US3286166A - Apparatus for non-destructive testing of enclosed relays for parasitic arc susceptibility by controlling the current-time integral of the arc - Google Patents

Description

3,286,166 SED RELAYS T. N. LOCKYER, JR APPARATUS FOR NON-DESTRUCTIVE TESTING OF ENCLO FOR PARASITC ARC SUSCEPTIBILITY BY CONTROLLING THE CURRENT-TIME INTEGRAL OF THE ARC Filed Jan. 2l 1963 INVENTOR. THOMAS N. LOCKYER, JR.

Nov. 15, 1966 .APPARATUS FOR NON-DESTRUCTIVE TESTING This invention relates to a relay test set and more particularly to an apparatus for the non-destructive -testing of enclosed relays for parasitic arc. susceptibility.

Small enclosed relays, such as miniature or microminiature relays, are often required to withstand voltages ofup to 1,000 volts between contacts and metallic case. However,relays of this type are subject to catastrophic failure if the case is` grounded and a parasitic arc develops between the relay contacts and the case. It has been found. that if the relay is not properly designed such catastrophic failure may occur `when `switching A.C. potentialsy as low as .25 volts. The failure occurs as the result of the generation of metallic vapor and metastable ions in the vicinity of the contacts by the arc formed between the contacts las they open. The vapor and the metastable ions provide a low resistance path between the relay contacts and the case, thereby allowing a parasiticarc to develop between the contacts and the case. vSuch parasitic arcing occurs when the case or enclosure of the relay or switchis positive with respect to the main arc'.- This parasitic arc phenomenon does not occur in unenclosed relays because the ionized contact material which is spewed out in the main arc is dissipated in the surrounding atmosphere and therefore does not provide a -low resistance path over Iwhich a parasitic arc may develop. A detailed discussion ofk causes of parasitic arc failures of relays used tov switch A.C. power is to be vfound in the article,y A Study of DischargeTransients `in Relays with Grounded Cases, by Nunn and Halbeck, presented at the 9th National Conference on Electromagnetic Relays, Apri'l 1961.

Although it lis desirable to test relays under conditions which approximate as nearly as possible actual operating conditions, difficulty is encountered in using an alternating current source for testing for parasitic arc susceptibility. The primary difiiculty in using an alternating cu-rrent source is the problem of synchronizing the contact opening with the desired portion of the A.C. waveform. As pointed out in the above-cited article, the Iparasitic arc susceptibility is related to the quantity of the metallic vapor and metastable ions in the vicinity of the contact which is in turn proportional to the current-time integral of the arc. Therefore if the current-time integral of an arc produced when switching a D.C. potential bears some fixed relation to the current-time integral of the arc under actual operating condition of the relay, the operation of the relay when switching an A.C. potential can be predicted with reasonable certainty. While test circuits for establishing the point at which various relays fail due to parasitic arcing have been proposed, the means provided for controlling the current-time integral of the arc have not been entirely satisfactory.

It is an object of this invention to provide a novel means for controlling the time duration of an electrical arc.

An additional object of the invention is to provide a novel means for controlling the current-time lintegral of an electrical arc.

A further object of the invention is to provide a novel non-destructive test set for testing the parasitic arc susceptibility of relays or switches.

United States Patent O 3,286,166 Patented Nov. 15, 1966 ICC Additional objects of the present invention Aare to provide novel -me-ans for (a) Extinguishing the main arc between electrical contacts, and

(b) Controlling the length of time which an arc persists between two points.

In general the present invention comprises a constant current D.C. source of supply connected across a pair of contacts, an integrating network connected in shunt with the contacts, a :signa-l controlled rectifier connected in shunt with the contacts and the integrating network, andmeaus connecting the integrator output to the control element of the rectifier for control of the operating state of the. rectifier. The rectifier -is connected to short circuit the electrical contacts upon conduction thereof. Means are provided for separating the contacts to initiate an arc therebetween. In the embodiment of the invention adapted for testing closed-case relays an auxiliary D.C. source of supply is provided which may be connected between the metallic enclosure and one of the electrica-l contacts. Means may be provided for measuring the potential of the auxiliary D.C. source of supply. An indicator such as a cathode ray tube may be provided for visually displaying traces representative of the main and auxiliary arcs.

In its preferred embodiment, the relay test set of this invention includes a suitable power supply for connection to t-he relay contr-ol winding for alternately opening and closing the re-lay contacts.v An adjustable D.C. constant current supply is connected across the relay contacts, and means are provided for connection of an auxiliary D.C. current supply (whi-ch is also preferably adjustable) Abetween the relay case and one of the relay contacts. A solid state -controlled rectifier is connected across the constant current supply as is also a series-connected resistor and capacitor, and the junction between the resistor and capacitor is connected to the control element of the rectifier. When the relay contacts are opened, a main Iarc is established between the relay contacts which main arc exhibits a substantially constant potential drop which drop provides an input to the RC network. When the voltage across the capacitor of the RC network reaches a pre-determined level, the collectorto-control electrode voltage across the solid state controlled rectifier reaches 'a level to cause conduction of the rectifier. Upon conduction of the rectifier the relay contacts are -short circuited through the rectifier thereby terminating the main arc. The duration of the main arc may be adjusted by -control of the RC time constant of the RC network. Means are provided for sensing and/or observing the main arc current-time integral and also any parasitic arc current.

Other objects and advantages of the invention together with a better understanding thereof may be obtained from the following detailed description together with the accompanying drawings, the single figure of the drawing showing a schematic circuit diagram of a relay test set embodying this invention.

Referring to the drawing, the relay to be tested for parasitic arc susceptibility, designated 10, is of the closedcase type and is shown including a pair of contacts 11 and 12, a control winding 13, and a metallic case 14. The control winding 13 of the relay is yadapted for connection to the output terminals 16| and 17 of a multivibrator 18 of the free-running type, which multivibrator is preferably adjustable in frequency by means of a frequency control knob 19. The multivibrator is energized by a suitable D.C. source 21 shown connected thereto, one end of the source being connected to the ground terminal 22. Since frequency adjustable free-running multivibrators are well known in the prior art, no further details vof the circuits of multivibrator 18 are shown in the drawings. In one practical circuit arrangement which has been built and tested, the multivibrator was adjustable in frequency within the range of 2 to 6 cycles per second.

A main D.C. source of supply 23 is connected across the relay contacts, the positive terminal 24 of the main D.C. supply being connected to the fixed contact 11 while the negative terminal 25 of the supply is shown connected to the mov-able contact 12 by way of ground terminal 22. The main D.C. supply source 23 is preferably of the constant current type rfor reasons which will become apparent in a description of the operation of the circuit hereinbelow. The current output from the main D.C.

supply source 23 is adjustable by means of a current control knob 26. l

A solid state controlled rectifier 27 comprising, for example, a silicon controlled rectifier functioning as a normally open switching means is connected in shunt with the relay contacts 11 and 12. The anode of the solid state controlled rectifier is connected to conta-ct 11 while the cathode thereof is connected to contact 12. For voltage transient protection .a Zener diode 28 is connected across the silicon controlled rectifier 27. An RC integrating network comprising a resistor 29 and any one `of a plurality of capacitors 31, 32, 33, 34 or 35 is also connected across contacts 11 and 12, the resistor 29 being connected to any one of the capacitors th-rough a switch 37. The mov-able switch contact 38 is directly connected to the resistor 29 while the lfixed contacts of the switch are connected to the individual capacii tors. The other end `of the capacitors are connected together and to contact 12 by way of the ground terminal 22. The movable switch contact 38 (i.e., the junction between the resistor 29 and any one of the capacitors 31-35) is connected through a lead Iwire 39* to the control element 41 of the silicon controlled rectier 27.

For observation of the main arc between the relay contacts 11 and 12, as the relay contacts are alternately opened and c-losed, an oscilloscope 42 of the dual-trace type may be provided. As seen in the drawing, one pair -of horizontal inputs 43 and 44 of the oscilloscope is connectedacross the relay contacts 11 and12.

An auxiliary D.C. supply source 46l is connected between the relay case 14 and the common ground terminal 22 through a current-limiting resistor 47. D.C. supply source 46 is also preferably adjustable by means lof a voltage control knob 48, and in one practical emb-odiment of the invention, the auxiliary D.C. supply source 46 is adjustable within a range of voltages between 0 and 1000 volts. A second set of vertical input terminals 51 and 52 of the oscilloscope 42 is connected across the series-limiting resistor 47 thereby providing a visual indication on the oscilloscope of any parasitic arc which may develop between the main arc and the relay case.

In operation, the normally closed relay contacts 11 and 12 are alternately `opened and closed as the relay winding 13 is alternately energized and deenergized by the output from the multivibrator 18, the frequency at which the relay winding 13 is energized being controlled by the frequency control knob 19 yof the multivibrator 18. Each time the relay contacts are opened, a main arc develops between the contacts 11 and 12 which arc is maintained |by the constant current lmain D.C. `supply source 23. The voltage drop across the main arc, i.e., between contacts 11 and 12, is proportional to the distance between the separated contacts. Therefore, -when the contacts are fully opened, the voltage across the contacts is substantially constant and nearly independent of the current therethrough. The current through the arc, therefore, is deter-mined primarily by the setting of the control knob 26 of the constant current type main D.C. supply source 23.

In the illustrated arrangement, the movable arm 38 of the switch 37 is shown connecting the resistor 29 to the capacitor 35. When the relay `contacts 11 and 12 are in the closed condition as illustrated, the RC integrating The -auxiliary network `comprising the resistor 29 and capacitor 35yis short circuited. When the relay contacts 11 and 12 are opened and an arc is developed thereacross, the arc voltage appears across the RC network and the capacitor 35 begins to charge through the resistor 29. After a period of time ydetermined by the RC time constant of the resistor 29 and the capacitor 35, and the voltage drop across the arc between the contacts 11 and 12, a sufcient voltage appears at the output of the RC integrating network across the capacitor 35 to trigger the silicon controlled rectifier 27 to a conducting state from the normally open condition. Le., the collector-to-control electrode voltage of the silicon controlled rectifier reachesv a suiiicient level to cause conduction of the rectifier. Upon conduction, the relay contacts 11 and 12 are short circuited through the rectier thereby terminating the mainarc. The resistor 29 is chosen of a suiciently small value such that when the arc is terminated and the relaycontacts 11 and.12 are still in the open condition, a sutiiciently large current ymay -be drawn therethrough to maintain the silicon controlled rectifier 27 in a conducting state even though capacitor 35 may partially dischargerunder such conditions.` Re-establishment of-'the arc during'the remainder of the test cycle during which the relay-contacts are open is thereby prevented. By way of example vbut not limita'- tion, the resistor 29vmay have a'resistancevalue ofl 4.7K ohms and the capacitors`31 through *.35 may have the values of 2, 4, 6, 8 and 10 microfarads, respectively. Thus, by control of the RC timel constant of the control circuit for the silicon controlled rectiiier'27, the time duration of the main are may be adjusted;

As mentioned above, the amount of metallic vapor'and metastable ions in the vicinitycf the contacts 11 and 12 during the arcing thereof has been found-to be proportional t-o the current-time integral'of the arc. The current of the arc may be adjusted by adjustment yof the control knob 26 of the main constant current D.C. supply source 23 while the time of the arc may be adjusted'by the time constant selected by switch 37. Thetrace ofl the "main arc voltage drop on the oscilloscope 42 provides a measure or indication of the current-time integral of the main arc, the trace of the main arc being designated 61 in the drawing. The slope, or chamfer, .61a of the main are during the initial portion of the illustrated waveform is dueto the increasing arc drop as the relay contacts are separating. Once the separation is complete, the arc drop voltage is substantially c-onstant. An amlmeter 62 may be included in series circuit with the main constant current D.C. upply source 23 for measurement of the current there rom. l

During operation of the relay, a parasitic arc may form between the main arc between the relay contacts 11 and 12 and the case 14 whereupon a current path for the auxiliary D.C. supply source 46 is completed, which path includes the relay case, the parasitic arc, the auxiliary D.C. source 46 and the resistor 47. When a parasitic a-rc is produced causing current tiow in the auxiliary circuit a voltage drop is created across the resistor 47 across which the second set of vertical input terminals of the oscilloscope 42 are connected. Thus, a trace off the parasitic arc, which trace is designated 63, is provided on the cathode-ray tube 66 of the oscilloscope. The resistor 47 is selected of a suciently large resistance value to prevent destruction of the relay 10 under test when a parasitic arc is drawn. A vacuum tube voltmeter 64 is shown connected between the case 14 and ground terminal 22 to provide a measure of the relay case voltage.

In testing the relay 10 for parasitic arc susceptibility, the output voltage from the auxiliary D.C. supply source 46 is increased manually by turning knob 48 during operation of the relay. If a parasitic arc forms between the relay case 14 and the lmain ar-c, current will flow in the auxiliary current circuit in the manner described above. This will produce the trace 63. The voltage necessary to produce the parasitic arc rrnay be read on voltmeter 64. The

sesame susceptibilityv to parasitick arcing, ldesignatedffK hereinbelow, may range from zero upwardly, with a small "K rating indicatingminim-um susceptibility t'o parasitic arcing and a larger numbensuchia's 8,'indicat`ing a greater susceptibility. 1 rPhe :following empirical formula.l 'may'f'be utilized in establishing the'A parasiticarc susceptibilty, K, of relays-tested vby the circuit of this'invention..

The susceptibility toparas'itic arcing, K; v.as determined by the above described 4circuit-p'roduces .a parent? curve for a familyof curves related by the current-time integral of the main arc." A "safe-"operating alternating current voltage for-relays or switches ativarious. .RtM.S..ourrent values in 1grounded A.`C. systems is determined, by :relating the current-time inte-gral of altrnating,'current waveform back tothe above 'parent'curve-`a", s` follows@ 707 Svrms where: Svms=the safe R.M.S. voltage forl grounded A.

tems,

e=base of natural logarithme, y Zt 103zamperemillisecond current-time integral for the alternating current which, in tu-rn, equals 2I peak A.C. 27rF It will be apparent that the test `circuit and operation thereof is in no way limited by the above formulas, the empirical Formula 1 and safe R.M.S.operating voltage Formula 2 -being included merely to indicate means for obtaining a uniform imeasure of parasitic arc susceptibility .and a safe voltage rating for the relay tested for worse-case conditions. Y

The invention now having been described in detail in accordance with the requirements of the Patent Statutes various changes and modications will suggest themselves to those skilled in this art. For example, any suitable RC integrating network may be employed, and where a variable RC time constant is desired, either a variable resistor or variable capacitor could be employed instead of the illustrated switching arrangement. Further, although the circuit has been described in terms of a novel relay test set for the testing of the parasitic arc susceptibility of relays, it will be apparent that the circuit has other applications. For example, the main arc circuit could be employed in an arrangement for shorting out circuit elements after a specified over-voltage has existed for a predetermined length of time. It will be apparent that the time at which the silicon controlled rectifier conducts to thereby short out the elements would automatically adjust to the various values of over-voltage; shorting the circuit to be protected more quickly for large overload values than for small values. In'such an arrangement, of course, the main D.C. supply source 23 could be of any type and would not necessarily be of the constant current type. Overload protection circuits of this general type are known as crowbar systems. In

addition, the main varccircuit arrangement could be employed in an electric resistance welding arrangement for control :fof the welding.1current.- Also,- in practice,` the parasitic arc may not only strike the case but may strike other?'relayrcomponentsV attached or connected thereto, such as the relay .poley piece, armature, or header. It willbe understood,.therefore, that reference to a parasitic .arc includes'not only arcing between the main arc and case i but also ,iarcing from the main arc to other relay components. It is intended that the above and other such changes and modifications shall fall within the spirit and scope of the invention as recited in the following claims.

I 1.A Apparatus for controlling the duration and current intensityy of '.ana'rcbetween firstand second electrical contacts so that said arc is extinguished when the time `integral'of thecu'rrent of'said arcreaches a predetermined value, comprising `a-.D.'C. constant-current supply connected in shunt with :said electrical contacts for establishing an arc between s'aid electrical contacts upon opening of said contacts,I

` normally open switching means in shunt circuit with Y -1 ,"said electrical contacts, lmeans responsive to the `voltage of said arc for pro- .1 `ducing a signal having a value dependent on said {volta-ge.I anden-the duration of Vsaid arc, said signal attaining a'given value when the duration of the arc fis such thatusaid time integral attains said predeterminedvalue, iandfmeans respon-siveto said signal for closing said switching means when said signal has said given value, 7thereby/:ttoshort'circuit said contacts and extinguish saidarc. 1

v2. The invention as recited in claim 1 including means for alternately opening and closing said electrical contacts.

3. Apparatus according to claim 1, wherein said normally open switching means comprises a solid-state controlled rectier including an anode, a cathode and a control electrode, and means connecting the anode-cathode path of said controlled rectifier directly in shunt with said electrical contacts;

said means for producing said signal comprises integrating means having an input and an output, means connecting said input directly in shunt with said electrical contacts, and means connecting said output between said control electrode and said cathode of said controlled rectifier, said integrating means producing across said output a potential difference the valuef of which depends on said voltage and said duration of said arc and attains said given value when said duration of said arc is such that said time integral attains said predetermined value, and

said means for closing said switch comprises said control electrode and said cathode of said controlled rectifier, said rectier being responsive to said p0- tential difference applied between said control electrode and said cathode to become conductive when said potential difference attains said given Value, said -rectilier short-circuiting said contacts and extinguishing said arc upon conduction thereof.

4. Apparatus for testing for parasitic arc susceptibility of a device which includes lirst and second electrical contacts enclosed in an electrically conductive enclosure, comprising means for alternately opening and closing said electrical contacts, a main D.C. source of Supply connected to said contacts for establishing a main arc therebetween upon the opening of lsaid contacts, means controlling the current-time integral of said main arc, and an auxiliary D.C. source of supply, connected between said conductive enclosure and one of said electrical contacts, for applying between said conductive enclosure and said one contact a voltage suiciently high to establish One C011* other voltage supplied to the other of said inputs,A 1

means connected tosaidiirst and second contacts for supplyingto said one input a voltage `proportional to the voltage between said contacts, means connected in circuit with said auxiliary sou'rce of supply for producing' a voltage proportional -to the current owingthrough saidfauxiliarygsource,

` means for supplying the last-named voltage -to said other input.

-6. Apparatus for testing for parasitic arc of separable electrical contacts enclosed ih-anfelectrically conductive enclosure comprising a .constantcurrent .D.-C. source of supply connected to two of said separable contacts, a solid state controlled grect'ier `including a control element, said rectifier being vconnected in shunt.circuit with said two contacts, an integrating networkiconnected in shunt circuit with said two contacts, means connecting theoutpu-t of said integrating network to'said: control element of said rectier for controlling the operating state of said rectifier, said rectiferproviding a short-'circuit between said two electrical contacts upon conductionof said rectifier, and' anaux'iliry D.'C. source of `supply/theonne-cted between said conductive enclosure-.and one-fof said two electrical contacts andk adjustable to :applyxbtween said conductive enclosure and said one contact aLIVoItage sufficiently high to -establish another arcl rbetween said main arc and said conductive enc1osure.- i

susceptibility f 18 Apparatus'according to claim 6, additionally includ- 1ng:,4 y means for'measuring the voltage between said 4one co`n` tf ltact and said-conductiveenclosure, 1 1 ,5 means,` having two inputs, for displaying a representation o f one voltage supplied to one of said inputs and for simultaneously displaying a representation ofganother voltage supplied to the other of said inputs, means connected to said two contacts for supplying the voltage betweemsaidcontacts to' said one input,

resistive means, connected in series Y'relationship with said auxiliary source of supply, for producing atvoltage proportional to the current owing through said f auxiliary source,'.and

means forsn'pplyi'n'g the last-'named voltage t o said other Rtflgllces Cited by the Examiner.

1 UNI-TED `STATES 1 PATENTS 25 20012.58v .-'301-.885 I ,-OIHER REii-:a-ENCS "GeneralElectric Company, Silicon-Controlled Rectifier Manial `Se"cond-'Edition,1961,"page 106. l if Richter, WJ- andBllioL AWL: "lri'lnstr'urnent for the De'- 1'39 termination of TContactMaking'and Breaking .Time,in

Transactions of vAmerican Institute of Electrical Engineering, January 1943*,1lvol. 62.

wLrER L. CARSON, Pfhm Exdminelry.

L. LEI'I, Assistant Examiner.

Claims (1)

1. APPARATUS FOR CONTROLLING THE DURATION AND CURRENT INTENSITY OF AN ARC BETWEEN FIRST AND SECOND ELECTRICAL CONTACTS SO THAT SAID ARC IS EXTINGUISHED WHEN THE TIME INTEGRAL TO THE CURRENT OF SAID ARC REACHES A PREDETERMINED VALUE, COMPRISING A D.-C. CONSTANT-CURRENT SUPPLY CONNECTED IN SHUNT WITH SAID ELECTRICAL CONTACTS FOR ESTABLISHING AN ARC BETWEEN SAID ELECTRICAL CONTACTS UPON OPENING OF SAID CONTACTS, NORMALLY OPEN SWITCHING MEANS IN SHUNT CIRCUIT WITH SAID ELECTRICAL CONTACTS, MEANS RESPONSIVE TO THE VOLTAGE OF SAID ARC FOR PRODUCING A SIGNAL HAVING A VALUE DEPENDENT ON SAID VOLTAGE AND ON THE DURATION OF SAID ARC, SAID SIGNAL ATTAINING A GIVEN VALUE WHEN THE DURATION OF THE ARC IS SUCH THAT SAID TIME INTEGRAL ATTAINS SAID PREDETERMINED VALUE, AND MEANS RESPONSIVE TO SAID SIGNAL FOR CLOSING SAID SWITCHING MEANS WHEN SAID SIGNAL HAS SAID GIVEN VALUE, THEREBY TO SHORT CIRCUIT AND CONTACTS AND EXTINGUISH SAID ARC.

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