PAGE No. 27

The Unit Automatic Exchange No. 14
January 1939

THE U.A.X. No. 14 is the largest “unit“ type exchange and supersedes the U.A.X. No. 7. It is designed for exchanges where the ultimate requirements exceed 200 but do not exceed 800 subscriber’s lines.

Five of the racks in a U.A.X. No. 14 have been specially designed for this particular type of exchange, the others are similar to those in use for main exchanges. The special racks are designated Units 14A, 14B, 14C, 14D and 14E whilst the remaining racks required to complete an exchange are the meter rack, meter pulse machine rack, trunk distribution frame, combined main and intermediate distribution frame, traffic recorder rack and test rack.

Usually the test rack is provided only when the number of calling equipments exceeds 500.

Unit 14A Equipment
This Unit accommodates 100 subscribers’ line relay equipments, 10 line finders, 7 selector hunters, 2 allotters and control relay sets and 20 final selectors, the last including I test final selector on every alternate rack. As the trunking equipments rarely justify the provision of more than 10 final selectors per unit, the top shelf, accommodating the last 10 switches of this type, is not fitted as a standard but can be added when desired.

The illustration to the right shows the front of the unit will make clear the disposition of the equipment referred to above.

Unit 14B Equipment
Accommodation is provided on this unit for five shelves of group selectors, each shelf taking ten switches. The shelf complete, consisting of shelf ironwork, selector banks, shelf jacks, connection strips and all associated wiring, is however an individual item and the number fitted is dependent upon trunking requirements.

The shelf jack wiring is arranged so that either 1st or 2nd group selectors can be
jacked-in as required.
Second selectors are provided only when the number of junction codes exceeds five, or where it is necessary to employ 2-digit junction codes.

The unit resembles in appearance the standard group selector rack used, in non-director main exchanges.

Unit 14C Equipment
The equipment of this unit is not of a standard character and varies according to the particular exchange requirements. It accommodates the equipment, mainly comprising “strip-mounted sets,” required for junction working.

These strip-mounted sets deserve special mention. They consist of one or more relay mounting plates and, in some cases, a uniselector in addition, and cover the whole of the equipment associated with a particular circuit. The plates concerned are linked together to form one unit and all components are connected by means of a local cable to form a complete item. A small connection strip is mounted at the rear of the strip-mounted set for the termination of leads requiring external access. A test jack is fitted on each of these equipments and access is given to it without removing relay plate covers.

The drillings on the rack are of a universal character and although the strip-mounted sets are of various sizes they can be fitted in any desired position.
The illustration to the right shows front and rear views of a unit 14C with a typical equipment.

Unit 1 4D Equipment
This unit accommodates the following miscellaneous and special service equipment.
Shelf G. 1 service observation equipment.
Shelf F. 4 test and plug-up line equipments.
Shelf E. 4 changed number equipments and 1 alarm delay equipment (6 seconds).
Shelf D. 1 service interception finder, 1 howler and 1 P.G. milli-ammeter.
Shelf C. 4 1 N.U. tone relay sets.
Shelf B. 1 test number, 3 service interception and 1 alarm extension relay set.
Shelf A. 1 test selector, 3 alarm delay relay sets and 1 howler relay set.

Although space for the service observation equipment is allowed on the upper half of the unit it is an individual item, arranged as a complete and self-contained panel, and is bolted direct to the uprights when required.

The provision of service interception equipment is also variable but the wiring for this is fitted as a standard.

When the exchange does not exceed 400 lines only two N.U. tone relay sets are fitted.

The illustration to the right shows a front view of a typical equipment, without service observation and interception sets.

Unit 14E Equipment
This unit is only fitted at exchanges which have routes to director areas. It has a capacity for mounting 30 route discriminating (junction) relay sets, 5 route discriminating (common) relay sets and 5 route discriminating (common) selectors.
The selectors are of the 8-level, 2-motion type, and function in a similar way to the BC selectors as used in directors.

An exchange is built up by a combination of the above described units and racks according to the number of subscribers lines, trunking and junctions required. The number of A units may be 1-8, i.e. one per 100 subscribers, B units, 1-4 according to the trunking requirements, C units, 1-33, as required for junctions, D units, one, E units, 0, 1 or 2 depending upon whether or not access is required to director areas, one meter rack, one meter pulse machine rack, one or two trunk distribution frames, as required, one traffic recorder rack, one combined main and intermediate distribution frame and 0 or 1 test rack as previously stated, together with a power equipment as described below.

As the buildings housing these exchanges will be provided with heating it has been found unnecessary to totally enclose the units in sheet steel cabinets as in the smaller types of U.A.X. equipment.
Open type construction has thus been adopted throughout and the rack design is as far as possible in accordance with the G.P.O. standard 2,000 type equipment.

The units and racks are all 8 ft. 6.5 ins, in height and the width of the A and B units is 4 ft. 6 in. and that of the C, D and E units 2 ft. 9 in.

Apparatus Details
The selectors and relay sets are of the B.P.O. 2,000 type and incorporate ballast transmission bridges and balanced tone feeds.

All relays used are of the B.P.O. 3,000 type except those for the subscribers’ line ringing machines of the tone circuits (L & K relays) which are the B.P.O. 600 type.

The miscellaneous equipment associated with a particular rack is mounted on the right-hand upright of that rack.

The power plant standardized for all U.A.X.’s No. 14 is of the Post Office parallel battery float type, in which the two batteries are permanently connected in parallel and to the exchange. When they have become discharged to the extent of 4% the charging plant is automatically brought into operation and supplies energy direct to the telephone system via appropriate smoothing equipment. The batteries supply any current required by the exchange in excess of the output of the charging equipment and are eventually recharged when the exchange load falls. When the
batteries are fully charged the charging unit is automatically shut down.

The voltage at the exchange bus bars is maintained between prescribed limits, during the charge and discharge cycles, by the use of counter E.M.F. cells which are automatically brought into operation under the control of a contact-voltmeter and a group of contactors.

The following equipment is provided

(a) Batteries. Two main batteries each consisting of 25 secondary cells ; a group of 8 counter E.M.F. cells of the alkaline type for voltage regulation purposes; a group of 7 counter E.M.F. cells of the lead acid type for P.B.X. 30v. and a positive battery of 37 Leclanche cells.

(b) Charging Equipment. For A.C. supplies two rectifiers (initially) are connected in parallel. These are of the metal or mercury arc type according to the output required, and are provided with suitable smoothing equipment.

For D.C. supplies two motor generator sets are provided, one being a stand-by machine. A single smoothing unit is provided for use with either machine.

(c) Control Panel. This panel accommodates all the apparatus associated with the automatic control of the charging equipment in addition to that which functions to maintain the voltage of the battery between the allowable limits. Included on the panel are contact voltmeters, contactors, ampere-hour meter, ammeter and all the associated relays and miscellaneous equipment.

(d). Ringer Panel. This panel accommodates ringing machines of the tone inductor type, ringing changeover equipment, ringing distribution circuit breakers, tone chokes and transformers and miscellaneous equipment. For an A.C. power supply one mains driven and one battery-driven ringer are provided; for D.C. supply both machines are of the battery-driven type.
The picture to the right shows a typical board assembly comprising control panel and ringer panel mounted side by side.

The subscribers line circuits terminate on the banks of 100-outlet 2-motion type line finders some of which are directly connected to first selectors while others are connected via selector hunters to a common group of first selectors.
A four-digit numbering scheme is employed for subscribers lines, the initial digit being either 2 or 3 which is used as a discriminating digit and is absorbed by the first selectors. Levels 1, 2, 3 and ‘0’ of the first selectors are connected to four groups of 200-line final selectors, so that by this arrangement a numbering range of 2000-2399 and 3000-3399 is available.

Level 9 is used to gain access to the automatic equipment at the 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 exchange 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 4, 5, 6, 7 and 8 are used for dialling out to adjacent exchanges. When it is necessary to provide additional dialling out codes a rank of second selectors can be provided and connected to one of the levels, e.g. if the 2nd selectors are connected to level 4, then this will mean that additional dialling out codes 41-40 are available.

The junctions from the parent exchange terminate on individual 1st selectors junctions from adjacent exchanges are given access to a common group of first selectors via selector hunters.

Junctions can be bothway as shown on the typical trunking diagram or unidirectional, the particular arrangement provided depending on the traffic conditions for the exchange concerned.

Local Call
When a subscriber originates a call the control relay set associated with the group in which the call is originated, connects itself via the allotter to a free line finder and causes the latter to search for the calling line. The allotter is arranged so that it routes calls via directly connected linefinders until all of these in the group are engaged, after which, calls are routed via the indirectly connected linefinders.

The control relay set and allotter are in use only during the searching time of the linefinder and immediately the calling line is found the allotter pre-selects another free linefinder in readiness for the next call.
When the linefinder finds the calling line the subscriber is extended through to a first selector and receives dial tone.

The initial digit dialled on a local call is either ‘2‘ or ‘3‘. Assuming that ‘3‘ is dialled then the selector steps to level ‘3‘, operates a discriminating relay and then releases. The next digit will be 1, 2, 3 or ‘0’ and the dialling of say ‘3‘ causes the first selector to cut in on level 3 and search for a free final selector.

The discriminating relay in the first selector, which was operated at the end of the first impulse train, now passes forward a signal to the final selector and causes it to connect the wipers ready for selection, in this case, in the 3300 group. If the first digit dialled had been 2 instead of 3 the wipers would have been connected for selection in the 2300 group.

The 3rd and 4th digits step the final selector wipers in a vertical and rotary direction respectively and the called party’s line is tested. 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 groups 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 once and conversation can proceed.

Call to Parent Exchange (Manual Board)
The calling party is connected to a selector as described above and dials ‘0’. This causes the selector to step to level ‘9‘ the tenth impulse being absorbed. The first selector searches for a free junction to the parent exchange and signals a manual operator at that exchange. When the caller is barred trunk calls (e.g. coin box lines) the operator receives a visual indication which informs her that she must collect the necessary fee.

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

Call to Dependent or Non-Dependent Exchange
The calling party dials the call digit to reach the required exchange, i.e. 4, 5 or 6 etc. If the called exchange is manual the operator is signalled and can complete the connection in the usual manner. If the called exchange is automatic subsequent impulsing operates the auto equipment at that exchange to select the desired number or to route the call to another exchange.

Route Discriminating Equipment
When a junction to another exchange is seized the route discriminating and multi-metering equipment associated with the junction equipment counts the dialled impulses and performs one of the following operations:-
(1) Connects 1, 2, 3 or 4 unit metering to the meter wire.
(2) Connects N.U. tone to the caller if the number dialled is a spare code or is barred to the caller.
(3) Provides manual hold.

In director areas the route restricting equipment is unsuitable for dealing with the required number of codes and in this case common equipment is provided which will cover up to 800 codes.

Incoming Calls from Parent Exchange Manual Board
The operator takes into use an outgoing junction and engages the first selector associated with the junction used. The operator can now dial any number on the exchange in the same way as a local subscriber.
If the called line is engaged the operator receives busy flash and tone. Provision is made so that the parent exchange operator can gain access to the engaged line for the purpose of offering trunk calls. This is achieved by the momentary operation of the ringing key, the call being offered to the engaged line and the subscriber is requested to replace the receiver. When the operator receives the clear on her supervisory lamp the ringing key is again momentarily operated to apply ringing to the wanted party’s line.

Incoming Calls from Parent Auto Equipment
The calling party dials the digits to gain access to a junction to the U.A.X. The incoming U.A.X. first selector is seized and subsequent impulse trains will cause the selection of the desired number as for a local call.

Incoming Call from Non - Dependent Exchange
When an incoming junction is seized the selector hunter searches for a free first selector. If the calling party dials before a first selector is found, busy tone is transmitted to line.
Assuming that the calling party dials after a first selector is found ther the U.A.X. equipment functions as for a local call and the desired line is selected in the usual manner.

Incoming Call from Dependent Exchange
On calls to local lines the equipment functions as for a non-dependent exchange incoming junctions. When the calling party dials ‘ 0’ a discriminating signal is passed forward to the incoming junction equipment which causes the selector hunter to search for a junction to the parent exchange instead of a 1st selector in the U.A.X.

Control Relay Sets
One control set normally serves each 100 lines associated with a particular unit, provision is made however so that in the event of failure of a control set the control relay set belonging to another 100 lines is arranged to serve both its own group and the group in which failure occurs. This is arranged by pairing units 1 and 2, 3 and 4 etc. these acting as partners. If failure of the control set serving unit I occurs then the control set for unit 2 will take over units 1 and 2 until such time as the fault on the control set for unit 1 has been cleared.
If the control set for unit 2 fails then the control set for unit I will serve both groups. Units 3 and 4 etc. are arranged to function in a similar manner.

P.B.X. Working. P.B.X.
facilities for groups of 2-10 lines are provided throughout and when required it is possible to cater for 2-20 line groups.

Alarm arrangements
The alarm arrangements follow non-director auto exchange practice, provision being made to extend urgent alarms to the parent exchange. An exception to standard practice is the P.G. alarm scheme whereby P.G. alarm lamps are fed via a contact ammeter, an alarm being given when a predetermined number of P.G.s occur.

Other Facilities
Provision is made for service interception, changed number equipment, meter observation, service observation, traffic recording and test facilities. For the latter, test selectors and test final selectors are provided to enable tests to be made from the parent exchange.

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Last revised: November 03, 2002