Reference / Paper · 1936
POEEJ July 1936: The Development of 2000 Type Selector Circuits
British GPO 2000-type Strowger selector design and circuits.
Reference / Paper · 1936
British GPO 2000-type Strowger selector design and circuits.
THE POST OFFICE ELECTRICAL· ENGINEERS• JOURNAL VOL. JULY, 29 PART .2. 1936 CONTENTS PAGR THE DEVELOPMENT OF 2,000 TYPE SELECTOR CIRCUITS-R. H. Chapman, B.Sc. (Eng.), and J. S. Groome RECRUITMENT AND TRAINING OF .. 87 . POST OFFICE WORI<MEN- E. J. C. Dixon, B.Sc. (Eng.), A.C.G.I., A.M.I.E.E. 94 AN OUTLINE OF THE PRINCIPLES OF ATOMIC PHYSICS. II. THE PHENOMENA OF IONIZATION IN GASES AND THEIR SIGNIFICANCE-F. C. Mead, B.Sc. (Hons.), A.R.C.S. 105 TELEGRAPH AND TELEPHONE PLANT IN THE UNITED IUNGDOM .. . 109 HARDNESS TESTING-Ceo. F. Tanner, M.I.E.E. 110 THE TELEPRINTER ANCILLARY-S. G. Lewis, A.M.I.E.E., and C. E. Easterling, B.Sc. (Hons.) THE 119 ... GUERNSEY-CHALDON ULTRA SHORT WAVE RADIO TELE- PHONE CIRCUIT-A. H. Mumford, B.Sc. (Eng.), A.M.I.E.E. . . 124 . THE KING'S HOUSE SUBSCRIBER-ATTENDED P.A.B.X. - W. A. ... Phillips, A.M.I.E.E. THE TEST DESI{ SERVING METROPOLITAN, NATIONAL 130 AND LONDON WALL EXCHANCES-W. Woodcock and H. L. Claxton ... 135 THE UNIT AUTOMATIC EXCHANGE No. 13-C. G. Grant and A. J. C. Henk 141 ... THE CARRIER SYSTEM No. 3-N. W. Lewis, Ph.D. (Eng.), A.M.l.E.E. .. . 152 CARRIER TELEPHONY II-G. J. S. Little, B.Sc., A.M.I.E.E. 156 NOTES AND COMMENTS 161 DISTRICT NOTES 162 THE INSTITUTION OF POST OFFICE ELECTRICAL ENGINEERS 167 JUNIOR SECTION NOTES 168 BOOK REVIEWS STAFF CHANGES ... 161, 169 ... 170 Price I/· net TIIE INSTITUTION OF POST OFFICE ELECTRICAL ENGINEERS .QVER 250,QOO YARDS OF POST· .OF-FICE TELEPHONE CABLES HAVE NOW .BEEN PERNAX SHEATHED AND ARE LAID ALL OVER THE UNITED KINGDOM AS SHOWN SCOTLAND. ·PLASTIC PERNAX TAPE Districts with over 500 yards:Edini..,i.•.,"'.: IS ALSO LARGELY USED Girvan Glasgow BY THE POST OFFICE ENGINEERING DEPT. Because it is WATERPROOF ADHESIVE, (but NOT STICKY) and EASILY USED. NORTH. IRELAND. Districts with over 500 yards: Carlisle Chester Malton Salford Districts with 0'-"'� 500 yards:Belfast - MIDLANDS. Districts with over 500 yards:Birmlngbam Martock Districts with over 500 yarda :Boaitor Heathfleld Holmwood Brlahton Reading Canterbury Tunbridge Well& Chichester Exmouth LONDON. Coring Grove Ferry Guildford PERNAX i3 manufactured only by CROYDON CABLE WORKS LTD., MITCHAM ROAD, CROYDON Who also manufacture VARIOUS PROTECTIVE DEVICES and PAPER INSULATED LEAD COVERED CABLES 'Phone : (4 lines) : Thornton Heath 2 882 iii . .e TELE PH ON ES Used all over the World by Administrations demanding the following features:- Complete range for all systems. Highest overall efficiency. . Maximum side-tone suppression. Robust cradle. Stainless steel dial. Long-life cords. High-insulation terminal strip. Complete tropical finish. THE GENERAL ELECTRIC COMPANY COVENTRY TELEPHONE WORKS Coventry 4111 Head (8 lines) Office : Temple Bar 8000 (70 lines) LIMITED ENGLAND Magnet House, " Polyphase, London " (Cables) "Springjack, Coventry." Kingsway, London, W.C.2 " Electricity, Westcent, London " ('Grams) IV. TURN ER DIFFERENTIAL MILLIAMMETERS C.P.0. No. 17 & No. 18 exacting demands for balancing currents multiplex telegraph circuits. ---- prod u c t of A quality ou tsta n d i n g merit ---- Also manufacturers of G.P.O. Detector No. 4, and a compre hensive line of instruments for precision electrical measure- /� CHILTERN WORKS, -� 55-73, TOTTERIDGE AVENUE, ---�hone : Hirh ments. Wycombe 301. Telerr•m• · G11,.. .01n. Hirlt Wyco.M. HIGH WYCOMBE, BUCKS. ENGLAND F•RR1tM1rll A.C./D.C. CIRCUIT TESTER This new moving coil instrument measures D.C. milliamperes, D.C. volts, A.C. volts and ohms. D.C. readings are first grade in accordance with B.S.I. No. 89 and A.C. readings are correct within 3 per cent. at full scale reading. The instrument is complete with self-contained I �-Volt "cell. RANGES D.C. MILLIAMPS 0-1, 7.5, 30. 150 and 750. 1000 D.C. VOLTS 0-3, 15, 150, 300, 450 and 600. ohms A.C. VOLTS 0-15, 150, 300, 450 and 600. 20 to 4000 cycies per second. per volt. A.C. MILLIAMPS 0-1. RESISTANCE 0-50,000 ohms self-contained and up to 0-30 megohms with external D.C. source up to 600V. DIMENSIONS 3�" x 4f' x 2!" Jj PRICE £5 I Ss. including substantial case and all accessories. Write for Li•t In. 1. FERRANTI _LTD. HOLLINWOOD, LANCS., London Office: Bush House, Aldwych, W.C.2. v. Ericsson Telephone Equipment is made at Beeston, Nottinghamshire. This fine factory keeps well ahead in telephone development and progress. A manufacturing and technical experience extending over 50 years is helping us all the time to maintain a high level of telephonic efficiency and reliability. We manufacture, supply and instal complete automatic exchanges of the "director" and "non-director" types for any number of lines. Informative Literature sent free. ERICSSON 67/73, TELEPHONES KINGSWAY, Telephone: Ho/born 3271/3 Ericuon Automatic Switch Board. LIMITED LONDON, Wor.ki: W.C.2 B•<1ton, Nott• TELEPHONE EXCHANGE EQUIPMENT XVI. • Every once in a while there comes hot from the presses a news item of an importance which ensures for it not only the instant, deep-seated interest of every reader, but a place in the annals of History. Of such a nature is the announcement of the introduction of the new Type 3 2A Strowger Selector, for it represents an ad vance in design and construction which marks the commencement of a new era in the field of automatic telephone switching. The Type 3 2A Selector is not merely an episode in automatic switching develop ment. six It is the successful culmination of years intensive investigation and research in the Strowger Laboratories at Liverpool - investigation and research • ''News'' which has now succeeded in producing a new mechanism finer even than its wellproven predecessor and, more important still, in providing yet further evidence of the superiority of the two-motion decimal principle of automatic switching. Future text-books of telephony-the History of the art of Telephone Communi cations-will mark 1936 as the year of the new Type 3 2A Strowger Selector. AUTOI Phone : Old Swan 830 MELBOUR� Phone; Temple Bar 4506 XVII. that TYPE 32A makes • THE NEW STROWGER (B.P.O. TYPE 2000) Prominent among the many outstanding advantages which the new Strowger Selector possesses over its predecessor are the following : Its overall size has been substantially reduced, which means that racks occupy less floor space and con siderable economies result in building costs and rentals. It has three times the bank capacity-as a point of in terest, the number of contacts in the bank is more than double that provided by any uniselector and greater than that of any power-driven selector. Its speed of operation has been almost doubled ... any one of 2,200 contacts being capable of selection in 600 milli-seconds. The mounting arrangements of the t1ATI C IE HISTORY! Selector are excellent ; it can be instantly withdrawn from position, and is self-locating when replaced, no latches, bolts or nuts being required. In addition, all adjustments can be made with the Selector in position. The design of the new Selector definitely ensures greater stability of the various adjustments, thus reducing main tenance costs. The new Strowger Selector has been adopted by the British Post Office, the South African Post Office, the Australian Post Office, the Indian Government, the Bombay Telephone Company, the Polish State Administration and other important Telephone Authorities. co. ELECTRIC STROWGER WORKS, SELECTOR LIVERPOOL LTD .. , 7. Telegrams 'Strowger' Liverpool HOUSE, ALDWYCH, LONDON, W.C.2. Telegrams 'Strowger' £strand, London viii. THE SYMBOL OF Rtri1tered Tradt Mark SERVICE PHOTOGRAPHIC For over half a century Ilford Limited have specialised in the manufacture of photographic materials for every industrial and scientific purpose. The high quality of these products has won for them a world-wide reputation for unvarying consistency ilnd they may be relied upon implicitly for use in every branch of research and record photography, oscillographic service as well as ordinance survey. Write for copy of booklet "Photography Work Scientific to an aid as ." ILFORD PLATES - FILMS - PAPERS MAKERS OF THE FAMOUS Made in England by LIMITED, ILFORD ILFORD, LONDON �s:(tIJ> & 5@hhrome RollFilms For quick and econo mical t ran s missio n te legraph The TELEPRINTER The British Post Office, fol progressive their lowing policy, have been steadily re placing Morse Apparatus by Teleprinters during the last few years. This change-over enables I 00 telegrams an hour to be dealt with on each circuit as approxi with compared mately 30 by the Mor�e System. Model No. 3a Tape Teleprinter Today there are over 3000 Teleprinters constantly at handling telegrams work with great efficiency and economy Model No. 7 Page Teleprinter CREED & COMPANY, LTD., Te/ephone Telegraph House Telex- Croydon 2121 CRQyDQ N Croydon Telex 1082 OPEN LETTER TO PARENTS Dear Sir a1· M.J.da m,- When your children first arrived they bro•Jgbt with them a wonderful lot of sunshine. Later you became proud of the intelligence they di1iplayed, but still later you became anxious as lo what would become of them in the fu rur e. Perhaps you were anxious when you visua• lised them as grown men l\nd women. Even with plenty of money it it not alwaya easy to select the right career, and 11 parent is 1ometimes inclined to ask cdvice of some relative and in ni nety• nine case; out of a hundred that relative knows nothing at all about the po11ibilities ol employment. Why not let me relieve you of !f.Dme of your anxieties? In fact, why not let me be their Father? We do not profeS9 lo acl as an employment agency, but the nature of 01.1r butineu compels us to keep an e y e upon tbe clas5 o f men and women that a r e wanted a n d w h o wants them. There are 1ome people who manufacture an artide and put it on the market to sell. We do not do tliat, we work i n exaclly the opposite direction, W e fi.nd out what employen want and w e train our atudenh t o 611 those job•. We have to be experts in the Maller of emplo>yment, progreu and rosp erit y. If you have a n y anxietiu a t a l l a ' t o what your 1ont a n d daugbten should be, write to me, or better •till, let them write to me personally--Fatherly Advice Department-and tell me their likes and dislikes, and I will give 1ound practical 11dvice 111 to the possibiliries of a vocation and how to 1ucceed in it. Yono sincere!)', THE HUMAN MACHINE (Sscrets of Succesa) � p It teaches you : How to Memorize , ..Self-confidence... How to De· velop Pe:rsonality... How to think • . • Bl.It you cannot earn money by thinking how to think. You must do something. You must study your job. Send for Free Particulars of the Course that interests you and at the same time ask for a free copy of the Human Machine. YOU TO INTO SKILLED A CAN YOU CHANGE MY EXPRESSION? Just try it for yoursdf. trace or draw tbe out line and then put in the features There are hundreds ol openings In connection with Humorous Papers, Advertisement Drawing, Poster1 Cataloauu, 'textile Deaign1, llluatrallona, etc. Accountancy Examinations In su rance Advertising & Sales Manage- Mathematics ment Matriculation. Metallurgy A.M.I. Fire E. Exam. Mining, all sublects Mining, Electrlcal ADplied Mechanics Army Certificates Engineering Auctioneers & Estate Agents Motor Engineering Aviation Engineering Motor Trade Banking Municipal & County Engineer! 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Calendan, C O L L EG E FOR A FEW S HILLINGS MONTHLY KEY POSITION. .. H A VE A TRAINING IN ALMOST AN Y CAREER NOW IS YOUR CHANCE CAN Boo k ff ,,,,.t ;{" ,,�1 set i•,,1u .nun ,.,eq11i•-.:111,,11!s tdJvr, 1C1rifr to tis 011 an.v sub}tcf HOW TO STUDY In your spare time when it suits VOU. You fix your own time, you do not GO to your There is nothing that a class-room teacher can show on a blackboard that we cllnnot show on a white p.iper. The lesson on a blackboard will be c\ea11ed off, but our lessons an., P.ERMANENT. A class.room teacher cannot give you a priva te word of encouragement, but a Cor respondence Tutor can do so when ever your work deserves It. On the otlier band he can, where necessary, p oin t your out mistakes PRIVATELY. TO STUDE NTS ABROAD LIVING or on the high seae, a good supply of lessons ls given, so that they may be -dona in their order, and despatched to us Jor examination an<l correction, They are then sent back with more work, and in this 1vay a co1t/inuo1u stream of work is always in transit from the Student to us and lrnrn us to the Student, therefore distance makes no difference. IT IS THE PERSONAL TOUCH EVERYDEPATMENT IS A COM· WHICH PLETE COLLEGE. COUNTS JN POSTAL TUITION EVERY STU. DENT IS A CLASS TO HIMSELF 60 per cent. of Commercial Art Work is done b\ "Free Lance Artists" who do tl1eu work at irnrric and sell it to the highest bidders Many Commer cial Artists draw "retaiuing lees" fron1 various sources, others prefer to work full-time employment or partnership arrangement. \Ve leach you not only how to draw what ia wanted b u t how to mal1c buyers w11nt whit you draw. Many of our student� who or_iginally took up Commcrcrnl Art as a hobby h_ave s1i:ice turned it into a full-lime paying profes sion with studio and staff of assistant artists: thne is no limit to the possibilities. Let us send Juli particulars for a FREE TRIAL and de1tails o f our course for your inspection, You will b e 1.:ndcr no hlii:ation whateve.i. o Art Dept. 24 studies--thc po�tman brings THEM TO YOU. Dept. 24 THf BfNNETI COLLEGE. SHEFFIELD x. Btandard ES CABL E N O H P E L TE SES O P R U P ll a Jot Our illustration shows a "Minor trunk" Cable in process of installation for the British Post Office, on the Slough-Ascot route. Our engineers are always available for the purpose of giving expert advice upon matters relative to the manufacture and installation of every known type of telephone cable or Complete Communication Systems. Bia11dard Telephones a11d Cables Limiied COMMUNICATION NORTH Telephone : WOOLWICH, ENGINEERS LONDON, E. I 6 Albert Dock 1401 THE POST OFFICE ELECTRICAL ENGINEERS' JOURNAL Vol. XXIX July, Part 2 1936 The Development of 2,000 Type Selector Circuits R. H. CHAPMAN, B.Sc. (Eng.), and J. S. GROOME A survey of the circuit changes introduced concurrently with the adoption of the new 2,000 type selector is given and the advantages and new facilities afforded thereby are enumerated. Inf roduction. N recent articles which have appeared in this I Journal, 1 the development of the new Post Office standard two-motion selector (2,000 type) has information interest. has been included which may be of In publishing this article the authors also wish to make an acknowledgment to the Automatic Electric Company, who produced the basic design been comprehensively dealt with from the point of from which the standard mechanism was evolved, view of ·the mechanical features, racking, mounting and to whom was assignee! the prime responsibility arrangements, etc., and it is thoug•ht that an account for the ci·rcuit development. of the circuit aspect may be of interest, particularly in view of the imminent installation of new exchanges employing this selector. The standardization of the forward rotary release Ba/a.11ced Tones. Some typical examples of ! he methods of tone application hitherto employed are shown in Fig. I, feature (already adequately described) rendered all and from i.nspection of these it will readily be appre existing standarcl two-motion selector circuits obso ciated that the speaking pair to which the tone is lete and new circuits had .therefore to be developed. connected is unbalanced in respect of tone potential. Since from the outset it was intended, where possible, This condition of unbalance :has always given rise to employ the new selector on extensions of existing to a certain amount of overhearing which has in some exchanges, radical departures from previous practice instances reached inconvenient dimensions, particu had necessarily to be restricted in considering the larly where star-quad cable has been employee!. development of the new circuits. Nevertheless, ----<>� opportunity has been taken to effect improvements in facilities and performance where possible. Some of It ·�-n-- HA these, suoh as balanced tones, are purely incidental and could have been introduced independently, where as others have only been made possible by utilizing "=f the following features ·which are of advantage to the circuit designer and are peculiar to the ne1Y DiBI mechanism :- Tone * 1. Vertical and rotary stepping by se:f-interrupted drive can rnadily be obtained, with conse 2. Owing to the robust mechanical design, the limitations in bank Busy Tone ..i. Group Selector quent saving in hunting time. previous sl capacity 2 no longer apply. 3. Certain restrictions hitherto imposed on the number and types of mechanically operated springs have been removed. ln the ensuing artiole 18.n attempt has been made to show the influence of the above characteristics on circuit development and to describe in general terms the mai.n innovations incoroorated in the new sta.ndard circuits. In a few instances some existing pri1nciples have been discussed for purposes of com parison and a 1 Vol. 29, Part 1. \"UL. XXIX. certain amount of relevant general FIG. 1.-ExrsTING ;\lETJIODS OF AI'l'L\'I:O.:G To:-.:E. April, 1936. G 87 will a l s o be e\·ident that the volume and quality nl' any particular tone rece ived var ied accord ing tu the method of application and therefore t;he non uniformity of the methods used gave rise to con siderable variation. In the 2,000 typ e circuits the tone is feel through a 570 ohm third w inding , wound over the 1normal line coils of a b ridge relay, usually the A or I relay. The tone wi·nding thus functions as the primary wi.nding of a transformer , of whic.11 the line coils of the relay constitute the secondar y Some typ i cal B ridg e relays «tre windings. illustrations m-e gi v e n in Fig. 2. normally sandwich-\\·ouncl and since the tone winding is \nrnnd on the outside or the coil, some small un balance is introduL-cd on account of the difference 1n normal!y used in group selector circuits and suffers from both the clisaclvantages of (!) and, in add ition, necessarily limits the hunt ing speed. 3. Relay inter-action drive, combined with a battery-testing relay (used in A. digit hu nt er, line finder circui ts, allotter etc.). This method was designed to obviate double con nexions a1nd employs a two-coil testing relay, one 1winding being of high resistance and the other (the hold winding) of low resistance. The free condition on the outlet is a r esist ance ba t t ery and the testing relay op c ra:tes to this with its coils in series. The hig1h resist ance coil is, on operation, short-circuited by the low resistance guards the outlet. winding , which then 'f.11eoretically, the relays should be adjusted so t.Jrnt, if two are applied simultaneously to the same outlet, both will commence to operate, but will not hold with their low resistance win d ings in pamllel. current adjustment specificat i on, thi s principle, is usually so the relay m ai ntain-. Group Seler:tor is di ffi cult G en e r.ally, tu .based A on stringent that manufacture however, and the holdi ng current required by the relay under circuit conditions, is greater than under conditions of maintenance tes tin g, owing to the short time of application of the operating current, and thus the current adjustment specification limits can be made wider .than theoretically necessary M 2 ,st code & JJ'oigit .. 1H selector Q �:j: will, ·'·· I � fo'��-1�-- _J '., f1<;_ 2.-i-IEW i\IE'l'llUllS UF Al'f'I.YING TuNE. niupling bet ween the to n e winding and t-he '1 wo coils of the line winding. In spite of this, the arrang e llll.'.nt is far sup erior to the sup ersede d mdhods, from an o ve Pl10aring poin t of view. The new balm1ced method of tune application is be in g introduced concu rr ently with the new selector o.n all circuits employing tones. l-I1111ti11g Arra11ge111c11ts, Battery Tcsli11g and Dri·ve Tripping. The standa rd methods of hunting and hunting control are enumerated below, together with some brief notes on t lwir v arious disa dvantages :- 1. Self-i11/crrupted dri·ve i11 co11j1111ctio11 wit/1 1111 ear/Ii-testing, i/ri11e tripping relay. This is the m e thod employed in the subscribers' uniselector circuit and in common with all earth testing circuits, allows the switch to stop on a disconnected outlet. Further, double connexions are not theo r etically pre cluded, although the unguard period, during which th ey can occur is sufficiently small to gi ve reasonable im m unity, in view of the relative improbability of simultaneous testing. 2. l?c/111• i11/cr-acliu11 dri·vc i11 cu11j1111ctio11 with a11 : ca1 /11-tesli11g relay. This arrang eme nt is 88 no and this is normally done. doubt, be appreciated that It the amount of relaxation ,permiss ible can only be determined by t ests and the construction of a suitable specification ba sed on t<he practical performance of a large number of samples is a matter of considerable diffi c u lty . l\'1ore over, in spite of the possibility of relaxing the theoretical adjustment limits, the specification is still strin gent .from a m anufa ct uring and mainte.nance point of view, particularly where, as is often the case, conditions of triple t esting have lo be met and the effect of d eri ve d networks has a:lso to be considered. In th e case of the 2,000 lype group selector, it was desirable that the principle of ear bh-tes t ing should be continued in order lo enable the .new s elector to be used on extensions of e x ist in g eq u i pmont, but, in order to take advantage of the high h un ting speeds possible with self-inte r rup ted drive, a drive tripping earth-testing relay arrangement was developed . :\Jthough the scheme is, of course, not immune from double eonnexions, the unguard period is less than heretofore. The essentials of the ar.rangement are shown in Fig . 3, relay H b eing· the drive tripping fIG. 3.-DmvE TIUPPING, Kiln)[ TESTING CIRCUIT. relay. Incidentally, this relay, <�!though used for . holdi1w, is not employed to sw1toh the speakmg wires 1rough, bhis function being performed b.y the C relay in conjunction with ·rotary off-norm3:1 s�nngs. . Relay H is operated over tts 500 ohm wmdmg as soon as the switch is stepped off normal. Relay C releases at the end of the vertical impulse train in the normal way and completes the rot•a:y self-.interru·I;1ted drive circuit. Relay H is ·held by its 500 ohm wmd ing until the rotary interrupter springs break, wihile tl the wipers a•re on the outlet to be tested. If the outlet is busy, H is maintained by the private ea;th during this " break " period, but releases and tnps the rotary drive if the line is free. The releas e of . . . relay H causes .C to •re-operate and switch the pos1t1ve . . and negative wires through. The chsconnex1011 period of the rotary interrupter springs is ?f th� order of 10 milliseconds, so that the release t1111e of H must be weLl wit.bin this period. By dispensing with the standard enla·rgecl core encl and using a nickel iron core to reduce eddy-currents and thus accelerate flux decay, a design of relay has been pro duced which will c�nsistently meet ·bhe required con dition with an adequate margin. It will be appre ciated that the preclusion of unwanted disconnexion periods on the private during r_otary hunting is essential to the soheme and necessitates rather more strincrent mechanical adjustments than would other wise required, but this disadvantage is more than off-set by the gain in hunting speed. l)e For other 2,000 type circuits, employing self interruptecl drive, a standard drive-tripping ba:ttery testing relay has been produced. This .relay is ex tensively used in the new line-finder circuit for the allotter drive, line-finder vertical hunting circuit and secondary control set switching circuit a.nd is normally a single coil 11 ohm relay with one change over unit. The free outlet condition is a 150 ohm battery and t.he relay is adjusted not to operate under dual testing conditions, although in these circum stances it wil. hold, once operated. The scheme, therefore, relies for its success on the .fact that two switches hunting over tihe same multiple, always step at slightly different speeds. Hence if two testing relays arrive at t.he same outlet simultaneously, both will step on unti.J the switches become sufficiently out of phase for one rela,y to operate before the other arrives. Since the operate time of the .relay to its break con tact is of the order of three milliseconds, in practice the switches seldom have to take more than two steps before the necessary phase displace ment is achieved. The unguard period is, of course, zero, since the outlet is busied as soon as the relay is applied. The conditions imposed on the relay can be more easily met than in the normal battery-testing case. Moreover, the impedance of the normal battery-testing relay rendered it unsuitable for use in a drive-tripping scheme required by self-interrupted drive. The new scheme offers one of the best solutions so far obtained to the problem of elimination of double connexions and hence has been applied in the majority of the cases where this aspect is of prime importance. A typical illustration of the principle is shown in Fig. 4, whic.h re presents the circuit element employed in tl1e line-finder secondary control ·relay set. SKT�- ± �{". "sA'J". j <l�JlDK-----J�� : 1: ·� 1so I ... , ' I 1 Secondary Allotter -4� IDK Fn;. 4.-DRIVING TRIPPING, B A TTERY TEsTI:-IG Crnct'IT. Impulsing. . The maximum junction resistance over wlu_ ch satisfactory impulsing can be obtained is, in the mam, dependent upon the performance of the B ,relay, the . reached when the make penods of the A limit beino"' . . relay contacts are so curtailed .that the ene_rg1zat10n o·iven to the B relay is insufficient to enable It to hold uring an impulse train. The standard specification for a 3,000 type B relay requires that it should, a ter saturation, be capable of remaining operated du.ring an infinite train of impulses of 79.5% breal � at 12 i. p.s. If nhis requirement is satisfied, correct 1mp�ls . ing can be obtained over a single _1,500 ohm 1unct1on . . from an auto-to-auto repeater, impulsed 111 senes with a 500 ohm subscriber's line, assuming that all other factors such as voltage, dial speed and rat.io, etc. are simultaneously adverse. vVhen tand_em j un tions are used, the permissi�le overall junction resistance must be correspondmgly <educed on account of the distortion introduced by each repetition. d � � 011 pre-2,000 type circuits, the requisite B relay release lag is obtained in the standa�cl manner by means of a copper slug, but even w1t.h � slug of . maximum dimensions, the f undamenta! 1mpuls111g requirement is, in cases of relays wi h heavy sl;1ring loads, difficult to meet without reducing the residual to such an extent that the .relay has a tendency to stick. In some circumstances a more satisfactory method of obtaining the necessary lag, is to arrange for the relay to release with its winding short circuited, but the efficacy of this method depends upon the winding space available on the ,relay core. Hit.herto, the B relays on grnup and final selectors � have had to be provided with a separate '' busy hold '' winding to enable the relay to be held ope �ated when the A relay releases during busy flash periods, and tJ1e space occupied by this winding is normally such that no advantage can be gained by the use of '' release to short-circuit '' principle. The use of the three coil A relay for purposes of applying balanced tone, however, has enabled the " busy hold " windi1w on the B relay to be dispensed with, since the A rel y can be held operated via the 570 ohm � windin o- during the application of busy flash to line ·via the ne windings. It has therefore been possible on 2000 type circuits to employ bhe short-ci.rcuited B relay and the arrangement adopted in the case of the The final selector group selector is shown in Fig. 5. arrangement is, of course, fundamentally the same. It will be seen that the operation of the A relay "'ii 89 c � +1.--Lj L c ( 1) vVhen a selector A relay is energized over a junction in series with a D and I relay loop, the operate lag may be such that the bunch ing time of the make before break contacts is sufficient to produce a false vertical step. (2) At the reintroduction of the D and I relay loop at ·the encl of an impulse train, an extra impulse may be given to the selector magnet owing to the momentary release of the A relay. FIG. 5.-GRoup SELECTOR, B RELAY CmcuIT. removes the short circuit from tJ1e B relay coil and allows this relay to operate in series with the spool YA and rectifiers :MRA. Relay C then operates via its 700 ohm winding and connects its 5 ohm winding in series with the vertical magnet, and the B relay coil. The small current flowing through the vertical magnet and 5 ohm coil of relay C has at this stage no appreciable effect. Impulsing takes place in the normal way and at each break full earth is applied to the C relay and magnet circuit, the B relay coil being short-circuited. The rectifiers MRA oppose the back KMF from the magnet set up during the make period of eaoh impulse, and thus prevent the consequent surge current in YA from becoming sufficiently great to prolong the magnet re: ease time appreciably. This precaution is necessary to reduce the possibility of failure of the vertical magnet to ,release after each impulse break under conditions of long line or high dial speeds. Two rectifiers in parallel are used in order to reduce the possibilty of deterioration pro duced by the overloads carried during the bunchi.ng time of the A relay contacts and during the release lag of B on clear clown. Although a full impulsing investigation has not yet been completed, preliminary tests show a con siderable improvement in performance under long line conditions. The performance of the switch is peculiar in that under extreme conditions of long line impulsing the targ·et shows two breakdown lines, one representing B relay failure and the other failure of the vertical magnet to release between pulses, brought about by the slugging effect and leakage path offered by the B relay circuit. The B relay failure is the limiting factor at dial speeds below about 12 i.p.s. and since this is the maximum speed allowed for under normal maintenance conditions, the magnet failure, although of interest, imposes no additional limitation on practical impulsing con ditions. It may perhaps also be mentioned that although the leak current and magnet surge also pass through the 5 ohm coil of C, the effect on the release lag of the latter is in most circuits, of no account. vVhile dealing with the subject of impulsing, it may perhaps be opportune to mention the closely allied effect known colloquially as '' pick-up.'' This term is applied to the following two effects which become apparent during impulsing over junctions and are due to the high impedance of the D and I relay loop in the auto-to-auto relay set, combined with the characteristics of t·he bridge condensers. 90 These effects impose additional limitations when fixing allowable impulsing conditions, since in some circumstances either may occur at lower values of junction resistance than would cause actual impuls The short-circuited B relay arrange ing failure. ment used in the 2000 t)'pe selector circuits eliminates the possibility of false impulses on seizure since the magnet circuit is not completed until both B and C relays are operated, i.e., until after the bunching period of the A relay contacts is passed. Although 'bhe second effect of pick-up may not permit full advantage to be taken of the improved B relay per formance to increase present dialling limits, the new arrangement will at least allow existing limits to be met with wider toleirances on B relay adjustment. Before concluding the remarks on the impulsing aspect, it may be of interest to draw attention to the method of producing the requisite C relay lag. This is obtained by short-circuiting the 700 ohm winding This feature instead of fitting a slug as formerly. has been included not for impulsing reasons, but in order to enable the relay to be used under either fast or slow releasing conditions according to require ments. In the circuit illustrated in Fig. 5 and also in the case of final selectors, the C relay 700 ohm winding is short-ciircuitecl only during impulsing and the relay releases in an unsluggecl condition on clear clown of the selector. T1his arrangement gives a ready means of providing the pr.ivate unguard on release, hitJrnrto obtained by the employment of release magnet springs. The ZOO-Point Two-Motion Line-Finder. The pre-2000 type two-motion line-finder has been found by experience to be not entirely satisfactory, particularly under conditions of heavy traffic, and the introduction of the new mechanism presented an opportunity to eliminate undesirable featuires and improve the routine test facilities. Before dealing with the salient points of the new circuit, it may be of 1interest to reiterate some of the essentials of the two-motion line finder scheme and consider some of the imperfections of the existing circuit. A simplified trunking diagram of the scheme is The exchange is cliviclecl into units shown in Fig. 6. of 200 lines, each unit being provided with a primary start relay set, two or three primary control relay sets and a number of primary line-finders. The latter are sub-divided into directly a.nd indirectly connected finders, the former being directly connected to •first selectors and the latter having access via the banks of secondary finders, to a common group of fir.st selectors serving the indirectly connected primary finders in a number of units. The secondary vious description of the time pulse feature, it will be evident that under conditions of complete congestion, arrangements must be "' ;ue.s m�cle to prevent start conditions becoming . effective on a unit if all channels from the latter are busy. PRl�AR� START RELAY '" The existing circuit was designed on the assumption that complete congestion on a unit would always be due to all primary finders (direct and indirect) in the unit being in use, i.e., off normal.. Lock outs could therefore occur during secondary FIG. working 6.-LTNE-FINDER TRUNKING SCHEME. when indirectly ( a) connected primary finders were available in the unit, but finders are cont,rolled by secondary control relay sets and secondary primary start relay allotter banks. sets reached from Secondary workino· brought into use only when the regular finders i unit are all busy. the is � the Normally, each primary control relay set serves a particular section of t,he group of 200 lines, but during periods of simultaneous calls . ' team working between the control sets is employed. On account of its importance, each primary control relay set incorporates a time pulse lock-out feature whioh operates if on any particular call the interval betwee� ·seizing the control set and releasing after connexion between the subscriber's line and first se:ector is established, exceeds 6 to 12 seconds. This prolonged control set holding time should normally occur only under fault conditions and hence the time pulse feature locks the control set out of service until released by hand, brings in an urgent alarm, and diverts the traffic into the other control sets. The circuit is so arranged that at least one control set serving the unit is always left in service. In the past, in several exchanges, the number of all secondarY. finders were busy, (b) indirectly connected pnmary finders were available, but the secondary finder .groups associated with them were busy although other secondary Condition groups in the division were free. (a) was relatively unimportant owing to the probability of complete secondary congestion being comparatively remote. however, is of fairly Condition frequent (b), occurrence during peak load periods in busy exchanges and has given rise to considerable difficulty. The gravity of the effect of a lock-out under congestion conditions was increased by the fact that the circuit arrangement precluded reversion to primary working on the whole 200 line unit until clearance of the lock-out. This feature has been removed in the 2000 type circuit. The 2000 type circuit makes ingenious use of a valve to meet the various requi·rements under con gestion conditions, the essentials of the arrangement being shmvn in Fig. 7. The valve is located in the lock-outs experienced has been found to be excessive, and in many instances the causes were not ,readily traceable to specific faults. Some of the defects responsible are given below, together with an indica tion of the manner in which improvement has been DS effecced in the 2000 type circuit. (1) Two control sets seizing the same line-finder. This \\·as possible on both primary and OFB -��ON:YS�S secondary equipments and resulted from the . difficu:ty in producing a suitable design of battery testing· relay. This difficulty has been dealt with in the section on ,lrnnting arrangements and as indicated has been PRIMARY START R.S. alleviated by the development of an 11 ohm This was in the main clue to the necessity for obtainino· � hunting by an interacting relay common t all line-finders in the unit, it being difficult to devise and maintain a suitable relay adjust ment to ensure satisfactory functioning with any line-finder. In the 2000 type circuit this difficulty has been eliminated by employing self-interrupted drive, which incidentally also gives faster hunting, thus reducing dial tone delay. .... ··. �r��ER FIG. TO TESTING RELAY PRIMARY CONTROL RS 7 .-LINE-FINDEH CIRCUIT E).IPLO\'ING v ALVE. primary control relay set, and indirectly connected line-finders in the unit are commoned to the grid via metal rectifiers MRB. Directly connected finders in the unit are commoned to relay RFB w,hich is main t�ined operated ·via resistanc� YA as long as any Resistance YA directly connected finders are free. is also the battery testing spool for the control set (3) I11correct seizure of primary control relay sets under congestio11 co11ditio11s. F � OFC ONTROLLEO B) battery testing drive cutting relay. (