US2469001A

US2469001A - Abnormal temperature and ground detector alarm system

Info

Publication number: US2469001A
Application number: US663282A
Authority: US
Inventors: Edward J Poitras
Original assignee: Individual
Current assignee: Individual
Priority date: 1946-04-19
Filing date: 1946-04-19
Publication date: 1949-05-03
Anticipated expiration: 1966-05-03

Description

Patented May 3, 1949 ABNORMAL TEMPERATURE ANDGRUND DETECTOR ALARM SYSTEM Edwar'a tji'oitras, noilisto'n, Mass. Application April 19, 1946*,Serial Norf63,282

. 7 claims. 1

y My-pre'sent invention relates toy detecting and signalingI abnormal-operational conditions, particularly those characterized by undue heating. -Being especially `applicable to electric alarm jsystemto bel actuated Vin-response -to such condition as anoverheatedvjournal box, the'invention finds important use -for railway carsand locomotives.A VIt has as a main object to make pro- Vision for readily indicating the existence of an 'unresponsive condition in :thefalarm system and foridetermining the'flocation and nature of 4the trouble. Y 4 4 lTn the drawing'illustrating one embodiment of a system inaccordance with theinvention:

Fig. lis a schematicviewand*wiring diag-ram 'of typical apparatus and'manner-of-arrangement in the system; and

Fig. 2 is an enlarged vcross-section of a -conductor` cableappropriate for use in `the system,

particularly the portion seen at therightin Fig. -1. Further by wayvofvlgeneral explanation, it Will be appreciated that in trainvoperation stringent `safety requirements make it highly desirable that railway signalsv shall be fail-safe. By this I mean that the 'signal system.v should be such that in the event of vbecoming unresponsive -to theexpected operation,the unresponsive condition shall 'be promptly evident to those-'in charge of operations.y :My Vinvention laccordingly'providesY for fail-safe operation and simplified servicingof an alarm system through novel Yconstruction and arrangement 'enabling al'systernatic check as -to the locationcause and nature of the operative interruption orfailure. As willbe apparent lfrom vthe following description in -connectionwiththe 1 drawing,l the invention' contemplates fcertain lsafe operation tests as tothe instant statusboth of the system as `a'whole and of various -component elements.

l Referring lnowtoig-l, a mainor'general-alarm circuit is-indicated vgenerallyat '3. This lmaybe understood for Aexample as-the'usual oranylpreferrecl alarm circuity of Vone or morel railway scars, generallyextending 'to the locomotive inthe case The ldetector fand conditionetestingv of ja train. means ofthe -system arel in this instance adapted '4to impose 'signals upon thegeneral alarmcircuit 3 als through leads 4 in'series connection with lthat circuit. This general or train Yalarm -circuit 3 `desirably hasgroundedshielding 2|, .25 similarly .ias vwill Abe explained withreference to unit -Bof Fig. l'an'dto Figf2. .'If desiredea yspecial alarm 'cir'cuit'na'y beprovide'din lieu ofthe-*regulartrain alarm circuit 3, and the detectorand fail-safe systemI to be clescribed-may -be v`employed for (C1. ivi-*311) electric alarm circuits other. than for railway use. It` will also be understood that the-system is7'susc`e`pti-bleof use where conditions otherthan ltemperature arefto .fbe vc'letected `or controlled, I.as

yfor instancefor war-ningof or response yto nconditions-fof abnormal pressures, excess or, deficient humidity and others.

. In Fig. 1 twormain units orsections areevident,

respectively designated as Ia whole at'A and .B.

The .unit A comprises a controlpanelt atr which the A dotted `line 6 :represents a' shielding `enclosure with grounding as at 1. Thefunit-orsectionB, to

l,be referred't'ovas the detector circuit,v` is typical of thecircuit .at the location from which the overheated orfother undesiredoperative conditionis to beidetermined, such yas a'hot boxat arcar axle. l. A group of Afourjournal boxes, for examplaare'indi- .cated in connection with'unitB. These `may be on one truck ofarailway carflon a number of trucks` of the-.given car, orondiiferent vcarsin aftrai'n. 'It isV contemplated with reference-to railway installations that a `panelsuchas represented-atie will vbe convenientlylocated 'at each e'ar and also .at the locomotive, Abut -if preferred `in some- -instancestwo ormore carsmaybe yarfrane'ed'for, signaling to an alarmstation at'one train, such 'as-thatlatl associated withthel-eads il,llto'the"train.alaim3, desirably arefplaced in "series[throughoutthe train. enables the complete train-to be continuously monitoredlin 'a'fail-safe system, in asimilar manner asfor .the

.'falarm at each; cariwi'threspect tothe individual yjournals. thereof. A 0 v v ALeads B Vfrom a suitable .power source `formthe alarm system are indicated at the leftin VFigrfl,

extending through fuses Slto a mainv power switch '-IU fatthepanel. 5. l.The power. source Kis yun- .lg'rounded as. usuallypreferred "in railway--uses,

a'nd'as will be apparentthe present systemfclesir- .ably includes provision for .detectinggrounds.

. yIlie advantages of an ungroundeclf-system Jinl-this v"connection will be evidentfromthesubsequent 'descriptionjnoting that in general v,oneground vlean occur `Without .causing trouble; at least two f'sim'ultane'ous Ygrounds having` to occur'hefore-an .inoperative conditioncanarise-in anyapart of the system. vThe main switch l=shown-as a `.double-'pole singlethrowv type. is 'adapted when fclosed. toy energize the 'Y entire Aapparatus .ofA fthe l panel Aor board 5 vandalso the circuit Tof-the one or more detectorunitsforsections B. Withl'ref- -erence to the control Aunit'Arrandptlne panel f5; it will 'be noted. that the'main switch il' vse1'ves`to separate two ,circuits at Vthe panel and alsoazthat one circiut portion at the panel, that appearing directly above the switch in Fig. 1, is not iniiuenced by the opening or closing of this switch. Considering iirst a typical detector circuit such as represented by unit B, at the right in Fig. l this comprises one or more and generally a series of thermal switches I2, one for each location to be protected against an overheated state, such as a hot box. Four such switches I2 are indicated in Fig, 1, as for example at the outer ends of the axles of a four-wheel truck, or they may be regarded as on different trucks. In railway applications there usually lwill be eight, twelve or other number of these thermal switches to each car. Each switch I2 normally is closed as seen in Fig. 1 and is designedk to openin response to rise in temperature. For railway use as referred to these thermal switchesvIZ are set to open when the corresponding journal box temperature reaches a point dangerous to train operation, which islusuallyr inthe neighborhood of of 350 F. The. individual switchesmay be of any known or preferred c`o1`1str1 1ction, but preferably are such as disclosed and claimed in the patent to W. J; Turenne No; 2,090,407, dated August 17, 1937, and are so indicatedy by the switch symbols IZofFigl.

The'thermal switches I2 of thegroup B, or of any number of groups associated'with a given control panel A, are electrically connected in series as by Y'the lineindicated at I5. Thus on heat atuatiorifofr any switch I2` the series circuit at `1? is. opened. Sas. previosly noted the series relation of the individual k,'iournal-located thermal switches `I2- totheircontrolpanel A is sim-l ilar as for the several panels A in series relation tothe general trainalarmw circuit 3 and the master panelrthereforjat thelocomotive or elsewhere.

Adjacentleach thermal switch I2 is a test pushbutton I3. Thesegare olf"`thenormally open circuit-makerwtype. ,Each' is shunted across the corresponding yswitch,I2and thus, when pushed has the` same `eiiect for-test purposes as the normal Closed statuant theressociated. Switch An Open Conditiprl,,ofariysyvitchv I2mrnay thereby be determinedl and' the, 'particular switch identified. Thatis, pushingjofwthe ybutton I3 foranyparticular open I switch I2 vree-establishes the circuit. A ,reversetesa fpruthennormal status of the detector circuit anduforlthe associated alarm circuits to badesribts icreasliness. to `respond to :an Werheated. ,.giiditign- 1S. )afforded as by means of a master pushbuttonjlnplaced in the detector line I ,at anyrconvenientpoint, as for example.' @the .eri'elgfi ihecbirol unit A- fes.s een.;in rie-g1; theipuaon, wis a Vnormally closed circuit-breakertype. Itmis adapted to Simula'iih opening'gfgatiermalswitch L2 and thus serv s -to* clieclrthemresponsiveness of the u system forsuch occurrence,

stallations such asy for railway rolling-stock are fslibectfio Sie'iefeppsrtins @editions and' varying weather @effectshf Accordingly the `invention importantly 'includes provisionior detecting wirarranged 'for 'automatic grounding in case of insulation failure.`4` Further, these cables, or at H'leastthe portions extending from a. car to the 'journal 'boxes 'as generally indicated at the regions" I'I'on'Fig.'1,"'are Iiexible, to afford the 4 desired non-rigid connections as between the car body and truck-borne parts.

In the illustrative example of Fig. 2 each conductor 2i), 23 is placed within a shield 2|, 2I, a metallic sheath-like enclosure or partial enclosure separated from the conductor by insulation 22. The one or more individually shielded conductor elements 2li- 2| are in turn enclosed in an insulating and protective rubberous or like case 23. Each conductor 20 and its shield 2I are so related that breakdown of the insulation 22 between them as the result of flexure, heating, aging, or other use condition Will cause the wire 20 to be shorted against the encompassing shield 2 I.

As indicated at the various points designated 25 on unit B of Fig. 1, also on Fig. 2, the shield elements 2i of the cable are grounded. And through the alarm circuits to be described this grounding of the conductor shielding 2I in its entirety is utilized as an element of the protective or fail-safe circuit here concerned, effecting a characteristic response in the event of shorting of a conductor 20 to its shield 2I as referred to. The points of grounding 25 for the shield are provided in such number and are so distributed, particularly at the regions II adjacent journal boxes, that a conductor shorting at substantially any location in the circuit of a unit B will give the desired warning indication. The same likewise applies to a grounding short in the train line 3 or Within the shielding of a panel A.

The grounded-shield line I5 of the detector unit B extends to the control unit A, herein through the master test button I6 previously mentioned and through fuses 29 to protect the circuit B for mover loads. In the unit A one circuit subject to the main switch IU comprises a relay 30, the coil 30a of which Vis in series with the unit B circuit, This relay controls the status of certain indicator or alarm means preferably both visual and audible and also controls the imposing of a signal onto the main or general alarm circuit 3, all in response to a call from some point ina detector circuit B. From the main power switch I0 one lead 32 runs to the coil 38a of the relay 3S, and thence to one side of the detector circuit I5. Another lead 33 from the other side of the power line at switch Ill is adapted for connection Withthe other side of the detector circuit I5, herein through the master pushbutton I6 previously mentioned.

Between a portion 32 of the lead 32 from the relay coil 36a and an adjacent portion of the other main lead 33 there is interposed a doublepole double-throw pushbutton indicated generally at 35. In the normal position shown the lower pairs 36 of two sets of pushbutton contacts are closed to connect the relay into the detector circuit B. The other contact pairs 31 shown in the normal open condition are serially connected to by-pass the lower contact pairs 36, for cutting out the detector circuit B and directly closing the circuit of relay 30 When the button 35 is pressed. Accordingly the button 35 provides a test for the over-all ready condition of the alarm circuits to be described, for continued or resumed Y normal operation thereof as when no overheated condition is present.

The relay 3!) responsive to the detector circuit B in turn selectively controls a plurality of The relay 3l] is double, having two armature members 40 and AI arranged for simultaneous actuation as indicated by the mechanical connection shown :tizi-reagent 'f dotted '5 at 12 Y 'Sprin'ginasfas l'at f4'3 'bias'es armatures 40,y 4| in "t ef iect'ionf -in --FgfL Tl'e relay\amature`-4 is`ado-uble-Lpole l-unime#thirsty type 'adapted to -cipe the lcir'euit across one pair 'Pof i contacts 44 'When' the elay coi1 30a is energized, and to make contac' another adjacent pair of contacts d'fupnde- *energizingof the relay A`andA freeing ci the armature to' the-"action of 'thespr'ing neans 43. Si-

IInult'ane'cnisly 'with I-tlie -"normal f relay-energized closing of the contacts i 44 "by t tlie armature dll, the other armaturen I normallyclo'ses-"a 4pa'irof contacts 4B for completing' the @circuit through l pose a vsignal onl the generali alarm lcirc'uitS.

The 'parallel indicator or alarm circuits *'s'ub- VjectI to the 'relay'f3ll at the panel'etendf between i a lbranch E-33 "of "theflead l33 and theVOtlierUIead "32. fFirst, f there 'is fa v`connect-ion 150 from Lthe branch 33 and'V tllrol'igh thev i6-Wen rclaycticts i yIM v(normally 4closed bythe iarrnatureAG) fand 'through a lamp 5| fand thenceto the Vothe'rlead V32. iablyf green as indicated, significant-,fof anormal I-safe or operative'con'dition yof"thehsystein -and "of "the ecli'iilmentfof which the overheated condition is to be detected. It Willfbegseen that With -tne relayc'oil V39menegizedfas under the lnormal I '-closed status-of the thermal *switches l2 I`the :green-iigiitil Yisbn. A

Second, an Aalternateparallel circuit between the branch3'3' andil the lead 32 :comprises: 'a connection 6|] through-the upperl contactslljipen when the relay is-energizedrand throughalamp 6I `to tne y'lead `-32. The `la'rnpjtl "isfof another "color, Ypreferably 'fred as indicative 'ofv "a j danger condition yatsorne region 'subiectjto-y the -thermal detection. Upon "de-energizingof wthe relayV coil i301; the circuitisvbrokenf'at the n'orrnally'clo'sed contacts 44, extinguishingftneigreen'light;and is finadeat th-e opposite contacts'jlijghting the 1lairipil (red). At tnese'trrie"tirnej'the4 circuit is broken at the relay' contactsd,"iattne'upper part "of the VFi'gfl diagram, thereby'irnp'osingfasignal oni the mainalarm f circuit 3 as -Aprevi'ously `rnien- -"ti'oned. l A

Third;theredtsrabiyl isan audible* aiarm' tir- 'fcuit arranged `in parallel'"Withl the `"Visual fred light circuit, for Iisirnultaneons operation j tliere- AWith. `In Fig. -1 l this comprises "a connection 1Jtill from the redflainpconnectionfil "through a Y buzzer 'or tne'flfike-65"and thencetl'rrou'gh an onmanual twitch-@Seattle lead 32. Thus, 4jatfs'umin'g `tne optional-aise switch "66 is `closed `the EbuclzeriiEi-is st'iundedat` any time when the danger 'Warningla'm'pl- (red) is on. Theswitcn'also enables the buzzer 65 to be turned 01T :Whenlthe idanger4v call has been observe'djslrould lit'bezlesired lto avoid vfurther audibility While' the 'trouble is y'recl'eiving attention. y

The apparatus atthe'panel 'as hereinillustrated further comprises the circuit previously referred to Yas'notsubjectlto tnernain` powersw'itch l0. This "circuit, Seen attliejleft inFig. '1,"comprises branchv leads'lllandi'; from' the 'respective 'sides ofthe mainfpotvver linel. 'Thebranch "10' f extends throug'n'a/further'lamp "il and thence lthrough a-fpair of contacts "l2 and to the branch .lead in. jrite-1am n iscf another distinctive "color Aas f'orexam'plev amber.' yThis mariberVs'ignal circuitA is arranged in 'association witha 4relay t d evic-e ind-icated-'generauy amato 'give 'warning of 't'lie presencebfa grounding' short" in tle sys- `This=la1npfis oa-distinctive color, 'prefer-v :f-tmjas infany detector?circuitfsuch'as the'u'nit "B by insulation failure `at a 'vconductortor'in the 'train circuit3,- or"at the pane1'5 `through theg'rounde'd'shieldy'l of the latter. Y 5 A Forfthis-purp'ose the relay 80 is of 'a diilerential -typewcomprising tw'o opposed normally balanced -colsfSL i82 having an equal number of turns opypositely Wound The circuit of the'relay' coils comprises a branch'll" from-the lead lll to one endof' one coil,nerein coil 8|. I Fromthe latter the circuitextends reversely through the other *c'il82 and thence to the other branch lead"10. The common'conn'e'cton of the coils is connected toground'as 'at-83.A -Norinallynfin the? absence of l'any' snortinget'oeground-'conditicn in thet circuits, the" two'coils' 8 l 82' balance'each other and there t is -zer'o'forcefeiective 'on the'relay armature; the latter therefore' normally 'opens' the `contacts .."l'2='as'shown,`un'der the influence' of fa spring, EandV the light '1| (amber) vis "on, signifying no gr'ounding"snort But 'should` anyA conductr "of the circuits come'into'electrical contact'with the v"enclosing grounded'I shielding through 'i'nnul'ation failure fas between Ia Wire "20 "and the shield ""2 I o therefor, 'or "at the "groundedshiel'ding ofi the 't panel '5,v 'theffdiierental 'rela'yfll is `thrown into nun/balance by reason "of "the current viio'wifng itnrough'only one' vof tlfre'coilsk SVI :or S2, vtlieother 1' lcoil "being 'in effect vIcy-passed by the grounding "c'on'dition. The diiTerential-relay ffarmatr'e "'35 *accordingly tcloses the circuit iat the contacts-"I3 f "and lthlolllgli the lamp I l (a'inlc'er) g' '0f1'ightig`- ,the flatter Tand indicating 4of the L'grounding occr- "rence 'Means desirably isprovided for testing the' described-grounding circuit, fori/determining the irea'di'n'essf'for re's'pnse'to anlinsulation `failure 1. "or fothersnorting to` ground." Vll1"0"rftl`1`is purpose I have indicated atth'e'lower left 4inlig 1""afsi'1i'gle- L.xItlrro-W pushbutton Sil havin'g contactsfi' connect- `iable'serially between groundxas at "SSa'ndl'one ."side `vof the main? 'power line "8,5 her'ein the `side to t which the lead-rsa1 is connected. "Thisx tt'buton -illlis nbrniallyopen. When depressed it closes at .=tlie.contacts 92 "a 'test circuit simulatiye`of a sli`orting to ground.` `Sincethis''circuit including 3 the.' test v'button @il yis vassociatied` With the'r lcircuit k rof'fthe dfferentialirelay '8m-a testv ground cndication isr created' thereat, responsiveness in this -relation being indicated by the attendantilignting foftheflampll'amber). v l y t Irol` thefforeg'ing itis jevident I"ilat"'l'tliif sys- :tern-of I'the inVe'nt'ion noyelly'`p1oyides fory 'groundf-edfshielding foi 'alarm conductorsiiandassociated equipment such "'asj'unctionboxes' and the' like in '-atmanner: to detect and Warn of' a groundi-n the system "Other important lfeat'ures of the l inv 'vention include the 'means y'"assci'ated Witnt-hr- 'nia-lly responsivefswitch'es as at journal boxes for #readily i checking yfasi to.' which? 1such box v 'is inl an overheated :stateg and various means r "testing 1 yas 'to *circuit `-conti'mity fand *as to "the Y over-all l f'operativenessof the alar'inisystein.

The voperation of tne 'cir'cuit "coinponerlts Will `4be evident from the foregoing descripti'onbf-'their 65, interconnections fand "Withreference t'o tli'eillus- V `trative embodiment of 1 the drawing. The "sevferal Alampsl and signals indicate'the-'corresponding conditi-ons Which Jmay arise,- While Imanipul'ation -of` the-described pusnbuttonstests the-`ass`ociated junction. TheV information available by "signals, both-in actual' operation 'and'4 intest' therefor, rnay Abe summarized thust,

With the power switch Hl 'open lampi If (amber) `should.. -beofi, indicating- -loperat-iveness Cof 4the local ground detector circuit, correct balancing at the differential relay 80 and proper operative status of all parts associated with the system portion 'Iii-86. A test for this may be had by the button Si? pushing of which should light the lamp 'll (amber) as indicative of readiness to signal a ground occurrence. Further, with the power switch it closed the lamp H (amber) should remain on, indicating no ground condition in the circuit beyond the switch lll. The test in this connection is the same as just stated.

With the main switch l closed, lighting of the lamp (green) indicates that all thermal switches l2 are closed (assuming test buttons I3, i6 and 35 are normal) and hence that there is no overheated condition at any region subject to the system. At such time general readiness of a circuit B may be tested by master pushbutton I, producing an effect analogous to opening of a thermal switch l2 and thereby changing from the visual indication at the lamp 5l (green) t0 that at the lamp 6l (red) and giving the audible signal at the buzzer E5, if in use.

Assume now that the system is in intended operation as upon a train, the main switch Ill being closed. The light 5| (green) is on, the lights 6l and 'll (red and amber respectively) are off and the buzzer 65 if in use is silent. If now the lamp 5I (green) goes off, the parallel lamp 6| (red) comes on, the buzzer E5 sounds and the remote signal as at the locomotive is given over the alarm circuit 3 (the buttons I3 and 35 being normal), then the indication is that a thermal switch l2 may be open under a hot-box condition. Thereupon a test of the actual condition may be made by successively manipulating buttons I3, until the lamp 5l (green) lights, the parallel lamp 6I (red) goes out and the buzzer if in use ceases. In this manner the particular overheated bearing is determined and isolated.

On obtaining a danger indication as last above mentioned, that is, by lighting of the lamp 6| (red) sounding of the buzzer and signalling to the locomotive, the operativeness of the alarm circuit may be tested as to whether the danger indication is due to something thereat rather than in the detector circuit, by manipulating button 35. This should re-establish the clear indication by relighting of the lamp 5| (green), showing the alarm circuits of unit A as being duly operative.

Thus all portions of the system as a Whole are subject to operative test under substantially any condition or combinations of conditions of the component parts, and maximum assurance of fail-safe operation is afforded.

Through the use of an ungrounded power source and the described provisions for grounded shielding encompassing the conductors in the various circuits, the system is made to include detection and signaling of grounds. And by reason of the ungrounded system fail-safe operation is promoted to the further extent that one ground can occur without interference with the continued intended functioning of the system. It is expected that the maintenance personnel will remove such ground before a second one has a chance to develop. For instance, the instructions to the operating personnel are understood to be that when the lighting of a lamp 'Il (amber) of a control panel A indicates a ground condition, the proper person will determine by the described test means whether the ground is in a detection circuit B under that control panel A and will remedy the fault immediately, before development of a second ground could cause one or more of the'thermostatlc switches l2 to be by-passed in such detector circuit B.

My invention is not limited to the particular embodiment thereof illustrated and described herein, and I set forth its scope in my following claims.

I claim:

1. In an alarm system for warning of an abnormal operating condition, an electrical switch responsive to the abnormal condition, an electric conductor in circuit with the switch, the conductor having grounded conductive metallic shield means substantially encompassing and normally insulated from it, an alarm device actuated in response to operation of the switch due to an abnormal operating condition, and ground detector means responsive to a shorted condition as between the conductor and the shield means.

2. In an alarm system, a detector circuit containing in series with it one or more conditionresponsive switches and connective conductors, grounded conductive metallic shield means individually encompassing and normally insulated from the conductors, ground detector means responsive to a shorting of a conductor to the shield means, and auxiliary manual means at the one or more switch locations for determining the continuing non-grounded status of the particular switch or switches.

3. In an alarm system, a detector circuit containing in series with it one or more thermal switches and connective conductors, grounded shield means individually encompassingly adjacent and normally insulated from the conductors,

. an alarm operable in response to action of a thermal switch of the detector circuit, a normally ungrounded circuit including a power source, a differential relay and a signal device controlled by the relay, and means including a ground connection to the relay whereby shorting of a conductor to the grounded shield means causes the relay to actuate the signal device.

4. In a railway journal-box alarm system, detector means comprising thermally responsive units serially interconnected through encompassingly insulated non-grounded electric conductors, grounded conductive metallic shielding encompassing each conductor, said conductors connectible to an ungrounded power source, alarm means for operating in response to action of a thermally responsive unit of the detector means, and a ground detector means responsive to a grounded condition of the electric conductors.

5. In an alarm system according to claim 4, the combination and arrangement in which the ground detector means comprises an ungrounded power source, and differential relay coils are connected across the power source and center tapped to ground.

6. An electrical alarm system particularly adapted for railway use to warn of an abnormal operating condition such as a hot-box, the system comprising, in combination, a general alarm circuit extending along one or more railway cars, a plurality of normally closed electrical switches respectively subject to the condition to be warned of at a like number of locations, electric conductors serially connecting the switches and having grounded conductive metallic shield means normally insulated therefrom, a relay also in series with the switches and with a power source, signal means controlled by the relay including local signal devices informing of a normal and of an abnormal status of the switches and conjoint means to impose a signal on the general alarm circuit,

and a relay and a signal device controlled thereby in accordance with the absence or the existence of a shorting of a conductor across to the shield means therefor.

7. In a system for warning of overheated railway journal boxes, a general alarm circuit extending along one or more railway cars, normally closed thermal switches in heat-subject relation to the respective journal boxes, insulated electric conductors serially connecting the thermal switches, grounded conductive metallic shield means encompassing and normally held out of electrical contact with the conductors by the insulation thereof, a relay and a signal device controlled thereby according to the absence or the existence of a shorting of a conductor to the shield means, a further relay also in series with the switches and a power source, and signal means controlled by the relay including local signal devices and conjoint means to impose a signal on the general alarm circuit.

EDWARD J. POITRAS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS