PAGE No. 15

Installation of the Ericsson House Exchange System
July 1937

A general description of the Ericsson House Telephone System with exchange facilities has already appeared in the Ericsson Bulletin No. 5 dated July 1934, but it is thought that a more comprehensive and detailed record of the advantages offered by this system, together with the methods of obtaining these facilities, would be of particular interest. To do this it is necessary to enlarge upon and even to repeat some of the original matter.

Prior to the consideration of the system in detail, it would no doubt be of value to review the telephonic requirements which brought about its adoption. Up to the time of its introduction a subscriber had his own particular needs catered for by three alternative arrangements:-

  1. A private intercom system, and one or more public service telephones.
  2. A private branch exchange (P.B.X.).
  3. A combination of a P.B.X. and a private intercom system.

The Ericsson system caters for all such conditions, and utilizing only one instrument affords every facility offered by any previous single system or combination of systems.

As constant mention will later be made of the fundamental parts constituting the system, a general definition of such equipment is desirable.

An Internal Extension Station is any station served by the standard telephone instrument having the full complement of local and exchange push keys.

A Main or Master Station is an internal extension station specially predetermined and allocated to handle the incoming and in certain cases the outgoing exchange traffic. in normal circumstances only one such main station is desired per installation, but (as on P.B.X. working) provision is made for extending night service to other extensions by means of a second master station. It is thus possible to transfer at will the supervision and distribution of exchange calls to this “second choice main station“. The main stations have in addition to the standard instrument an Auxiliary Apparatus Unit in which is housed the necessary exchange line and transfer equipment, together with an audible alarm.

An External Extension Station is a station equipped with a standard C.B. or auto telephone and is connected to the main cabling by means of a 2-wire cable. This station can be rung by all instruments but can only make call to internal extensions via the main station, at which special auxiliary apparatus is fitted according to the number and type of lines equipped. This facility of incorporating an external extension is of great value since it affords a means of communication to a point some distance from the main installation where the cost of installing standard multiple cable would be unduly expensive.

The equipment is available in two sizes for accommodating:-

(a) One exchange line and five internal or side stations (5 + 1) system.

(b) Two exchange lines and ten internal or side stations (10 + 2) system.

While these will normally cater for the usual requirements, in exceptional cases the provision of a sixth station on a five line system, or an eleventh station on a ten line system is desirable. This is accommodated by utilizing the “home“ button on each instrument for calling the additional station. On either size of installation one external extension station may be fitted in lieu of an internal extension.

In order to facilitate initial installation and subsequent maintenance, both the instruments and units are equipped with plugs and cords, the incoming cables being terminated on jacks. The jacks are standardized so that it is possible to plug in the smaller equipped instrument into an installation wired for a capacity of 2 exchange lines and 10 side stations. Similarly if it is desired to add an external extension at any point, the appropriate auxiliary unit may be jacked in to replace the standard exchange line auxiliary unit. This interchangeability can readily be appreciated by reference to figure 1 (below) showing the plug point connections.

Fig. 1 - Connections of jacks and plugs of the instruments and auxiliary units

Extension bells wired from the jacks may be provided. This provision, for extending the exchange line or external extension alarm bell, enables incoming calls to be answered at other points should the main station operator be absent. On installations having two exchange lines a common alarm bell serves both exchange indicators, so that when the alarm circuit is extended it becomes necessary to test both lines at the normal internal extension. A similar test is necessary when the main station unit caters for one exchange line and one external extension.

Fig. 2 - 10 + 2 instrument

The 10 + 2 size Instrument consists of the push-button mechanism enclosed in a moulded Bakelite case as shown in figure 2, the exchange keys numbered 1 and 2 being coloured red and the local keys 1 to 10 black. The mechanical arrangement of the key movement is such that under normal circumstances the depression of any key automatically restores any previously operated local key. If an exchange key is in the operated position the depression of a second exchange or local key will partially restore the first exchange key which takes up the “hold” position leaving a 600 ohm hold coil across the exchange line. A special green conference key is included which renders the normal tripping mechanism inoperative when it is desired to set up a conference call. Two special triggers are provided above the exchange buttons on the (10 + 2) instruments in order to allow the complete release of either exchange button where “holding” both exchange lines. The necessity of such a provision becomes apparent when it is realised that the whole of the exchange key spring-set can otherwise only be restored to normal by replacing the hand-microphone. Should both exchange lines be in use the result of replacing the handset would be to clear both exchange lines, hence the provision of individual releasing triggers. Slip-in paper labels are provided at the side of the keys for designation purposes. A diversion from the standard practice has been made in mounting the buzzer externally on the instrument plug. This allows any adjustments to be carried out without interfering with the casework of the instrument. A standard size of case is used for both the 5 and 10 line instruments.

 The special auxiliary or transfer units are also housed in a standard Bakelite case as in figure 3, the equipment varying according to the particular use of the unit.

Transfer Units N.9000 (P.O. No. 1) and N.9002 are suitable for C.B. and L.B. working respectively and may be used on 1st and 2nd choice main stations on systems having one exchange line and up to 10 side stations but not an external extension station. They may however be used on 2nd choice master stations, on an installation which includes one external extension, by the addition of an indicator N.4798.A (P.O. No. 400K) in place of the dummy normally fitted.

Transfer Units N.9005 (C.B.) (P.O. No. 1A) and N.9006 (L.B.) cater for the first choice main stations on installations having one exchange line, one external extension and up to 4 internal extensions.

Transfer Units N.9001 (C.B.) (P.O. No. 2) and N.9003 (LB.) are suitable for use as first or second choice units on installations having 2 exchange lines and 10 internal extensions. They may also be modified by the addition of the indicator N4798.A for use as second choice master stations when an external extension is incorporated in the system. In this case an additional unit N.9004 is fitted at the first choice main station.

Transfer Unit N.9004 (P.O. No. 3) is purely an external extension unit for use in conjunction with units N.9001 and N.9003 for 1st choice main station working on systems having 2 exchange lines, I external extension and 9 internal extensions; or in conjunction with units N.9000 and N.9002 when more than 5 internal extensions are required on a system including one exchange line and one external extension.

Owing to the comparatively small number of standardized units, minor modifications become necessary when certain of the special facilities are desired at any particular station. This has been catered for in so simplified a form that modifications can readily be carried out on site to suit any particular requirement.

Units N.9000, N.9001, N.9002, and N.9003 are fitted with a “dummy indicator“ which is replaced by indicator N.4798A.  when required for external extensions. Provision has been made in the local cable form for this additional indicator.

Fig. 3 - A typical Auxiliary Unit Fig. 5 - Transfer Unit (N9001) terminal block

When a second choice main station is fitted, the exchange lines and the external extension indicator circuit can be transferred to this station, by throwing the appropriate keys of the first choice main station auxiliary unit or units. To provide for this facility it is necessary to remove the strap connections on the first choice units. These straps are clearly shown in figure 5, which illustrates the terminal block of a transfer unit with 2 exchange lines. Certain of the transfer unit key labels which are engraved both sides should also be reversed to indicate the transfer position.

While one of the main features of the system is that all exchange calls are normally secret, it may be desirable and advantageous for one or more instruments to have monitory or supervisory facilities: e.g., for trunk offering purposes. Straps are provided on the terminal block of each instrument as shown in figure 6. When supervision facilities are required strap 1-2 is removed and strap 2-3 substituted. This permits the operation of the exchange line instrument relay over a local, circuit irrespective of the condition of the exchange line. Normally this relay can only operate and access be obtained to an exchange line, when the exchange line is disengaged.

Fig. 6 - Terminal block of the 10 + 2 instrument

An automatic dial would normally be supplied with each instrument for automatic working, but should automatic facilities not be required at the outset, the dial can be omitted and supplied as and when required. The dial can be connected on site by means of a flexible cord and the strap removed from terminals 1 and 2.

The transfer units as standardized by the British Post Office work on the common battery (C.B.) system, and where these are connected to public exchanges utilizing local battery (L.B.) working, special auxiliary units are fitted in the lines at the exchange. It is realised however that such modifications are not always possible, especially in countries abroad, so that suitable units have been specially designed for working direct to local battery exchanges.

The main condition governing the necessity of L.B. units is the question of providing a suitable battery supply from the main exchange. If such a supply can be obtained from the main exchange then standard C.B. units can be used at the subscribers’ premises, and a special unit fitted at the main exchange to give the correct call and clear conditions depending on the type of L.B. exchange.

If the main exchange is fed by primary cells it is not practicable to provide the battery supply from this source, so that a local battery must be provided and L.B. units installed. These units are similar in external appearance to the C.B. units but the equipment is not standardized. When L.B. units are required, each installation is examined in detail by the Ericsson research and engineering departments and units are designed according to the prevailing local exchange system, namely, C.B.S. No. 1, C.B.S. No. 2, C.B.S. No. 3 or magneto. When working to a magneto public exchange a special magneto auxiliary unit is fitted at the main exchange to provide the necessary ringing current.

Thus is can be seen that both calling and clearing is carried out in the same manner irrespective of the type of public exchange, i.e. automatic calling and clearing is given even on magneto systems. This is a great advantage insomuch that on magneto working it is quite common for the subscriber to fail to give the ring off when an exchange call is concluded. On the Ericsson system a through clear is given by replacement of the hand microtelephone.

While the above brief description gives the broad outline of L.B. working, the system is so flexible that it can readily be adapted for any particular local conditions.

 In common with the instruments and units, special type junction boxes have been designed giving excellent facilities for cross connecting the various cables. These boxes are made up in two sizes, 30-way and 48-way, the former being used throughout the five line system and also as an auxiliary box when required on the ten line system, which is normally served by the larger size. The smart and pleasing external appearance of the moulded Bakelite cases is shown in figures 8 (shown to the right - 30 way - with junction box strapped and cross connected for instruments No's 2 & 3) and 9.

Provision for the termination of four multiple cables is provided and the lids of the boxes have thin Bakelite “break-ins“ which may readily be removed to permit the entry of the cable at either end of the box. A further deviation from standard multiple boxes of the past has been incorporated, in that the cables are terminated on individual terminal strips and the necessary commoning done by means of special square-section bare wire supplied with the boxes. When jumper connections are required they are carried out in switchboard wire, a typical example of a completed box being shown in figure 8. Each of the four terminal strips may be individually removed from the moulded base and if desired the cable can be connected to the underside prior to being cleated down. Cable bonding clamps on the underside of the strips are connected by means of the bonding strip (figures 8 and 9) on the upper side, and where the cables are led in at opposite ends of the box the two bonding strips should be connected by suitable switchboard wire. Three point fixing of the base plate is standardized and together with the three rubber feet allows the mounting of the box on uneven surfaces without any undue strain on the Bakelite moulding. Separate cellastoid labels are supplied as required for designating the various cables.

Two special sizes of cable have been introduced for the system N3390 being for 12 pair and N3391 for 20 pair. These cables are to British Post Office specification and consist of twisted pairs of conductors, No. 23 S.W.G. having a uniform coating of pure tin, enamelled, the covering of each individual conductor being two lappings of cotton laid on in opposite directions. Standard colours are used for the outer cotton lapping and a helical lapping of white tape laid over the outer layer of conductors, the whole cable being waxed and lead sheathed over all giving an external diameter of approximately 0.52" for the 12 pair and 0.64" for the 20 pair. At this point it may be well to stress the necessity of a twisted pair cable in order to ensure that cross-talk is reduced to a minimum. When the smaller size cable is led into the larger size box it is necessary to pack the cable with a thin lead strip in order to obtain a satisfactory bonding grip.

The layout of an installation will naturally depend to a great extent on the particular design of the establishment, and this should be given careful consideration with a view to obtaining the most economical cable runs, etc. If a system is not fully equipped in the first instance, provision should be made for extending the multiple to further junction boxes. Two typical layouts for 5 and 10-line systems are shown in figures 10 and 11.

Fig 10 - 5 line system Fig 11 - 10 line system

In order to illustrate how the details necessary for the cross connections of the junction boxes are obtained, a pictorial diagram of a 5-line system comprising 5 internal extensions and 1 external extension (No. 5) is given in figure 12, the junction boxes being designated with the figure numbers described hereafter. Figure 13 shows in detail the cross connections necessary to serve instruments No. 2 and 3 (see also figure 8). In both cases terminals Nos. 11 and 12 (HL and R) on the instrument strips are connected to the multiple pairs corresponding to these particular instruments, i.e. No. 2 to 15 and 16, and No. 3 to 17 and 18. Thus it can be seen that on a normal 5 and 10-line installation the A and B terminals on the instrument strip, corresponding to the station’s own number, would be disconnected from the main cabling. When a 6th or 11th station is fitted this particular pair of terminals is connected by switchboard wire to the 12th or 20th pair of the multiple cable, and the terminal strip serving the additional station has its HL and R terminals (Nos. 11 and 12) strapped to the 12th or 20th pair as shown in figure 14. The main station junction box is represented by figure 15. In addition to the multiple and instrument cables this box feeds the auxiliary unit and, if desired, a second choice master station auxiliary unit as shown dotted in figure 12 (right). An external extension is included in the layout in lieu of No. 5 internal station and is connected to terminals 29 and 30 of the auxiliary unit strip. The HL and R of this auxiliary unit (terminals Nos. 11 and 12) are strap connected to No. 5 pair of the multiple cable. Normally an internal extension is rung by the application of an earth to the ‘B‘ line at the calling station, thus operating the D.C. buzzer of the instrument. In the case of the external extension, which is fitted with a polarized bell, it is necessary to convert the application of the earth into a suitable ringing current. This is done by the external apparatus unit which, as far as the cross connecting for HL and R is concerned, becomes equivalent to No. 5 instrument. The exchange line is also terminated on the main junction box.

Fig. 12 Pictorial diagram of a 5 line system with one external extension
Fig. 13 - Commoning for teles 1 & 2     |     Fig 14 - Commoning for teles 4 & 6
Fig. 15 - Commoning of terminal block for main station
Fig. 16 - Circuit diagram of the telephones
Fig. 17 - Connections of instrument terminal strip to bar direct access to exchange lines Fig. 18 - Connections of instrument terminal strip to totally bar exchange service
Fig. 19 - Pictorial diagram of a 10 + 2 system showing the use of two junction boxes for the main station

When certain special facilities are required at any particular station, modifications are made to the junction box strappings.

Any station may be “barred direct access“ to the exchange lines (except at the discretion of the master station) by a modification of the strap connections on the junction box serving the multiple to the barred station. By reference to the instrument circuit figure 16 it will be seen that the operation of the instrument line relay depends upon an earth being supplied over the D wire. To prevent the operation of this relay in order to bar direct access to the exchange line, it is only necessary to remove the earth from the "D" line of the particular instrument in question. The "D" line of the instrument is strapped on the junction box to the "D1" line, to which an earth may be applied at the discretion of the main station by depressing the push button provided on the main station auxiliary unit. This junction box connection is shown in figure 17.

When it is desired to completely bar a station from the exchange service the multiple wires C, D and D1 are not cross connected to the multiple strip and the C wire of the instrument is permanently strapped to earth as shown in figure 18.

This has the effect of giving the normal engaged signal if the exchange button is pressed in error. A similar earth strap is required on the second exchange line "C" wire on a 10 + 2 system when only one exchange line is connected to the system.

The cross connecting of a 10 + 2 system follows the same principle as that of the 5-line, except that, at the main station it may be necessary to fit a 30-way auxiliary junction box in addition to the 48-way main box, owing to the number of cables to be accommodated. Such a case is illustrated in figure 19 where two auxiliary apparatus units are required in addition to the instrument. It will be noticed that the instrument cable is taken from the main box and the units from the auxiliary box. By this method the smaller type box can be used and connected by means of a l2-pair cable, the necessary cross connections being as shown in figure 20 (below).

Fig. 20

The power is normally obtained over a power lead from the public exchange but where this is not available a battery of primary cells or a trickle-charged accumulator set may be employed. The system is designed to operate on 24 volts but the factor of safety is sufficient to permit its use on any voltage between 18 and 28 volts. The maximum current consumption is approximately 1.3 amps. for a fully equipped (10 + 2) installation. A 10 uF condenser should be connected across the power lead in order to reduce to a minimum any crosstalk due to battery feeder resistance.

On completion of the installation the apparatus should be disconnected by means of the plugs and jacks and the following tests applied. All the A lines should be commoned and tested to the B lines with a 250 volt Megger, care being taken to remove the straps from the A and B lines of the battery and earth pairs. The pairs should then be bunched together and tested to earth. Under both conditions the insulation resistance should not be less than 1 meg-ohm. Continuity of the cables should be tested and also the efficiency of the bonding clamps to earth.

After jacking in the apparatus it is recommended that the following tests be made at each instrument:-

  1. An exchange call to be made on each exchange line at each station.
  2. A local call to be made to each station on the system.
  3. A conference call to be made to include all instruments.
  4. The exchange lines to be ‘busied’ and an engaged test made by depressing each exchange key.

A complete cabling diagram should be drawn out showing the cable runs and junction boxes, with details of the instruments fed from each particular box, and any special feature appertaining to an instrument. The diagram will definitely be of great assistance should it be necessary to locate any cabling faults which may arise after the system has been in service some time.

Finally, it cannot be too strongly emphasised that it is of the utmost importance to ensure that everyone concerned in the use of the installation shall be fully conversant with the facilities available and the correct method of obtaining them. in this way the maximum efficiency will be obtained from the Ericsson House Exchange System which undoubtedly represents the latest solution to the problem of internal telecommunication.

A complete list of the instruments, transfer units, junction boxes and cables is tabulated below:-

Code No. Description P.O. Code
N1668 C.B. Set for 1 exchange and 5 extensions Telephone Intercom. No. 1
N1669 Auto Set for 1 exchange and 5 extensions -
N1670 C.B. Set for 2 exchange and 10 extensions Telephone Intercom. No. 2
N1671 Auto Set for 2 exchange and 10 extensions -
N9000 C.B. Transfer unit for 1 exchange line Unit Transfer No. 1
N9001 C.B. Transfer unit for 2 exchange lines Unit Transfer No. 2
N9002 L.B. Transfer unit for 1 exchange line -
N9003 L.B. Transfer unit for 2 exchange lines -
N9004 External Extension Unit Unit Transfer No. 3
N9005 C.B. unit for 1 exchange and 1 external extension Unit Transfer No. 1A
N9006 L.B. unit for 1 exchange and 1 external extension -
N3310 30-way junction box Box No. 1
N3311 48-way junction box Box No. 2
N3390 12-pair lead covered cable Cable E and C 12 pr./10
N3391 20-pair lead covered cable Cable E and C 20 pr./10


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Last revised: October 14, 2019