 410 and in Diagram LD 175, which is pasted inside the cover.

The complete unit weighs approximately 14 lb. (6.3 kg).

The Relay-unit Q 415 is provided when an inter-switchboard or private circuit is included in a HES No. 3 installation, and is fitted in addition to Relay-unit Q 405 or Q 410.

Relay-unit Q 415 contains a number of circuit elements, which may be strapped according to the type of signalling being used on the circuit.

The wiring details of Relay-unit Q 415 are shown in Diagram. Q 415 and in Diagram ID 180, which is pasted inside the cover.

The connections required when using the Relay-unit Q 415 are shown in Diagrams Q 441 to Q 447. Signalling limits for this unit are the same as for a Unit, Auxiliary Apparatus, No. 97 and are given in P1061.

The unit wiring and multiple cables together with the required strappings shown in Diagram Q 441 are terminated on two Strips, Connection, No. 121D mounted back to back; these are all soldered connections.

The complete unit weighs approximately 15 lb. (6.8 kg).

The Relay-unit Q 516 is fitted to all HES No. 4 installations, and contains the components and circuits which control the lamp signals and exchange line ringing signals at a Telephone, Intercom, No. 4/1 fitted at a multiple station. Relay contacts within the unit are also available, when required, to extend signal and start conditions to the relay-unit of non-multiple extensions or private circuits.

Current through each of the series connected exchange line engaged lamp circuits of a Telephone, Intercom, No. 4/1 is controlled by a transistor circuit. The transistor is mounted on a heat sink which also carries the associated resistors, capacitor and rectifier; this complete assembly has been coded Regulator No. 4A.

The Regulator No. 4A is fixed by two screws to the hinged plate and joined to the circuit by three flexible spade-ended leads. Care must be taken to ensure that these leads are correctly connected otherwise the transistor may be seriously damaged. The regulator is adjusted during manufacture to pass 104mA (- or + 12 ma) under load conditions, and should any component become faulty, the complete regulator must be changed.

Mounted on the baseplate of the relay-unit is a clip to hold a Connector No. 203C55C, which is a 55-way connector that has been provided with a number of straps between certain sockets, and is intended for use at multiple stations where a Telephone, Intercom, No. 4/1 has been disconnected for repair or replacement. By replacing the telephone socket with a Connector No. 203C55C the series circuits of the installation are maintained and service is continued at all other stations. The connections provided in the Connector No. 203C55C are shown in Diagram Q 510.

Wiring details of the Relay-unit Q 516 are shown in Diagram Q 516 and in Diagram LD 192, which is pasted in the cover.

The complete unit weighs approximately 12 lb. (5.4 kg).

The Relay-unit Q 519 is provided at a HES No. 4 installation to connect the two-wire line of a non-multiple extension to the multiple wiring.

Circuits in the unit provide the non-multiple extension with most of the facilities of a multiple station; these facilities are fully described in C3031.

A Strip, Connection, No. 121M provides the terminating point for the unit wiring, multiple cables and strappings as required. Straps that may be needed should be connected on the permanent wiring side of the Strip, Connection, No. 121M; and the multiple cables terminated on the opposite side.

Details of the wiring are shown in Diagram Q 519 and in Diagram LD 195, which is pasted inside the cover.

When a Relay-unit Q 519 is provided, a Control-unit Q 535 or Q 537 (see para's 21 and 22) is always fitted at the main station and the operation of the Relay-unit Q 519 should be studied in conjunction with the operation of the Control-unit Q 535 or Q 537. Circuit elements and explanatory, notes are shown in diagrams in the Q 5... series.

The complete unit weighs approximately 17 lb. (7.7 kg).

The Relay-unit Q 524 is provided at a HES No. 4 installation to connect the two wires of a private circuit to the multiple wiring.

The unit contains circuit elements which may be connected by strappings on the Strip, Connection, No. 121M, to terminate the circuit at the HES No. 4 according to the type of signalling to be used. The cable connections and straps required for various signalling groups are shown in Diagrams Q 561 to Q 567.

Wiring details of the unit are shown in Diagram Q(L) 524 and in Diagram LD 194, which is pasted inside the cover.

Signalling limits for the Relay-unit Q 524 are the same as for a Unit, Auxiliary Apparatus, No. 97 and these are shown in P1061.

Straps should be connected on the permanent wiring side of the Strip, Connection, No. 121M and cabling should be terminated on the opposite side.

The complete unit weighs approximately 17 lb. (7.7 kg).

Certain facilities that are available to both HES No. 3 and No. 4 installations are provided by fitting small relay-unit into a Telephone, Intercom, No. 3/1 or No. 4/1 and modifying the instrument cord connections.

These units consist of a Relay No. 16/... or Relay No. 23/6 mounted on a metal mounting plate which is secured over a bell gong.

Other components may also be included, terminated on `polytags' pressed into the metal mounting plate.

The various relay-units for fixing inside Telephones, Intercom, are described in para's 16 to 19.

The Relay-unit Q 408 is provided in a Telephone, Intercom, No.3/1 when the instrument is to have restricted exchange service.

The unit consists of a Relay No. 16/1 and two diodes (Valves, Electronic, CV 7040) on a mounting plate (Part l/DMO/102), and has spade-ended flexible leads for connection to the telephone circuit.

Diagram Q 408 shows the wiring of the unit and the connections within the telephone.

The Relay-unit Q 409 is provided in a Telephone, Intercom, No.3/1 when the instrument is to have monitoring facilities.

The unit consists of a Relay No. 1612 wired with spade-ended flexible leads on a mounting plate (Part 2/DMO/102).

Diagram Q 409 shows the wiring of the unit and the connections within the telephone. Relay-Units No. 408 and 409 are mounted over the right-hand side bell gong.

The Relay-unit Q 527 is fitted in a Telephone, Intercom, No.4/1 when the station is to have restricted exchange service.

When exchanges service restriction applies to only one line the Relay-unit Q 527 should be used and connected to the appropriate line.  When both lines are to be restricted the Relay-unit Q 527 should be connected to the line on position one of the telephone and a Relay-unit Q 528 (see par 19) connected to the line on position two.

The Relay-unit Q 527 consists of a Relay No. 16/1 and two rectifiers (Valves, Electronic, CV 8308) on a mounting plate (Part 3/DMO/102), and has flexible spade-ended leads for connection to the telephone.  Details of the wiring and telephone connections are given in Diagram Q 527.  The Relay-unit is mounted over the left-hand side bell gong.

The Relay-unit Q 528 is fitted in a Telephone, Intercom, No. 4/1 when both lines are to have restricted exchange service. The unit includes a mounting plate (Part 1/DMO/102) for mounting over the right-hand side bell-gong but in all other respects the Relay-unit Q 528 is identical to the Relay-unit Q 527. Details of the Relay-unit Q 528 are included in Diagram. Q 527.

When a non-multiple extension or private circuit is fitted at a HES No. 4 installation a control-unit must be fitted at the main station.  This unit provides the circuit calling and clearing supervisory, and exchange line testing circuits.  There are two sizes of control-unit, Control-unit Q 535 which caters for one circuit and is described in par 21, and Control-unit Q 537, which caters for up to four circuits and is described in par 22.

The Control-unit Q 535 is a desk unit measuring approximately 3.5 in (90mm) wide x 5.5in (140 mm) front to back and 2.75 in (70mm) high; it is fitted adjacent to the Telephone, Intercom, No. 4/1 at the main station.

The unit is contained in a Mounting D 92155 using a cover (Part 1/DCO/672, Grey), and a face plate (Part 1/DPL/2136).  The 'EXCHANGE LINE' lenses are Caps, Lamp, No. 79C, Red and the 'EXTENSION' lens is a Cap, Lamp, No. 79C, White. The lamps used are Lamps No. 2-45V.  The lamps and lenses are easily replaceable but other components are soldered in and should not normally be changed in subscribers' premises.

The cord associated with the unit is a Cord, Inst, No. 18/04AJ, Grey, 72 ins (1.8m) and this is terminated with the cabling onto a Block, Terminal, No. 37..., both must be ordered separately.

Diagram Q 535 shows the wiring details of the unit and Q 1015 explains the facilities of the unit.

Control Unit Q535

The unit is available in grey only.

The Control-unit Q 537 is a desk unit measuring approximately 6.75 in (170 mm) wide x 5.5 in (140 mm) front to back and 3in (75 mm) high; it is fitted adjacent to the Telephone, Intercom, No. 4/1 at the main station.

The unit is contained in a Mounting D,92199 using a cover (Part 1/DCO/676, Grey) and a face plate (Part 2/DPL/2135). In addition to the parts contained in Control-unit Q 535 (see par 21) the Control-unit Q 537 contains four key circuits which are used to connect non-multiple extensions and private circuits for intercom calls. When used at an installation with 2nd choice main station facilities, the Part 2/DPL/2135 is changed (see C3036).

This unit is issued without a cord and Q 3015 describes the conditions governing which cord should be fitted.. When a Cord, Inst, No. 18/04AJ, Grey, 72 ins (1.8 m) is fitted, a Block, Terminal, No. 37... must be used to terminate the instrument cord and cable. When a Connector No. 1046A or No. 1052A is fitted, a Box, Connection, No. 6A must be used to terminate the connector and cable.

Diagram Q(L) 537 shows the wiring details of the unit and C3031 explains the facilities of the unit.

Control Unit Q537 Control Unit Q537 Second Choice

A Telephone No. 710 is used to terminate the non-multiple extension. The components to be ordered separately and fitted in the telephone are fully described for the HES No. 3 in Diagram Q 420, and for the HES No. 4 in Diagram Q 550.

Telephone No. 710 fitted with buttons appropriate for a Non-Multiple Extension

Standard magneto bells, d.c. bells and buzzers are used when extension bells etc are required. Diagram Q 419 shows the connections for HES No. 3 and Diagram Q 545 shows the connections for the HES No. 4.

A Receiver, Watch, No. 8T and a Hook, Receiver, AG are used.

Particulars of dial labels for Telephones, Intercom, No. 3/1 and No.4/1 are given in C3034 and C3036 respectively, and the labels are described in M0010.

When issued, Telephones, Intercom, No. 4/... are fitted with station identification labels (Labels No. 469/1 and 2 and these are described together with Labels No. 469 in C3036.

There are two instrument cords and two connectors available for use on HES No. 3 and No. 4 equipment.

Cord, Inst, No. 20/03AJ, 72 ins (1.8 m) is normally fitted to Telephones, Intercom, No. 3/1.

Cord, Inst, No. 25/04AJ, 72 ins (1.8 m) replaces Cord, Inst, No. 20/03AJ, 72 ins (1.8 m) when extra conductors are needed.

Both cords have at one end 41 ins (115 mm) spade-ended conductor tails for Connection to the telephone, and at the other end, have the spade-ended tails formed into two rows of ten. Each row is held in position by strips of flexible plastic, which are moulded over the shanks of the spade terminals. The remaining five conductors of the Cord, Inst, No. 25/04AJ, ..., 72 ins (1.8 m) are left free. The plastic 'harness' assists in quick and accurate connection of the conductors to the Block, Terminal, No. 37...

Each cord is stocked in three colours: black, grey and ivory, and in two lengths: 72 ins (1.8 m) and 120 ins (3.0 m).

The Connector No. 1046A is normally fitted to a Telephone, Intercom,, No. 4/... and may be fitted to a Control-unit Q 537 if required.

The Connector No. 1052A replaces Connector No. 1046A when. extra conductors are needed.

Both connectors have at one end, 41 ins (115 mm) spade-ended conductor tails for connection to the telephone or control-unit. At the other end, soldered to the conductors, is a Connector No. 203C55A, which is a 55-way socket for connection to the Box, Connections, No. 6A.

Both connectors are stocked in three colours, black, grey and ivory, and in two lengths, 72 ins (1.8 m) and 120 ins (3.0 m) Connectors, with 120 ins (3.0 m) cords are coded Connectors No. 1046B and No. 1052B.

The cable used for HES No. 3 and No. 4 installation is Cable, Distribution ... (12W, 24w or 41w), /0.5 mm. The size and type of cable to be used is given in C3034 for the HES No. 3 and C3036 for the HES No. 4.

The HES No. 3 and No. 4 are mains operated installations, and standby batteries are not provided. A Power-unit No. 51A is provided for the HES No. 3 and a Power-unit No. 52A for the HES No. 4.

The Converter, Ringing, No. 7, which is mounted inside a Power-unit No. 51A or No. 52A is normally provided when a local ringing supply is required for a non-multiple extension or private circuit. Power-unit No. 100A/1 and 100A/2 for use with HES No. 3 and 4 respectively are equipped with Converters Ringing No. 7 and should be provided at installations initially requiring a ringing supply.

Formerly EI Telephones, Stations, Q1016

Vol. 56, Part 1 (1963)


A Simplified House Exchange System
House Exchange System No. 3
By F. L. RANDALL and S. A. L. WARD

House exchange systems provide subscribers with a service that is proving increasingly popular, hut the two systems at present in use cannot meet modern transmission requirements and are expensive to install. Some of the novel features of a new type of installation that will replace the smaller of the existing systems are described.


HOUSE exchange system provides intercommunication facilities between a limited number of stations as well as direct exchange-line service at each of these stations, without the assistance of a central switching position. Ideally, a house exchange system combines the facilities provided by a house telephone system or an extension plan with those of a small P.B.X. by enabling users to select for themselves, by a simple operation, the required facility.

The telephone instruments for an installation of the above type must thus contain function-selecting and signalling arrangements that are easy to manipulate but which cannot be mis-operated to the detriment of the service at other stations of the system. The mechanical problems created by this requirement were first solved satisfactorily in the house exchange systems introduced by the Post Office in 1935.  These two systems. House Exchange Systems (H.E.S.) No. 1 and 2, cater, respectively, for one exchange line and up to six extensions and two exchange lines and up to 11 extensions and have proved increasingly popular with subscribers.

Several disadvantages have, however, become apparent in the techniques used in H.E.S. No. 1 and 2.
These disadvantages include the following:-

  1. The transmission circuit is not adequate for exchange lines having the limits assumed for modern cable-planning schemes.

  2. The internal multiple cabling arrangements, relying on a multiplicity of junction boxes, are costly in materials and installation time.

  3. Failure of the power supply prevents access to the exchange line except from a non-multiple extension.

  4. Considerable desk space is occupied by the telephone instrument and the auxiliary switching units.

These disadvantages have been overcome, existing house exchange system facilities have been retained and additional ones added in a new system, H.E.S. No. 3, catering for one exchange line and five stations. The new system is based on a 700-type intercommunication telephone derived from the Telephone No. 710 and coded Telephone, Intercom, No. 3/1, which is available in three colours and in versions suitable for manual or automatic exchange areas. Many of the principles used in the H.E.S. No. 3 are also to be incorporated in a larger system for two exchange lines and 10 stations (H.E.S. No. 4) now under development as a replacement for H.E.S. No. 2.


In order to appreciate the facilities offered by the new system it is necessary to distinguish between the various types of station that can be associated with an H.E.S. No. 3 installation.

Multiple Station.
A multiple station is one equipped with an intercommunication telephone and linked to other multiple stations by a 21-wire cable.

Main Station.
The main station is a multiple station that:-

  1. always receives a ringing signal from incoming exchange calls,

  2. assists other stations not directly connected to the multiple to originate local calls, and

  3. controls certain additional facilities that are described later.

Non-Multiple Extension. A non-multiple extension is fitted if a station is located beyond the practicable limit of the multiple cabling, e.g. in another building Such a station is equipped with a Telephone No. 710 and is linked to the main installation by a 2-wire line.

As already mentioned, the new system can provide for one exchange line and up to five stations. A non-multiple extension may replace the last multiple station, and private circuits to a P.B.X. or another house exchange system can replace other multiple stations.

The following are the principal facilities afforded by H.E.S. No. 3:-

Exchange Service.
Exchange calls are secret, and outgoing service is available from any station which is not specifically barred, including the non-multiple extension. Access to the exchange line is obtained by operation of a locking press-button with receiver-rest restoration. Calls, incoming or outgoing, may be held or transferred at any station, including the non-multiple extension. Incoming exchange calls normally ring a standard magneto bell included in the intercommunication telephone at each multiple station, and calls can be answered by any station. The bell at most multiple stations can be silenced by a locking bell off key, but at the main station this key is permanently disconnected from the bell circuit and may be used to control other facilities.

Normal exchange service continues to be available at all stations if the installation power supply fails.

Individual Exchange Lines.
A multiple station can be connected to an individual exchange line or P.B.X. extension not available to other stations; these stations can share a common line or extension.

Local Intercommunication Calls.
A multiple station can call any other station by means of a non-locking press-button; buzzer signals are given at multiple stations and magneto-bell signal at the non-multiple extension. The non-multiple extension can only call the main station, which assists by signalling the required station. Local calls are not secret.

Conference Calls.
All stations can take part in a local conference call; this facility is not available on exchange calls.

Lamp Signals.
The exchange-line signalling circuit includes a supervisory lamp, coloured red, which:-

  1. flashes in time with exchange-ringing cadences,

  2. glows steadily when the exchange line is answered,

  3. flashes approximately twice a second when the exchange line is being held by any station, and

  4. glows steadily when a station takes over or is ready to accept a transferred call.

A clear supervisory lamp glows when the local speech circuit has been seized by any station.

Station-Engaged Signal.
A multiple station that attempts to call an engaged station operates its own buzzer.

Night Service.
The non-multiple extension can be given incoming exchange service by means of switching provided by the main-station instruments. Two-way exchange service continues to be available at all multiple stations.

Extension Bells and Buzzers.
Standard magneto extension bells can be fitted at all stations, provided that the total number of bells does not exceed five; one extension bell may also be fitted at the non-multiple extension. An additional buzzer can be connected to each multiple station for local-call signalling.

Association With Other Installations.
An H.E.S. No. 3 can be connected to another house exchange system or a P.B.X. either as a subsidiary installation or by a private circuit. In some arrangements, night service can be given over the private circuit.

Operator Recall.
When connected as a subsidiary to a P.B.X., operator-recall signals can be given from any multiple station, and the non-multiple extension can recall the H.E.S. No. 3 main station, which can relay the recall signal.

Exchange-Call Barring.
As an optional facility, any multiple station can be denied exchange service except at the discretion of the main station. Also, any station can be permanently barred from exchange service.

Exchange-Line Monitoring.
One multiple station can, if required, monitor exchange calls from other stations while exchange calls from the monitoring station remain secret.

Subscriber Trunk Dialling Facilities.
One subscriber’s private meter can be fitted to the installation. Perm¬ anent or optional trunk barring can be fitted at any station.


With the object of achieving economies in equipment and installation costs several departures from techniques used in earlier systems have been introduced into the H.E.S. No. 3.

Series-Multiple Exchange Circuit
To avoid the need for a line-connecting relay in each telephone, without sacrificing the secrecy feature, a series-connected exchange-line multiple has been adopted. With this arrangement the exchange line is normally fed into and out of each station in turn, beginning with station 1; priority of access can, however, be given to any selected station by introducing the exchange line into the multiple at the required point and returning it to the normal sequence by means of spare conductors in the multiple cable.

Single Intercommunication Speech Circuit
Only one speech circuit for local intercommunication is provided in the multiple. Power for the telephone transmitters is fed to this circuit through a double-coil high-impedance relay, the instruments being connected in parallel across the circuit during local conversations. This arrangement also provides a simple conference circuit. The non-multiple extension has its own power feed, but uses the common circuit for local calls. Signalling for local calls is over a multiple conductor exclusive to each station. The use of a common speech circuit working on central-battery principles avoids the need for a power-feeding transmission bridge in each telephone.

Smaller Multiple
The H.E.S. No. 3 requires a minimum multiple of 17 conductors, a reduction compared with the earlier system. As a standard 21-wire cable will normally be used for the multiple, spare conductors will be available for additional facilities, e.g. exchange-line monitoring or for bunching with the power feeds to reduce voltage-drop at stations supplied by long cable runs.

Dual-Purpose Terminal Block
A new type of terminal box (coded Block, Terminal, No. 37A) is used both as a multiple-cable junction box and as a connexion point for a multiple station. The new terminal block contains a double-sided terminal strip with two rows of 10 connexions and five additional terminals between the rows; the terminal strip is housed in a case moulded in toughened polystyrene and having rectangular knock-out sections for cable entries and for securing the standard cord grommet. The case has a snap-on lid.

Multiple cabling is connected to the under-side of the terminal strip; the upper-side accommodates the spade tags of the instrument cord.

Each intercommunication telephone is equipped with a 20-way cord 72 in. long. The spade tags at one end of the cord are moulded into two flexible plastic strips with 10 tags in each; the spacing and numbering of the tags correspond with those on the terminal strip in the block. The instrument is connected to the multiple wiring by loosening the terminal screws to give sufficient clearance for the spade tags to be pushed under the cupped washers on the terminals, the screws then being re-tightened. The use of an instrument cord with preformed terminations makes for rapid and accurate connexion of a multiple station telephone at low cost. Twenty-five-way cords are also available; these have 20 preformed connexions and five free conductors to cater for additional facilities.

This method of connecting multiple-station telephones avoids the use of separate junction boxes, with their cross-connexions and individual feeder cables to instruments, which were previously connected by plug and socket.

Wall-Mounted Units
The power-feeding relay for the local speech circuit and the signalling and switching relays are housed in units designed for wall mounting, replacing the table mounted transfer units of earlier systems. Three types of unit are available:-

  1. for installations with multiple stations only (Relay Unit Q 405),

  2. for installations that include a non-multiple extension (Relay Unit Q 410), and

  3. for an installation connected to another subscriber’s installation by a private circuit (Relay Unit Q 415).

A standard installation contains either a Relay Unit Q 405 or a Relay Unit Q 410; Relay Unit Q 415 is added as required.

Fifty-Volt Power Supply
Use of a 50-volt d.c. supply simplifies relay design and allows for extended signalling limits on internal calls. Power unit No. 51A 4 provides the required d.c. from a mains supply; a secondary-cell battery is not required. The maximum current taken is about 750mA.

A.C. Ringing Supply
A ringing-current input to Relay Unit Q 410 is necessary so that intercommunication calls can be signalled at the non-multiple extension and exchange ringing repeated when the extension is on night service. The Relay Unit Q 415 also requires ringing current when 17 c/s or 25 c/s a.c. signalling is used on a private circuit. These requirements are met by a mains-driven static converter (Converter, Ringing, No. 7) added to the power unit when required. The converter delivers 60 volts r.m.s. at 25 c/s and overcomes the difficulties previously experienced in providing an adequate ringing supply for the bell of the non-multiple extension.

Simplified Installation of Apparatus
Most of the apparatus items forming an H.E.S. No. 3 installation are connected by screw terminals; this also applies to the additional units added as required and to those parts that may have to be replaced during maintenance. Only in the Relay Unit Q 415, which contains a large number of variable connexions, are soldered joints necessary at the time of installation.


Intercommunication Telephone
The intercommunication telephone has standard 700 type transmission components and is therefore suitable for use on exchange fines of up to 1,000 ohms loop resistance. Push-button-operated contacts required for function-selection and signalling are contained in a single unit of box construction mounted between the receiver rest pillars and secured to the chassis of the telephone by four screws. Most of these contacts are permanently wired to an 18-way terminal strip (Strip, Connexion, No. 155 B) also mounted on the receiver-rest pillars immediately behind the regulator and secured by spring-loaded studs; this terminal strip is additional to the usual one on the base of the telephone. Six flexible leads with spade tags connect the remaining push-button contacts to the rest of the telephone circuit, so that the whole assembly comprising switching unit and terminal strip may be readily removed for maintenance.

The switching unit contains eight sets of comb-operated contacts similar to those used in the 1,000-type key. 5 As in the Telephone No. 710, the appropriate set of contacts is operated by plungers under the control of press-buttons on the top of the telephone case.

Six press-buttons are fitted, four of them being in the two right-hand positions; these are the station-calling buttons that each operate one set of contacts. Each of these buttons is marked with the number of the station that is called when it is pressed. As there can be five stations in the installation and only four calling buttons are available, the numerals are engraved in removable inserts that can, if necessary, be turned over to display the numeral 5.

The exchange-line button, engraved exch, operates three sets of contacts, two plungers being mechanically coupled to do this. The exchange button has three positions of rest: (i) normal, (ii) fully operated, and (iii) intermediate. Position (i) is taken up when the handset is on its rest, position (ii) when the telephone is connected to the exchange line, and position (iii) when the exchange line is being held and the telephone connected to the intercommunication circuit; restoration is by operation of the receiver rest. There is also an over-press position that puts a recall condition on the B-wire of the exchange line while maintaining the conditions appropriate to position (ii). A rocker-bar underneath the exchange-line plunger prevents operation of the button until the handset is removed. This device prevents interruption of the series-connected exchange line circuit by accidental operation of the button.

The press-button that cuts off the internal bell occupies the left-hand position; it can be locked in the operated (bell off) condition by depressing the button and then sliding the top part of it to the left. At the main station, the set of contacts that this button controls is disconnected from the bell circuit so that there is always one bell in the installation that will ring on incoming calls. The contacts may be connected to suitable terminals within the telephone and used to control other facilities, e.g. exchange barring or night service to the non-multiple extension. When the press-button in this location is used for other purposes, the bell off button is replaced by another button suitably engraved.

Lampholders of the M.E.S. (Miniature Edison Screw) type are fitted to, and in front of, the receiver-rest pillars. A lamp with a clear bulb is fitted in the right-hand holder and indicates when the local speech circuit is in use. A red-tinted lamp, indicating when the exchange line is in use, is fitted on the left. A red bulb was chosen, despite a small reduction in light output, in preference to a clear glass bulb with a coloured opal. Two colourless opals in the case present a better appearance, when the lamps are not lit, than one coloured and one colourless; careful contouring and polishing of the inner surfaces of the opals has helped to mask the unlit lamps, which are mounted immediately behind them. The lamps are of a nominal 0.25-watt rating at 6 volts, but have been selected to give a reasonable life, with satisfactory illumination, over the range 5.6-7.2 volts, thus accommodating some variation of the voltage drop in the multiple cabling. The lamps are fed, through a voltage-dropping resistor, from the 50-volt d.c. supply.

The intercommunication telephone includes a 6-volt d.c. buzzer for signalling incoming intercommunication calls. The buzzer is mounted on a synthetic-resin-bonded-paper plate that also carries the voltage-dropping resistor and spark-quench filter, the whole assembly being riveted to an extension of the cord-grommet clip shaped so that the buzzer assembly rests on the telephone case, thus helping in sound distribution.

An exchange-line barring or monitoring facility can be added, when required. Two small relay units, each using a miniature relay, are available to provide these facilities; they are designed to mount on the right-hand bell-gong pillar, above the gong, using the existing fixing screw. Flexible leads with spade tags connect the unit to the main terminal strips at the rear of the telephone.

Relay-Unit Assembly
The larger relay units, which provide signalling and speech current for local calls or connect a private circuit to the system, are all the same width as the power unit, so that a neat vertical assembly of all common apparatus is possible. In the relay units, all components are fitted to a hinged plate that swings away from the wall mounted plate to give access to relay wiring.

Non-Multiple Extension Telephone
The telephone No. 710 used at the non-multiple extension is fitted with two press-buttons, engraved exch and extn, respectively, which are associated with mechanically-linked plungers. The extension plunger and spring assembly lock, connecting the telephone circuit to the 2-wire line and so to the intercommunication circuit; they are released by operation of the receiver rest. The exchange button also operates the spring-set associated with the other button, momentarily earthing the loop circuit to send a discriminating signal to the relay unit that switches the 2-wire extension to the exchange line if it is free. If this dual-button arrangement were not used, the main station would be called whenever the handset at the extension was lifted to originate an outgoing exchange call.


A simplified schematic diagram showing the way in which two multiple stations are interconnected is shown in below.

Relay Unit Q 405
Relay IL (Fig 4) is the control relay for the local speech circuit and also forms the transmission bridge. The relay operates to a telephone loop and contact IL1 lights the intercommunication-circuit-engaged supervisory lamp in multiple-station instruments.

When the exchange line is being held, earth potential is extended from the holding station to the FL relay via contact FL2. The relay is self-interrupting, the rate of pulsing being controlled by the presence of a 25 pF capacitor to approximately 2-3 pulses/second.

Relay RG is connected in the series magneto-bell circuit and contact RG1 lights the exchange-line supervisory lamp in unison with the periods of ringing. With the RG relay connected in the unit to the B-wire of the exchange line, the ringing circuit is completed by strapping at the far end of the multiple to the A-wire.

Exchange Calls
Connexion to the exchange is obtained by lifting the handset and pressing the exch button, so operating the KX and KH contacts (Fig. 4). The KX contacts switch the telephone to the exchange line and light the exchange-line supervisory lamps (LP2) at each multiple station. The KH contacts disconnect the exchange line from the succeeding stations in the multiple. This breaking of the series connexion ensures that only one station is connected to the exchange-line at any time, and also interrupts the magneto-bell circuit so that it does not shunt the telephone during dialling and speech.

Exchange Call Held
If a station wishes to hold an exchange call and speak to another station, the appropriate calling button (KRA, KRB, KRC, or KRD) is pressed. The movement of the KR plunger concerned causes a section of the latch plate associated with the telephone switching unit to release the KX contacts while the KR contacts complete the calling circuit. The KX contacts restore the telephone to the intercommunication circuit, switch earth potential from the exchange-line supervisory lamps and apply it, via KH contacts and the D-wire in the multiple to the FL relay, which starts to pulse; the exchange-line supervisory lamps flash in unison. Another KX contact in series with the KH contacts, which are still operated, connects a 200-ohm resistor across the exchange line as a holding loop. When it is desired to resume speaking on the exchange line, the exch button is pressed, re-operating the KX contacts. Both KX and KH contacts release when the handset is replaced.

Transfer of Exchange Calls
When it is desired to transfer to another station a call that has been received and held at one of the stations, the exch button at the receiving station is pressed, causing the exchange-line supervisory lamps to glow steadily at all stations, the earth connexion from receiving station KX contact being applied to the C-wire.

If the receiving station precedes the transferring station in the multiple it will immediately receive the call, but if it follows the transferring station it must wait for the handset at the transferring station to be replaced and so cause the exchange-line to be extended. The change from flashing to a steady glow from the exchange-line supervisory lamps (due to the operation of the KX contacts) indicates when the receiving station is ready to accept the call.

Station-to-Station Calls
Lifting the handset automatically connects the telephone loop across the local speech circuit, operating relay IL. Contact IL1 lights the intercommunication circuit supervisory lamps at multiple stations. Pressing the non-locking KR button for the required station connects earth potential via the multiple to the buzzer of the called station. The use of only four press-buttons, as already described, makes it necessary to interchange the buzzer signalling wires at each station. When the called-station handset is lifted, both instruments are in parallel across the intercommunication circuit. The engaged condition is maintained until both stations clear and the IL relay releases.

Should the called station be engaged, the receiver rest contact (CS1) at that station will connect the engaged signal to operate the buzzer of the calling station via the multiple.

Conference Calls
A conference call is established by pressing the KR buttons of the required stations and, on answering, all instruments are in parallel across the intercommunication circuit, with speech and clearing conditions the same as for a station-to-station call.

Non-Multiple Extension Calls
If a non-multiple extension is fitted. Relay Unit Q410 (see picture below) is used to replace Relay Unit Q 405. In addition to incorporating the functions of the latter. Relay Unit Q 410 contains the circuits necessary for the operation of the non-multiple extension.

On local calls the non-multiple extension presents a loop to the line when the handset is lifted and the locking extn button, KM, is pressed. This loop operates the A relay through which the extension receives its power feed for speech. Contact A5 extends an earth to the buzzer at the main station, which, on answering, operates the IL relay; contact IL2 disconnects the calling earth.

For incoming intercommunication calls, signals from multiple stations operate the RR relay, which applies 25 c/s ringing current to the non-multiple extension. To call the exchange the non-locking exch button, KE, is pressed, also operating the KM contacts. When the exch button is released the KE contacts restore, leaving the KM contacts operated. By this arrangement, pressing one button completes the telephone loop via the KM contacts and also applies a discriminating earth potential via the KE contacts. The earth potential operates the differentially-connected D relay in the extension line so preparing the change-over from the local speech circuit to the exchange line. When relay D releases, the EX relay releases and relay S operates to exchange-line current to complete the connexion. When the non-multiple extension handset is restored relay S releases and causes relay EX to re-operate, thus restoring the circuit to normal.

Pressing the exchange button while the telephone is connected to the exchange line re-operates relay D, and contact D1 operates a recall relay (RC) that places a hold loop across the line. On the release of relay D the non-multiple extension is reconnected to the local speech circuit and the main station is automatically called. The non-multiple extension can be reconnected to the exchange line by re-pressing the exch button.

When an exchange call is transferred from the non-multiple extension to a multiple station, the exchange button is pressed at this station and the exchange line is switched into that instrument and disconnected from the non-multiple extension. This releases the S relay and, when the handset is replaced at the non-multiple extension, the circuit is restored to normal.

Night Service
Incoming exchange calls at the non-multiple extension are controlled by the press-button at the main station, the contacts of which extend an earth via contact RG2 to operate the RR relay (see picture above).

Power Failure
As will be seen, the multiple stations do not rely on relay switching for exchange service. In the event of a power failure the non-multiple extension is automatically connected to the exchange-line multiple by the release of relay EX (see picture above).


Where a house exchange system with not more than five stations is required, the H.E.S. No. 3 offers a cheaper, more compact, modern installation with a wider range of facilities than the earlier H.E.S. No. 1.


The authors gratefully acknowledge the co-operation of Ericsson Telephones, Ltd., in the development of the new house exchange system.



BACK Home page BT/GPO Telephones Search the SiteGlossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: January 19, 2022