T.4353, T4358, T4363, T4365 & T 4366

By F. HARTLEY and F. E. COMYN, Engineering Department
Automatic Telephone and Electric Company Limited, Liverpool
Taken from 'The Strowger Journal' July 1938

Tables are given of the equipment quantities and dimensions of the five basic new designs and of the auxiliary special services that can be provided. The general assembly and various design features are outlined. The circuit of the battery eliminator is given, also the numbering schemes, and various operational features are described. The article concludes with some notes on the special service facilities.

There are five basic sizes of the recently developed series of private automatic exchange (P.A.X.) equipments, which are suitable for use both at home and abroad. The relays and automatic selective apparatus are of the British Post Office designs and, together with the wiring, have a semi-tropical finish.

A feature of special interest is that each of the five basic sizes is assembled as a completely self-contained metal-cased unit and includes a battery eliminator for operating direct off A.C. mains, no batteries being required. The battery eliminator is omitted where the supply mains are D.C. and duplicate batteries are used instead.

Table I gives the initial and maximum equipment quantities and the dimensions of the five basic sizes. The initial quantities of line equipments and the connecting links can readily be increased in accordance with customers’ orders, but not, of course, beyond the maximum capacity of any particular P.A.X. The line equipments are available in groups of 5 lines. Actually, the design is such that the additional items can be fitted at any time, convenient provision thus being made for any extensions that may be required later on.

Provision is also made for a comprehensive and useful range of special services, any or all of which can readily be supplied initially or at any future date by the addition of various apparatus components. In the case of the 10, 25 and 50 lines P.A.X.’s, the special services apparatus is arranged to be accommodated, when required, in an extension at the top of the cabinet. On the 100 and 200 lines P.A.X.’s there is sufficient spare space within the cabinets to accommodate the special services components. Table II contains a list of the various special services and indicates which of these are available on each size of P.A.X.


Type Initial
Height Width Depth
10-2 10 2 10 2 3' 0" 1' 3" 1' 4"
25-4A 15 3 25 4 4' 6" 2' 0" 1' 4"
50-7A 30 5 50 7 4' 6" 4' 2" 1' 4"
100-10A 60 7 100 10 5' 6" 4' 6" 1' 4"
200-20A 60 7 200 20 5' 6" 4' 6" 1' 4"

Table II

No. Type of Special Service 10-2 25-4A 50-7A 1000-10A 200-20A
1 Secretarial X X X X X
2 Key-calling X X X X X
3 Conference key-calling X X X X X
4 Priority X X X X X
5 Tie-line X X X X X
6 Fuse alarm X X X Always fitted Always fitted
7 Release alarm - X X Always fitted Always fitted
8 Code call using morse signals  - X X X X
9 Code call using coloured light signals - X X X X
10 Fire alarm - X X X X

The automatic apparatus is composed of B.P.O. Type 600 and Type 3000 relays, B.P.O. type uniselectors and Type 32A (B.P.O. Type 2000) selectors. In the case of the Type 10-2 P.A.X., 12 point uniselectors are used as line finders and final selectors.

In all cases the P.A.X.’s are fully equipped with contact banks and wired for the ultimate capacity. The line and cut-off relays are strip mounted in groups of five pairs, as shown in Fig. 3. In the case of the Type 25-4A and Type 50-7A P.A.X.’s, the apparatus required to be fitted for an additional connecting link merely comprises a uniselector mechanism, with condenser and spark quench resistance, as line finder and a Type 32A final selector.  For the type 100-10A and Type 200-20A P.A.X.’s each additional connecting link merely requires the jacking-in of a Type 32A finder and also a final selector. Any additional connecting links that may be required can therefore be fitted very readily.

The ringing and tone apparatus is of the vibratory type and provides tones very similar to those standardised by the British Post Office, including dial tone, ring-back tone and busy tone. The ringing and tone apparatus comprises B.P.O. Type 3000 relays and vibrators and a transformer and choking coil unit.

The equipment is assembled on an iron framework and enclosed in a metal cabinet  The automatic apparatus and the battery eliminators are contained in top and bottom sections of the metal cabinets which have been specially designed to exclude all dust and foreign matter.

The metal cabinets comprise nine parts in the cases of Types 10-2, 25-4A and 50-7A P.A.X.’s. As the Types 100-10A and 200-20A P.A.X.’s have two-part front and rear panels, the cabinets for these have a total of eleven parts. The outer surfaces of the metal cabinets are finished in crystalline black and have chromium plated fittings.  The 25-4A is shown below.

As previously mentioned, the special services apparatus for the three smaller P.A.X.’s is arranged to be accommodated in an extension at the top of the cabinet. In these cases the apparatus is assembled as a unit on a small iron framework, which is arranged to be bolted to the top member of the main framework of the P.A.X. The top of the cabinet is, of course, first removed and is replaced by a metal band and two side pieces, which are bolted to the extension framework. The top of the cabinet fits on the top of the extension framework and fits closely on to the two sides. The top extension has also two panels, each being fitted with a lock. The whole of the special services apparatus is thus completely enclosed.

The front and rear panels are removable and their edges are tapered to correspond with a similar formation of the top, bottom and sides of the cabinet. Each of these panels is fitted with a lock, the tongue of which registers in a tapered slot. These arrangements ensure that, when the panels are locked in position, the tapered surfaces are brought into close relationship, thus effectively excluding dust or foreign matter. The action of locking also secures the aprons in position.

The intersection panel and aprons effectively separate the automatic apparatus and the battery eliminator, whilst the louvres in the side provide for the free ventilation of the eliminator section.

The battery eliminators, provide for maximum outputs of 1, 3, 5 and 8 amperes, the output voltage being automatically controlled within the limits of 45-53. Mains voltages of from 100 to 250 volts are provided for by means of suitable tappings.

There are two separate output supplies, one being specially smoothed for transmission feeding circuits and the other, less smoothed, for operating purposes. Ammeter links are provided and enable a portable ammeter to be inserted whilst the P.A.X. is in operation without causing any interruption of the service. 


(a) Type 10-2 P.A.X.
Designed for single digit operation, 0-9, the circuit arrangements are composed of three sections, namely, the line equipment, the connecting link equipment and the tone equipment.

The second connecting link is automatically made available during the time that the first link is engaged. In the event of the fuse of a link being blown, the particular link is automatically busied out of service. The two line-finders are relay-driven, a single relay serving both line finders in turn.

(b) Type 25-4A and Type 50-7A P.A.X.’s.

The numbering schemes for these two P.A.X.’s are as follows:-
Type 25-4A  20-39 & 41-45
Type 50-7A  20-69

In both cases 60 and 69 reserved for code-call service.

The line finders are preselected in turn by means of an allotter, which automatically steps forward as each call is switched through and continues to step until a free line finder is seized in readiness for the next call. The circuit arrangements are such that, in the event of the fuse serving the operating supply to a connecting link being blown, the associated line finder is “busied” and is accordingly passed over by the allotter. Should all the connecting links be simultaneously engaged, continuous rotation of the allotter is automatically prevented.

Tie-line service can be given over one or more groups of tie-lilies by means of either one or two additional digits.

(c) Type 100-10A and Type 200-20A P.A.X.’s Numbering Schemes.
Type 100-10A  01-99 with 80 and 89 reserved for code call.
Type 200-20A  200-299 & 300-399 with 280 and 289 reserved for code call.

The line finders on both of these P.A.X.’s embody the Type 32A (B.P.O. Type 2000) selector mechanism and have 100  points. The final selectors on the Type 100-bA have 100 points, whereas on the Type 200-20A the final selectors are of the digit absorbing wiper switching type.  The allotters function in the same manner as described above, with the exception that, unless both the operating and transmission supply fuses on any connecting link are intact, the associated line finder is automatically” busied” and is thus passed over by the allotter, when hunting.

Various alarm signals are provided for on both of these P.A.X. ‘s as follows:-
Fuse Alarm - delay = Nil
Line Finder Alarm - delay = 9 seconds
Release Alarm - delay = 12 seconds
Called Subscriber held alarm (C.S.H.) - delay = 25 seconds

These alarms are indicated both visually and audibly. The line finder alarm and the C.S.H. alarm are each indicated by a lamp having a white opal cap, and the fuse alarm and release alarm are indicated by lamps having red and green caps respectively.

The alarm lamps are accommodated on a small panel adjacent to the fuse panels inside the cabinet. This panel also accommodates a two way telephone type key and a push key. The former key, when actuated in one direction or the other, is for the purpose of routining the line finders on either the 1st or 9th levels. The push key is for cancelling the alarm bell, which commences to ring immediately any alarm signal is given.

The special service facilities available with this new series of P.A.X.’s provide important additional means for improving the convenience and efficiency of inter-departmental communications between the staff and personnel of offices, works, factories, hospitals, institutions, etc. The following notes may therefore be of interest.

(a) Secretarial Service
The secretary’s cabinet is shown to the right and contains an eyeball indicator, a buzzer, a push key and a switching key. Fig. 12 shows the general wiring arrangements.

Normally, all incoming calls for the principal will be received at the secretary’s telephone, the switching key being normal. The secretary, upon ascertaining that an incoming call is for the principal, operates the switching key to the local position and also momentarily operates the push key to actuate the principal’s calling buzzer. When the principal replies, the secretary is able to offer the call without the calling party being able to overhear. If the principal accepts the call, the secretary changes the switching over from the local position to the through position, thus connecting the calling party with the principal.

When “through,” the eyeball indicator on the secretary’s cabinet operates so long as the principal is on the line, but releases when the principal replaces his hand-combination. “Through” calls are usually fully secret, i.e., the secretary is unable to listen-in. When desired, however, the facility of permitting the secretary to listen-in on “through“ connections can readily be provided by simply adding two short strapping connections inside the secretary’s cabinet. When the principal desires to speak to the secretary or to make an exchange call, he merely raises his hand-combination and this actuates the buzzer in the secretary’s office. Conversation is immediately established by the secretary operating the switching key to the local position. A call from the principal to the exchange can then be set-up by the principal himself, after requesting the secretary to operate the switching key to the “through” position. Alternatively, the required call can be set up by the secretary and subsequently transferred to the principal.

During the time that the secretary’s telephone is un-attended, the switching key would be left operated in the “through” position, thus ensuring that any incoming calls are directed to the principal’s telephone.

(b) Key-calling Service
This is a particularly convenient service for a general manager or senior official, since, without dialling it enables immediate connection to be made with the line of any one of several subordinates. It is immaterial whether or not the required line is engaged; immediate connection is made in either case.

The keys for this service are compactly assembled in a small cabinet as shown as shown to the right.  This cabinet provides for key-calling to a maximum of ten different lines. The eleventh key is for ringing purposes and also for holding purposes as explained in the following paragraph.

(c) Conference Key-calling
This is an extension of the above simple key calling facility and enables a conference by telephone to be established rapidly by merely actuating some or all of the keys and then momentarily operating the common ringing key.

By means of the hold key the convener of the conference can withdraw his telephone for the purpose of dialling any non-conference line, whilst still leaving the conference intact. Restoration of the hold key gives immediate re-access to the conference. The general manager or convener of the conference can disconnect any line from the conference by merely restoring the appropriate key.

(d) Priority Service
This service enables a number of senior officials to have the privilege of securing immediate connection with any other telephone instrument, even if this happens to be engaged when dialled. The additional wiring required for the key-calling facility is not necessary for priority service.

(e) Tie-line Service
Tie-line service enables two or more P.A.X.’s in different localities to be operated together as a single unit, all the telephone instruments having full access to one another.

(f) Code Call using Morse Signals
This is a particularly useful service for works and organisations having many departments, since it enables telephonic communication to be established rapidly with any senior official whose exact whereabouts are not known. The system operates by morse code signals, which are usually produced both visually and audibly by means of lamps and bells or hooters. In all there are 22 code call numbers and corresponding code call signals, one of which is assigned to each senior official. The code call numbers include a prefix of either two or three digits, according to the size of the P.A.X., followed by the digits individual to each official.
When, then, a code call number is dialled, the corresponding morse signals are caused to be given in all the main departments. The required party is put into immediate communication with the calling party by simply proceeding to the nearest telephone instrument and dialling the recognised two or three digit common reply number.

(g) Code Call using Coloured Light Signals
This is generally similar to (f) except that groups of five differently coloured lamps are arranged to be illuminated in any one of twenty-two combinations, in place of the morse code signals. The coloured lamps are intermittently flashed at regular intervals and a bell can, of course, be used to give an audible signal as well. The signals are initiated and replied to in precisely the same manner as described above for (f).

(h) Fire Alarm Service
The fire alarm special service and includes a multi-core cable between the automatic equipment and the indicator cabinet installed at some central point. The cabinet usually contains an indicator or a lamp for each telephone line. In order to give a fire alarm signal, an assigned fire alarm number, followed by the digit “0”, is dialled from the nearest telephone. The fire alarm hooter or bell is then sounded and the indicator or lamp corresponding to the telephone line from which the alarm was given is operated.
The fire alarm signals, when once actuated, cannot be cancelled except by means of a reset key at the indicator cabinet. This ensures that the caller need not necessarily wait to speak to the central indicating station after dialling.

This system can also be further extended to give “group“ indications, i.e., the particular building, floor or area from which the fire alarm was given.

(i) Other Special Services.
The available special services are not necessarily limited to those described above since the flexibility of the P.A.X. design also makes possible the provision of the following:-

  • Patrol watchman’s service.
  • Party-line working.
  • Revertive calling between parties on the same line.
  • Automatic special selective ringing by executives.

An extract from a 1951 brochure

“A.T.M.”, “STROWGER” PRIVATE AUTOMATIC EXCHANGE (P.A.X.) SWITCHBOARDS comprise the full switching, ringing, tone and power equipment required for private automatic telephone exchange systems.

The whole of the apparatus is enclosed in a metal cabinet which has a crystalline black enamel finish.  The standard switchboards are suitable for use under sub-tropical conditions and equivalent switchboards suitable for full tropical conditions can be supplied. Power for operating the switchboards may be obtained from secondary batteries or from A.C. electricity supply mains through a battery eliminator (rectifier), designed in the case of the 10, 25 and 100 P.A.X., (home contracts), for fitting into the switchboard cabinet. In the case of the 50 line P.A.X., (home contracts), and in tropical conditions for the 10, 25, 50 and 100 line P.A.X. a special rectifier is supplied to mount outside the cabinet. For alternative power equipments see Section C.C.

Telephones of standard automatic common battery type may be employed unless they require other than the standard
16 or 25-cycle signalling current or a dial ratio other than 66% break to 33% make.

Loud-speaking Telephones can be used with these switchboards, and will be quoted for on request.

The following Special Services are available:-

(1) Key Calling.
(2) Conference Key Calling.
(3) Secretarial.
(4) Priority.
(5) Tie Line.
(6) Code Call.
(7) Fire Alarm.
(8) Party Line Working.
(9) Discriminatory Ringing.
(10) Fuse Alarm.
(11) Release Alarm.
(12) Watchman’s.

These special services can be fitted to all P.A.X. switchboards except the 10-line size (T4353), which is not arranged for 6, 7 and 12.

The standard maximum combinations of special services for each size of P.A.X. are given in Table I; any variations desired require special consideration. Yes = X in the table below.

Table I
Maximum Combinations of Special Services

No. Type of Special Service 10-2 25-4A 50-7A 100-10A 200-20A
1 Secretarial X X X X X
2 Key-calling for 10 or 20 lines 2 4 6 8 10
3 Conference key-calling - note (i) 1 1 1 1 1
4 Priority - note (ii) X X X X X
5 Tie-lines 1 2 4 5 10
6 Fuse alarm X X X Always fitted Always fitted
7 Release alarm - X X Always fitted Always fitted
8 Code call using morse signals  - X X X X
8a Code call using coloured light signals - X X X X
9 Fire alarm - X X X X
10 Discriminatory Ringing X X X X X
11 Party Line working - note (iv) X X X X X
12 Watchman's - - - - -

(i) A conference is limited to 10 participants who may be any predetermined 10 of those connected to the conference key calling cabinet.
(ii) Standard priority service can interrupt key calling priority and vice versa.
(iii) Code Call (Audible) and Code Call (Visual Colour Code) are alternatives.
(iv) The following variations of party line service are available:-

  • 2-Party selective ringing (standard), each party requires a separate number on the P.A.X.
  • Omnibus circuits with selective ringing; maximum 10 instruments; special consideration required.
  • Revertive calling may be applied to (a) or (b); special consideration required.

(v) Watchman’s Service is available for each P.A.X. except Type 10-2, but as individual requirements vary, special consideration must be given to each case before a quotation can be made.

Table II
P.A.X Switchboards - Capacities and Equipment

Height Width Depth
T4353 - 10-2 10 1 2 10 2 3' 0" 1' 3" 1' 4"
T4358 - 25-4A 15 2 3 25 4 4' 6" 2' 0" 1' 4"
T4363 - 50-7A 30 4 5 50 7 4' 6" 4' 2" 1' 4"
T4365 - 100-10A 60 5 7 100 10 5' 6" 4' 6" 1' 4"
T4366 - 200-20A 60 10 14 200 20 5' 6" 4' 6" 1' 4"

1. One local telephone circuit is occupied by each lie line equipped. With 1 digit working, a complete level of 10 numbers is required for from 1 to 10 tie lines.

2. Telephone instruments are not included. See Section E for suitable types.

3. Each conversation occupies a connecting link. Therefore with 4 connecting links, 4 simultaneous conversations are possible, that is, 8 telephones in use at the same time. A code call occupies 2 connecting links.

4. If top extension for special services is added to T4353, T4358, or T4363, the height of the cabinet is increased by 1-ft. 2-in. (356 mm.) (see Fig. 6). Top extensions are not made to cabinets of T4365 and T4366.

5. Weight equipped as indicated. An average equipment of special service apparatus adds approximately 50-lbs. (22.7 kg.) to weight of switchboard.

6. T4365 is the first unit of T4366 and may therefore be extended up to 200 lines.

7. For further information regarding any type see relative brochure.

T4403 For extension of 5 lines to T4358 or T4363.
T4404 For extension of 5 lines to T4365 or T4366.

T4370 Additional link including final selector for T4358 or T4363.
T4405 Additional link including final selector for T4365 or T4366.

For 10-line P.A.X. switchboard T4353:-
T4373 - Tie line for interconnection with another P.A.X. switchboard.
T4374 - Second tie line.
T4375 - Fuse alarm.
T4376 - Key calling to 20 lines for one master station. 
T4377 - Key calling to 10 lines for one master station.
T4378 - Conference key calling with 20 lines for one master station.
T4379 - Conference key calling with 10 lines for one master station.
T4368 - Secretarial service (one required for each line furnished with secretarial service). (See Fig. 8.)
T4369 - Priority service.
T3815 - Top extension to cabinet (1 group of 7 space units).
T3816 - 2nd top extension to cabinet (1 group of 7 space units).
T3817 - Auxiliary cabinet (2 groups of 7 space units).

For 25-line and larger P.A.X. switchboards T4358, T4363, T4365 and T4366.
T4371 - Tie line, 1-digit, for interconnection with another P.A.X. switchboard.
T4372 - Tie line, 2-digit, for interconnection with another P.A.X. switchboard.
T4380 - Fuse alarm.
T4381 - Release alarm.
T4382 - Key calling to 10 lines for one master station.
T4383 - Conference key calling with 10 lines for one master station. Key calling cabinet is illustrated in Fig. 7.
T4384 - Secretarial service (one required for each line furnished with the service). (See Fig. 8.)
T4385 - Priority service.
T4388 - Code call audible for 22 codes. See Note (a).
T4406 - Code call visual for 22 codes for T4358 and T4363. See Note (a).
T4407 - Code call visual for 22 codes for T4365 and T4366. See Note (a).
T3818 - Top extension to cabinet of T4358 (1group of 12 space units).
T3819 - Auxiliary cabinet as for T4358 (3 groups of 12 space units).

Note (a) - Suitable bells, buzzers, horns, lamps, etc., to give code signals, are listed in Section H; they are not included above.
Note (b) - Space available on T4365: 1 group of 8 space units. On T4366: 2 groups of 8 space units.

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

Last revised: October 06, 2012