US2696603A - Two-wire alarm or control system - Google Patents

Description

Dec. 7, 1954 EKMAN 2,696,603

TWO-WIRE ALARM OR CONTRUL SYSTEM Filed June 10, 1952 Jecf/b/IZ I W IH IN VENTOR Q AAMQ W BY QM} ATTORNEYS United States Patent 6) 259E603 K TWO-WIRE CONTROL SYSTEM Olaf. Ingemar Harald'Ekrnan; Stockholm, Sweden.

Application June 10 1952,=-S'e'rial-'No; 292,706 10 This invention relates-awia ztwo-wire alarmaon control system and more particularlytoian alarmiwoncontrol system, in which the. two wires forma continuous long: lOOP{ circuit which originates at =and return 'ztto-a -.centra'lv sta tion, and the long loop is subdivided into a number of seriallly .connected sections :bytdouble pole switcheswhich are, shunted at thecentral station side :by conductive pedances.

Control and: alarm circuits oi the two wire -type and with shunt impedances at sectionswitchesaredescribed in a patent to Hans T; Nicou; No. 2,294,043,-August; 25, 1942, and in my copendingapplication Serial No. 290,432; filed May 28, 1952. A number of two=pole thermo-con tacts -or-:thermal switches are. arrangediin: each: section of the. loop' circuit, andaboth -wires s are opened when a]. thermo-contact is :opened automatically bya predeter--- mined temperature. rise or when ones ol the section switches is opened manually; A storage battery-flatthe central station normally establishes direct. currents; in: parallel. through the two wires and; after:openingsofj-the' loop circuit, the two: terminals-lat each end of the l0op circuit are connected to the opposite terminals of -a cur rent source. The. impedances at =the-section'- switches are-- now in parallel in each of the'wire -ci'rcuit's;-at opposit'e" sides of the opened section-and the number of impedances' at each: side of the opened switch is measured by bridgemethodsto identify the section of the'loop circuit WhiCh 1s open.

Theprior practice has been to employ capacitiveirn-- pedances atithe section switches as there appeared to'-'- be no practical solution for the contradictoryde'signrequire-- ments that the section .irnpedances should 'be' of high ohmicresistance to: avoid ,an eifectiv'e short cir'cuiting of the two wires of. a long-loopwhich includes-a number 0f sections, andlshould be of lowohmic resistanceto insure ameasurable.distinction between the resist-ance-ofa -single impedanceand therelativelylowinsulation resistance of a long loopcircuitlocated in a region of-high'humidity acid fumes or other adverse conditions. With'capacitive section impedances it is-of :course necessary'to provide'asource of alternating current toenergizethe measurin'g circuits; The apparatus employing capaoitive impedances has been satisfactory and is currently in-use, "b'ut-the-re'i quirement for a source of alternating current for the irr'rpedance-measuring bridges=can be eliminated and other advantages can be had if the 'sectionimpedances are otresistive or inductive type.

Objects of the present invention are to prov idealarm and/ or control circuits of two-wire shunt-impedance type in which the shunt impedances are conductive Objects are to providetwo-wire talarm ortcontrol: circuits with section impedance of conductive type, and which are free from the limitations and disadvantages attendant upon the prior long loopcircuits employing conductive impedances. More-specifically;- objects-are to-provide alarm or control systems of the charac'terstated-in which the section impedancestake the "form ofa resistance or aninductanceinseries witha rectifierjand the.;circuit arrangement'is suchthat-the impedances are non-conductive during normal operationsof: the circuit but conductive to reversed polarity current upon a two-wire break. in the loop circuit.

These and other objects "and -the" advantages of the invention will be apparent from the following specification when taken with the accompanying drawing in which the single figure is a fragmentary circuit diagram of an 30 automatic fire alarm circuit embodying the invention.

I 2,696,603 Patented Dec. 7, 1954 In: :thedrawing-,3 theirefren'cetcharacters ZZZ-"and" :Jx-identia. fyntheetwoz'wires. of- :attlongz'loopaciricuit:whichuisrtdivided into aanumberfiof; sections; for examplesvten:sections-I40 X, by s'a nurnbersof SECtiOllilUl'litS oraboxes :81 which :pttefienably; eachvincludea nermallwclosedidouble pole switch S twhich may beeopened manually; to: actuate \the :alarm'. systern;..- There;is-.-'a=:stations indicator-at the start of each:; section, it being assumedsfor convenience of: description-1 thamthe: loop: circuit starts :at" the positive'vterminalc of a storage battery ll-at thezcentral' station andsiretu'rns" to; the negativei terminal; oft-thew battery. A number; of. thermor-contaets Ta'areiin seriessin th'e' two-wire-tline't in. each. section,'..thetnumb'er of the .thermot-cbntaets:depend ingi :upon'nthei E1Hgihli of I the 5 section iandszthe condition .or characterof thezpremi'ses thiioughuwhichi the sec'timriex tendspz. The thermoi-contacts Ti are '.'doubleifpol: nominally. closecl switches: which open automatically Wheirsthe' tema' perature exceeds a preselected value, and theY-preferablyv areeot zthe'styperdisclbsediand claimed in: myprior? Patent No.:'2;5 99:44 1"; granted-J une13, .1952.

Af-relaywRai is .-in. series in a-uwireaadjacenta'the startof the loop circuit, and a relay Rb is similarly in series'siniwire b -atth'e'end of the loop circuit These relays are closed-"manually 'to place th'e-'isystern operation; and have holding'cont'acts '12a -andr12brespctively for maintainingfthemienergized so'long ias there'is no'breakiin thet wires ia and 'b respectively; and not short-circuitof th'e:

wiresi=onteach other; Relays Rd and'iRbxeach ha ma. plurality of sets' of contacts for signaliand cont-rolffpur poses; including sets-13dland fl3b respect-ivel3 -With fron't contacts operative; when the relays are 'energize'dgl. toi complete the w acir'cuifi to the negativeiterminahof the; batterytll i and the "-wire v b circuit to: .the'vpositive battery terminal; Wirea of 'tlie loop circuit: is. returned: directly to the negativetbattery'terminal'but a voltage' droppingre sistanceR is 'betweem the front: con'tact ofi set 13b andv the positiveaerminalr At each fire alarmbo'x or station indicator Si; the cen tral statioii side ofthe switel'r-S is-fbridg'ed 'by a==co'nduc-* tive impedance CL-in series with a-rectifier RT of=:dr.y plateor disk typethe-rectifier beingyso polarized as to block current -fl'ow frorn wire b'to w-ire a when; as above: stated;wire b is-at the higher positive'potentialp The conductive impedance rnay be an inductance or iron cored choke or, as I illustrated, may be a: resistance.

As;isknown=in 'two-wire systemsof this general type, the-relays Ra and Rb Will each-=b-de-energizedupon a' br eak -in wires a and b,- respectively; and relay R b will be-released-by a-short circuit between the -two='w-ir'es;: Relay Rtt'is not released byashort-circuit as-it will then be -shunted-by the resistance-R whichis of about 3,000 ohins resistance whereas the resistance r of 5 relay Pia 'is abOut LSOO 'Ohms. As abovenote'cl; the relays have-sets of switch contacts; hotshown, for closing signalcircuits toindieate--therelease of either'relay; The station attendant is thus advised of a fault on the system-and should-take appropriate; steps to locate" and remove it:

A fireala'rrn signal is to be sounded only'in the event of a-sirnult'aneous break of both 'wiresg-eitherby a m'anual opening of aswitch Sat' one-of the 6 station 'unit's --SI or-by the' automatie' opening of onebfthe 'thermo-contactors- T by an abnormally high temperature}- When this occurs, the ends of the wires are" separated from the battery ter minal's-1by 'the relays"; and connected' into- Wheatstone bridges which'g when balanced,- identify the particular sec tion which-has opened; there being a bridge for the portion of the loop at the left'of the br'eak and a similar bridgegnot shown', arthe right side of 'thebre'a-k} The circuit. arrangements: for" establishing the' measuring: ci'r;- cuitsar'e 'as follows: The backcontact ofset 12ft: of'rel'ay R2: is" connected by a lead 1410 junctiomA' oi bridge. W} and'the. back contact of-set 13'b"ofi'elay 1% is. connected by a lead '15 ito anradjacent'bridge junction B. Similarly, the backbontacts of switch sets 12b of relay Rlifand 13a" ofrel-ay 'Rtz are"connect'ed"toileads'14; 15" to the other." bridge, The bla'desof "sets 12a and 131; are connectedj'to the Wir'esa andb, respectively, andth'e 'simult'aneousre lease of both relays thus connects the lefthand portion of the opened line into the bridge W as one side arm thereof.

The two adjacent arms BC and CD are comprised by impedances of equal ohmic resistance and which preferably are the windings of relays 16, 17 respectively having contacts in appropriate fault signal circuits, not shown. The arm DA of the bridge is an adjustable 1mpedance or station selector SS comprising ten conductive impedances, specifically resistances r when the conductive impedances CI of the several stations are of resistive type as shown. A contact blade may be turned by a knob d to connect the resistances in parallel, step-by-step, until the bridge is balanced by the same number of resistances r as there are conductlve impedances CI in the bridge arm AB.

The cross-arm AC of the bridge contains a relay 18 which drops out at bridge balance, the relay having contacts in signal circuits, not shown, for indicatlng that condition. The negative terminal of the battery 11 is connected to junction B of bridge W by a lead 19, and the positive terminal is similarly connected to junction D by a lead 20. The bridge W is thus continuously energized and the relays 16, 17 will release to give a fault signal in case the bridge should be de-energized.

The method of operation of the described alarm system is as follows, it being understood that the circuit diagram illustrates the circuit in normal operating condition.

It is to be noted that the positive potential on the wire a beyond the relay Ra will be relatively low, say of the order of 2.5 volts or less when battery 11 is a 24 volt battery, as the resistance of the relay Ra may be of the order of 1500 ohms and, in any event, constitutes the major portion of the total series resistance of the wire a circuit. The positive potential on the wire b along the several sections will be substantially higher in view of the voltage drops across the relay Rb and the series resistance R. If the resistance of relay Rb is about 1500 ohms and that of resistance R is about 3,000 ohms, the positive potential on wire b along the several sections of the loop circuit will be about 8 volts. The relative polarities of the wires a, b reverse, however, when connected into the bridge upon an opening of a switch S or a thermo-contact T. The wire b is then at zero potential since it is connected through leads and 19 to the negative terminal of the battery 11. Wire a is at a more positive potential and the rectifiers therefore are conductive.

The requirements for reliability in operation and sensitivity of open-section identification are thus satisfied by conductive shunt impedances CI since the provision of the rectifiers RT imparts a substantially infinite resistance to the impedances so long as the system is in normal condition, and the reversal of the polarities of the wires a and b upon an opening of the two-wire loop circuit substantially eliminates the rectifiers RT from consideration and each shunt path assumes its normal resistive or inductive impedance. Values as low as 1,000 ohms, which is convenient for clear station identification, are entirely practical with no risk of a false indication of a short between the wires.

While only resistive impedances CI have been illustrated, it is apparent that an inductive type such as iron-cored inductances or chokes may be employed. The latter have the advantage that the long loop circuit may then carry telephone or supervisory signals. It is to be understood that a plurality of loop circuits of the described type may be associated with a central station, and that various refinements in the bridge balancing impedance, as disclosed in my copending application, may be employed without departure from the spirit and scope of my invention.

In order to compensate for the ohmic resistance of the lines between the station indicators SI, especially in the case where such lines are very long, it is preferable to reduce the impedance of the station indicators progressively in the direction away from the first station at the beginning of Section I and in such manner that the amount of resistance from the beginning of the line to each station indicator plus the impedance of such station indicator will always be of equal value and hence correspond to the equal impedance characteristics of the several impedances r included in the station selector SS.

I claim:

l. A control or alarm system of the type including a normally closed two-wire circuit connected as a loop across a direct current source at a central station, shunt impedances connected between the wires of said circuit at spaced points to divide the circuit into a plurality of sections, relay means included in each wire of said two-wire circuit and de-energized upon an opening thereof, means additional to said relays developing a normal potential difference between said wires along the loop circuit, and means operative upon a simultaneous opening of said relays for connecting into a direct current measuring circuit one end portion of the two-wire loop circuit and simultaneously to reverse the polarity of the potential diiference between said Wires; characterized by the fact that said shunt impedances each comprise a conductive impec ance in series with a rectifier polarized to block Fcurr nt flow through the impedance by the normal potential difference developed between said wires prior to the simultaneous opening of said relays.

2. A control or alarm system as recited in claim 1,

v wherein each conductive impedance is of resistive type.

3. A control or alarm system as recited in claim 1, wherein the relay in one wire of the loop circuit constitutes the major impedance in that Wire circuit and the potential on the wire at the sections thereof is substantially that of the source terminal remote from the relay, a resistance is included in the other Wire at the central station and remote from its relay, whereby the potential of said other Wire along the sections thereof is intermediate that of the source of current.

4. An alarm circuit of the type including two wires each connected across a direct current source and with two-pole switches spaced along the two-Wire circuit and dividing the same into a plurality of serially arranged sections, said wires extending adjacent each other through the region to be protected, means establishing a normal potential difference between said wires throughout the lengths thereof, and impedances connected between said Wires adjacent to each switch and similarly located with respect to the switches and the current source; each impedance comprising a conductive impedance in series with a rectifier polarized to block conduction at said normal potential difference between the wires and means operative upon the simultaneous opening of both wires to reverse the polarity of the potential difference between said wires.

5. In an alarm or control system of the type includmg a two-Wire loop circuit connected across a source of direct current, relays in the respective wires at opposite ends of the loop circuit, shunt impedances across the wires and dividing the loop circuit into a plurality of serially arranged sections, means additional to said relays developing a potential difference between the wires along the loop circuit, said shunt impedances each including a conductive impedance in series with a rictifier polarized to block current flow, said relays inc uding contacts operative on simultaneous release of the relays by an opening of both wires of the loop circuit to connect one set of ends of the two Wire circuit into a measuring circuit and simultaneously to reverse the polarity difference between the wires of the two-wire loop circuit.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,537,211 Wootton May 12, 1925 2,052,057v Reid Aug. 25, 1936 2,294,043 Nicou Aug. 25, 1942 2,452,942 Lord et al Nov. 2, 1948 2,456,499 Fritzinger Dec. 14, 1948 2,556,363 7 Lord et al. June 12, 1951 FOREIGN PATENTS Number Country Date 909,728 France May 16, 1946

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