B,P,O. Unit Auto Exchange (U.A.X,) No, 12 Type
January 1938

Prior to 1929 small automatic exchanges for country areas were in the experimental stage and therefore no standard equipments were available. The necessity for standardized units to meet the rapidly increasing demand for automatic service in these areas was apparent, and in the early part of 1929 the British Post Office (B.P.O.) installed the first standard type exchange known as “Unit Auto” No. 5. This had capacity for 100 lines and was connected by junction lines to a nearby exchange, the latter being known as the parent exchange.

All local calls on unit auto No. 5 could be completed automatically, but calls to and from other exchanges were routed via the manual board in the parent exchange.

The success of this type of unit was rapid and by the end of 1930 over 300 exchanges were installed.

In order to cater for a larger number of subscribers, unit auto No. 6 was introduced; this provided similar facilities to No. 5 but had capacity for 200 lines. At a later stage it was found necessary to provide an even larger type equipment known as unit auto No. 7 with capacity up to 800 lines.

As already stated above, subscribers on unit auto Nos. 5 and 6 can only dial to subscribers on the same exchange, subscribers on other exchanges being obtained by the caller dialling 01 and obtaining the services of an operator in the parent exchange. When the B.P.O. decided on a programme of general automatization, it became desirable that subscribers should be able to dial calls to other exchanges, and consequently it was necessary to provide a unit auto exchange that had facilities which would enable the subscribers to dial subscribers on other exchanges in the area. To cover this demand unit auto No. 12 was developed, and replaces No. 5.

Unit Auto No. 12 has capacity for 100 lines inclusive of subscribers and junctions and, as the title implies, is built up in unit form. In order to provide for various equipments three different units are manufactured, namely: auxiliary, 12A and 12B.

Auxiliary Unit
The picture to the right shows the front and rear view of the auxiliary unit which accommodates the main distribution frame, (shown in the lower portion with heat coil and fuse mountings removed) the testing equipment, and relay sets containing the common equipment, such as, ringing, tones, time pulse and multi-metering equipment. The ringing and tones are generated by vibrating relays and the time and meter pulses are obtained by means of interacting relays and uniselectors. The telephone shown at the side of the unit is used as a service instrument or for testing lines, as required.

Figure 2 Figure 3

12A Unit
On the left of figure 2 is shown the front view of a 12A unit. Three relay mountings accommodating a total of 25 line relay equipments together with the allotter and routine test relays are fitted on the bottom shelf. Immediately above are fitted the line finders and the allotter switch. Above these switches are the subscribers and traffic meters while the next shelf accommodates the selectors. Capacity for 4 line finders and selectors and one allotter is available.

At the top of the unit are the terminal strips for inter-cabling and cross connection purposes. Removable panels are arranged on both sides of the top compartment to provide a clear enclosed run for inter-cabling between the units when lined-up.
On the right of figure 2 is shown a rear view of the 12A unit. The top channel type shelf accommodates the junction relay sets for parent or other exchanges and the lower shelf is for the associated multi-metering relay sets. Capacity is provided for 4 relay sets per shelf.

On the left of figure 3 the unit is shown with the gate, on which the line relays and line finders are mounted, opened out thus giving easy access to the wiring for maintenance purposes, etc. The right hand view shows the unit totally enclosed with all covers in position.

12B Unit
The picture to the right shows (left) the front view and (right) the rear view of a12B unit. This is arranged on similar lines to a 12A unit except that it is smaller in size and accommodates only 20 line circuits while its width will allow for only 3 selectors and relay sets being accommodated on the shelves.

It will be appreciated that as these units are installed in unheated buildings it is necessary to take every precaution to ensure that, as far as possible, the equipment is protected from varying atmospheric conditions.

On previous unit type exchanges the main frame (M.D.F.) was of the open type and suffered from corrosion of the metal parts. By incorporating it in the auxiliary unit with the other apparatus mentioned

above, this trouble will be very considerably minimized or even eliminated altogether.

Incoming cables enter the auxiliary unit through holes cut in the removable wooden false-floor which is then sealed with a special compound.

Between the units, when lined-up, a wooden gasket is fitted to make the joint airtight and provide a through cabling space in the top compartment.

The cabinets are constructed of angle iron, sheet iron and wood on the cavity-wall principle.

The recesses for the doors are fitted with a pliable circular rubber insertion, against which the doors are clamped by means of metal plates and thumb screws.

The overall dimension of the cabinets are:-
Auxiliary and 12B units 6' l0" high x 1' 7.5" wide x 1' 9" deep.
12A units 6' 10.25" high x 2' 0" wide x 1' 9" deep.

A complete 100 line exchange consists of one auxiliary unit, two 12A units and two 12B units lined up from left to right (front) in the following order:- AUX; 12A; 12B; 12A; and 12B.

If 50 lines are required the two right hand units are omitted while if only 25 lines are wanted the last three units are omitted leaving an auxiliary unit and one 12A unit.

Apparatus and Finish. The line circuits employ 600 type relays; all other relays are of the 3000 type. The meters are the 100 type, and the group and final selectors are the Strowger type.

Standard finish is provided for the various pieces of apparatus, but enamelled wire is used for all conductors in cable forms and jumpers.

Where an A.C. supply is available a single battery is provided with automatic charge and discharge arrangements.

If the supply is DC a double battery scheme is installed with a dynamotor for charging. In cases where no public supply is available a similar arrangement is provided except that the dynamotor is replaced by a petrol engine set.

A standard type building with internal measurements of 14' 0" x 7' 7" and a clear height of 8' 9" is used to accommodate all equipment. This is similar in size to that provided for unit auto No. 5, so that if it becomes necessary to replace a No. 5 unit by a No. 12 unit, the same building can be used without difficulty.

The picture above shows typical trunking arrangements. The subscribers line circuits terminate on the banks of 50-point line finders which are connected to selectors of the 2 motion Strowger type. The lines are divided into two separate 50-line groups each group having an allotter which, on the origination of a call selects a free line finder and causes it to search for the calling line.

The incoming side of the relay sets also terminates on the line finder banks and incoming calls via these relay sets are handled by the auto equipment in a similar manner to a subscribers call.
The first digit dialled is used for discriminating between local and junction calls. The digit ‘2’ is prefixed to each subscribers number and a multiple range of 200-299 is available.

The digit ‘9’ is used to gain access to the equipment at an automatic parent exchange. Dialling ‘0’ routes the call via the group of junctions connected to level ‘9’ but a discriminating signal which results in the parent operator being signalled is passed over the junction. By routing level ‘9’ and level ‘0’ traffic via the same relay sets an economy in junctions is effected.

Levels 3, 4, 5, 6, 7 and 8 can be used for junctions to other exchanges. In the case illustrated level ‘8’ is shown connected to a non-parent exchange.

Local Call
When a subscriber originates a call the allotter causes a line finder to search for the calling line. The allotter is in use only during the searching time of a line-finder and immediately the line finder seizes the calling line the allotter releases and is available for further calls. The allotter is arranged so that a different line finder is seized each time a call is originated, this feature preventing a faulty circuit from seriously interfering with service. The linefinder extends the call to the selector, which transmits dial tone to the line. If the calling party fails to dial before approximately 1 to 5 minutes have elapsed, the selector is released and the calling subscriber remains locked to his line circuit until the receiver is replaced.

Assuming that the caller dials prior to forced release conditions occurring, then the initial digit 2 causes the selector to step to level 2 and release. The 2nd and 3rd digits step the selector in a vertical and rotary direction respectively and tests the called party’s line. If the line is engaged the caller will receive busy tone. When the called line is in a P.B.X. group the selector will search for a free line in the group and return busy tone to the caller only if all lines in the group are engaged. If the called line is free, ringing is connected and ringing tone transmitted to the caller in the usual manner.

When the called party answers the calling party’s meter is operated and conversation can proceed.

Call to Parent Exchange (Manual Board)
The caller is connected to a selector as described above and dials ‘0‘. This causes the selector to step to level ‘9‘, seize a free junction to the parent exchange,
and signal a manual operator at that exchange. In the event of the caller being barred trunk calls (e.g. coin box lines) the operator receives a visual indication which informs her that she must collect the necessary fee. The bank wiring on level ‘9’ is ‘slipped’ so that on repeat calls a different junction is seized and consequently faulty circuits will not prevent access to a junction which is in working order.

Call to Parent Exchange (Auto Equipment)
In this case ‘9’ is dialled and the call is routed over the parent exchange junctions to an incoming selector at the parent exchange. The caller is now able to dial to subscribers in the parent exchange, or to exchanges connected to the parent exchange.

Call to Non-Parent Exchange
The caller dials the call digit (e.g. 8 in picture above) and seizes a junction to the required exchange. If the exchange dialled is a manual exchange the operator is signalled and can connect the caller with the desired line.

If the exchange dialled is automatic, subsequent impulsing operates the auto equipment to select the desired number on that exchange or to route the call to another exchange.

When a caller dials a digit which results in a junction to another exchange being seized, the multi-metering and route restricting relay set associated with the junction relay set functions and performs one of the following operations

(1) Connects the appropriate fee lead to the meter wire. Provision is made for 1, 2, 3 or 4 unit metering.

(2) Connects N.U. tone to line of the number dialled is a spare code or is barred to the caller.

(3) Provides manual hold conditions on a call to a manual board obtained via a distant auto equipment.

If the multi-metering and route restricting relay set is unsuitable for dealing with the required number of codes, additional equipment is provided to cover a larger number of codes. This equipment is common to all multi-metering relay sets and if a call is originated to the common equipment while it is in use on another call, busy tone is returned to the caller.

Incoming Calls from Parent Exchange Manual Board
The operator plugs into a jack in the outgoing junction multiple and dials the required number. The operator does not listen for dial tone; the linefinders are homing type switches and the incoming junctions terminate on the first choices on the line finder banks and therefore it is unlikely that dialling will commence prior to the linefinder finding the calling line. If however this does occur busy tone is transmitted to the operator at the conclusion of dialling.
If the operator dials a free line, ringing is connected in the usual manner and ringing tone is transmitted to the operator.

If the called line is engaged the operator receives busy flash and tone, but provision is made so that the parent exchange operator can gain access to the engaged line, for the purpose of offering trunk calls, etc. by the momentary operation of the ringing key. The call is offered to the engaged line and the subscriber is requested to clear the line. When the clear is received on the cord circuit supervisory lamp the ringing key is again momentarily operated to apply ringing to the wanted party’s line.

Incoming Call from Parent or Non-Parent Auto Equipment
The linefinder finds the calling junction and on receipt of the dialled impulses the auto equipment functions in a manner similar to that on a local call.

Alarms and Test Number Equipment
Alarm extension arrangements are not provided on this type of board but provision is made so that an operator at the parent exchange can dial a test number and ascertain, by the tone received, the condition existing at the U.A.X.

The above description gives a general idea of the layout, facilities and operation of unit Auto No. 12 which is now in use throughout this country and is giving good service. It may be of interest also to mention that there are two similar types of equipment available viz:- unit auto No's. 13 and 14.

Unit auto No. 13 has capacity for 200 lines and replaces unit auto No. 6 while U.A. No. 14 will replace U.A. No. 7 and has capacity for 800 lines. Both of these exchanges are provided with the 2000 type selector. The transmission feeds are of the ballast type and balanced tones are provided.


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Last revised: November 03, 2002