US2643370A - Electric circuit checking equipment - Google Patents

Description

June 23, 1953 w. J. LAWRENCE ELECTRIC CIRCUIT cmacxmc EQUIPMENT Filed April 26, 1951 IN V EN TOR. ZDUZLOJIZ Jlawrenoe BY A T TORNE Y Patented June 23, 1953 ELECTRIC CIRCUIT CHECKING EQUIPMENT William J. Lawrence, Pittsburgh, Pa, assignor to Westinghouse Air Brake Company, a coi'lflllfa-v tion of Pennsylvania Application March 26, 1951, Serial No. 217,618

8 Claims. (Cl. 340-213) This invention relates to a circuit checking means and more particularly, to an apparatus that may be adapted to be associated with a control circuit of electro-pneumatic brake apparatus employed on a train of cars, and to selectively signal integrity or lack of integrity of the said control circuit and accompanying electrical devices and pneumatically actuated electric controllers associated therewith.

Electro-pneumatic brake systems in use on present-day railway trains, employ control wires or circuits which extend from the locomotive or control car at the head end of the train to all the cars in the train. These control circuits usually include two magnet valves, the application magnet valve and the release magnet valve, on each car of the train, the magnet valves being controlled through the train wire circuits by a pneumatically operated switch or controller which is normally located on the locomotive.

The desirability of continuously indicating the integrity or lack of integrity of the electrical control circuits and of the magnet valves and pneumatic controllers has been previously rec ognized, and various means and methods have been proposed and employed for this purpose. An arrangement for this purpose is described in the pending application, Serial No. 102,836, filed July 2, 1949, of C. M. Hines, now Patent No. 2,573,442, which is assigned to the assignee 01' this invention.

In the above mentioned pending application, the integrity check is made, while the brakes are released, by use of a Wheatstone bridge are rangement, and while the brakes are applied, by use of a current-responsive relay in a common return wire circuit of the train. However, any momentary fault in the release control circuit and crease of current in the release control circuit will not cause the circuit-checking apparatus to effect a fault indication.

The'above objects together with other objects which will be made apparent in the subsequent description of my invention, are obtained by apparatus to be hereinafter described when read in connection withthe accompanying drawing, wherein the single figure shows a simplified diagrammatic view of an electro-pneumatic control apparatus embodying my improved circuit-checking and signalling system.

While a simplified control apparatus embodying only one control circuit is shown in the drawing for the purpose of illustrating the nature and utility of my novel circuit-checking system, it will be understood that the system illustrated and described herein is intended to and may be applied to function with any similar electric control the common return wire circuit which causes the current in the current-responsive relay to decrease below its normal predetermined value effects thev same fault" indication as does a serious prolonged fault.

It is an object of the present invention to provide an improved circuit-checking apparatus 01' the type disclosed in the above mentioned application, characterized in that the occurrence of fault in the release control circuit or in the common return wire does not cause the circuit checking apparatus to give an indication of a fault unless the fault is of prolonged duration.

It is another object of my invention to provide an improved circuit-checking apparatus of the type referred to in the foregoing object, and,

iurther characterized in that any momentary deapparatus embodying a plurality of electrical controlling units in a plurality of control circuits. Therefore, even though this circuit-checking system is for particular use in electro-pneumatic brake control systems usually having two or more control circuits, it will be described herein, for the sake of simplicity, in connection with only one control circuit, taken to be the release control circuit.

The control circuit shown includes a source of direct current voltage, such as a battery I, a battery supply wire 2, and a battery return wire 3,. a plurality of magnet valve devices represented by the release magnet valve coils 4, 5, 6, 1, 8 and 9 (one for each of a corresponding number of different cars), a switch device l0, representing the usual pneumatically operated master controller switch, for completing a circuit to energize said magnet coils, and a common return wire I I.. It should be understood that any num ber of magnet coils may be checked, but for purposes of illustration, only six such coils are shown herein.

The circuit-checking and signalling apparatus shown in the drawing may be embodied in or attached to a suitable casing or panel board I2, and comprises a two-winding relay iii, a twowinding relay l4, and a single winding relay l5, each of these said relays having several contact members for completing circuits which are decontaining a Wheatstone bridge and coding equipment (not shown) which functions to check the integrity of the control circuit and, incidental thereto, supplies a pulsating D. C. voltage to a wire t l-a, when the control circuit is not energized. Cabinet 34 represents and includes the Wheatstone bridge arrangement and coding relays and circuits used and described in the mentioned pending application Serial No. 102,836 of C. M. Hines, but since such details are not required for an understanding of my present invention, cabinet 34 is shown in outlineform only. It should be understood that suitable connections (not shown) are provided between the Wheatstone bridge and coding equipment in cabinet 3% and the control circuit such that the equipment is automatically disconnected from the control circuit when the controlcircuit is ener gized and thus automatically ceases to supply a pulsating D. C. voltage to wire 34a at such time. Thus, so long as the switch device ii] is in its open position andthe mag-net coils l, 5, 6, l, 8 and 9 consequently deenergized, the circuitchecking apparatus in cabinet 34- will check the integrity of the control circuit in the manner explained in the mentioned pending application, Serial No. 102,836. That part of the checking apparatus which functions while the control circult is energized is depicted in detail in the dra wing since my improvement is concerned directly therewith. The operation of this portion of the checking equipment will now be described.

To condition the apparatus for operation, the operator first closes a two-pole switch 9 to establish a circuit which may be traced from the plus terminal of battery 1 by way of the battery supply wire 2 to the 13+ terminal post 20 on the panel 12, thence through the switch contact member 2| of the switch IS], a B+ wire 22, a wire 23, a current limiting resistor 24, a wire 25, the lower winding 26 of the relay I i, a wire 27, a wire 28, and a wire 29 to a B- terminal. post 30 on panel l2, and thence to the negative terminal of battery I by way of the battery return wire 3. The completion of this circuit causes the a relay H! to be energized to its picked-up position, thereby closing the contact member '3! and moving contact member 32 of the said relay to its upper closed position. With the relay l4 energized to its picked-up position, the operator now closes a switch 33 which establishes a circuit for energizing relay !5, which circuit may be traced from the plus terminal of battery I by way of the battery supply wire 2, a wire 2a, the bridge and coding arrangement in cabinet 34 which delivers pulsating voltage to a wire 34a, a contact member 32 of relay l 4 in its upper closed position, a wire 35, a wire 35, the closed switch 33, a wire 31, a wire, 38, a current-limiting resistor 39, a wire 40, winding 4| of relay I5, a wire #2, the B- wire 43, a closed contact mentber 44 of switch l9, a wire 45, and a, wire 29 to the B- terminal post 30, and thence to the negative terminal of battery I by way of the battery return wire 3. The completion of this circuit causes the relay l5 to be energized to its picked-up position, thereby closing the contact member 46 and moving the contact members t1 and 48 to their respective upper closed positions. After the contact member 46 of relay I5 is closed. the operator mayopen the switch 33 and relay l5 then maintains its own energiz-v ing circuit by the stick circuit completed through the closed contact member 46 of said relay, which parallels switch 33.

In parallel with the winding of relay i5, is branch circuit consisting of wire 4%, resistor wire 5i, a condenser 52 and a Wire 53. Condenser 52 is thus charged simultaneously upon energization of the winding of relay l5. Accordingly when the circuit for energizing the winding of relay I5 is interrupted, condenser 52 discharges current through the winding of relay ii to maintain relay l5 picked-up for a short time interval after the energizing circuit is interrupted for a purpose hereinafter made apparent.

So long as the relay I5 is energized to its picked-up position, contact member 48 thereof is actuated to its upper closed position, thereby completing a circuit for energizing the allclear lamp IE which indicates an absence of fault in the control circuit consisting of battery 1, battery supply wire 2, and the battery re turn wire 3 while the switch I0 is open. This circuit may be traced from the B+ wire 22 to the contact member 48 in its upper closed position, thence by wire 55, the all clean-lamp l6, and wire 58, to the 3- wire 43.

Also, when the relay [5 is energized, the contact member 41 of relay I5 is thereby actuated to its upper closed position to complete a circuit for energizing relay l3 to its picked-up position, thereby causing the contact member 6 2 of relay It to move to its upper closed position, and a contact member 65 of relay l3 to move to-its closed position. This circuit may be traced from the 13+ wire 22 by way of wire 51, contact member 4? in its upper closed position, wire 58, closed contact member 31 of relay It, wire 59, the lower winding 68 of relay l3, wire resister 52, and the wire 63 to the 3- wire 42.

The energization of relay l3 to its picked-up position and the consequent movement of con. tact member 64 to its upper closed position. con1 pletes a loop circuit in parallel with the lower winding 60 of relay l3, which circuit consists of a Wire 66, a resistor 61, a wire 68, contact member 54 in its upper closed position, wire 59, condenser 70 and a wire 1|. Condenser 70 is thus charged so that when the circuit just described for energizing relay l3 becomes interrupted, the current discharged from condenser Iii through the delay or loop circuit continues to energize the winding of relay [3 and cause the relay to remain picked-up for a short time interval, for a purpose hereinafter made apparent.

The switch H) represents a contact member of the usual master controllerde'vice. If the switch ill is open, the release control circuit shown herein is denergized (brake release position of the switch [0), and if the switch I0 is closed, the release control circuit is energized (brake application position of said switch).

The conditions described thus far are the normal conditions of the circuit checking and signalling apparatus with the switch Ill open and no fault existing in the battery supply and return circuit of the magnets 4 to 9.

Assuming that no fault exists in the circuit to be checked, the magnet coils 5, 6, l, B and are energized by closing the switch H3, and the following conditions result:

Switch 10 is effective when closed to complete a circuit for energizing magnet coils 4, 5, 8 and t. This circuit extends from the battery supply wire 2, by way of switch 10, a wire it, the various magnet coils 4, 5, 6, I, 8 and 9, the common return wire I l to the common terminal post '53 on the panel l2, and thence by wire M, resistor 15, a wire 16, wire 29, and B terminal post 30 to the battery return wire 3, thus energizing the said magnet coils to. perform, their brake control function, which is the closing of the exhaust ports through which fluid under pressure is released to effect release of the brakes. Connected in parallel with the resistor and subject to the voltage-drop across said resistor is a circuit consisting of a wire ll, 3, variable resistor or rhecstat is (having a manually operable contact arm 19, a wire 80, the upper winding 8! of relay [3, and wire 16. The contact arm 19 of resistor 18 is positioned manually (as will be explained) to indicate the number of magnet coils (in this case the number of cars on the train, assuming one magnet coil per car), as shown on an escutcheon plate 82. The voltagedrop across the resistor 15, with no faults in the control circuit is sufficient to cause the winding 8! of relay [3 to be energized so as to maintain the relay picked-up when the contact arm 59 of resistor it is positioned, as indicated on the escutcheon plate 82, in accordance with the actual number of magnet coils in the circuit. The positioning of the contact arm T9 of variable resistor it is such that with no fault in the control circuit, the resistor may be adjusted in correspondence with an addition or subtraction of the number of magnet coils in a circuit, so as to regulate the voltage energizing the winding ill of relay l3 and limit said voltage to an amount insufficient to cause relay 3 to pick-up, but sufficient enough to maintain the relay l3 pickedup after said relay is picked-up by some other means as will be explained hereinafter. Said adjustment of resistor '18 in accordance with the number of magnet coils is indicated. on the calibrated escutcheon plate 32.

a fault occurs in the circuit energizing magnet coils l, 5, 6, l, 8 and 9, such as would cause a reduction from the normal current in the common wire H and a corresponding reduction in the voltage-dropacross the resistor it below that required to energize the winding 8| of relay l3 sufficiently to maintain relay l3 picked-up, relay it will drop-out by reason of the concurrent deenergization of the lower winding 69 of relay E3 in a manner presently described. Thus, unless a fault occurs in the control circuit, relay :13 will remain energized and picked-up. while control circuit is energized.

Switch iii is also effective, when closed, to complete another circuit, which circuit may be traced n. the battery supply wire 2, by way of switch ill, wire 12, wire t3, terminal post 84, wire 85, the top winding 86 of relay i l, wire 2%, wire 28, and the B- terminal post Ell to the bat= tery return wire 3. This circuit energizes the top winding 86 of relay i l in a direction opposite that in which the lower winding 26 of relay M is energized. The opposing energization of the two windings S5 and 26 will efiect movement of the relay Hi to its dropped-out position, in which the contact member 35 is open and the contact member 32 is moved to its lower closed position. When the contact member 3! is opened, the circuit for energizing the lower winding 60 of relay i3 is thus interrupted thereby leaving the previously energized top winding 8: as the only means for maintaining relay 5% picl ed-up.

The contact member 32 of relay it is effective in its down closed position to interrupt the circuit previously described whereby pulsating D. C. voltage is supplied from the cabinet 34 for energizing the relay [5, and instantly thereafter to establish another circuit for energizing the windmg of; relay 15 by way of the 3+ wire 2-1, closed contact member 65- of relay l3, wire 81, contact member 32 in its down closed position, wire 3'5, closed contact member 46; 3i, wire. 38, resistor 39,, wire- 41],, winding 4| of relay t5, and wire- 42 to the B- wire 43. During the: short time interval which the contact member 32 of relay l4 is changing from the upper closed position to its lower closed. position, the relay i5 is maintained energized toits picked-up position by the current discharging from condenser 52 through the loop circuit consisting, of wire 51, resistor 5G, wire 49, wire 3-8, resistor 39, wire 40, winding M of the relay [5, wire 42', and wire 53. Prior to the expiration of the delayed. drop-out period of relay 5,. the energizing circuit for the winding of; relay l5, as previously traced, will have been established by contact member 32 in its lower closed. position, theneoy keeping the contact members of relay la in their respective picked-up positions.

As previously explained, if a fault occurs in the control circuit resulting in the reduction of current in said circuit, the voltage-drop then existing across resistor 75 is insufficient to maintainv the relay l3 picked-up and the contact mem. bers of said. relay are thus restored to their dropped-out or down positions. With contact member 65 in its down position. the circuit revious-1y described for energizing relay i5 is thereby I interrupted and thus, after a short time interval (time during which the condenser 52 discharges through the loop circuit previously described) the relay 15 will be deenergized and its contact members will move to their respective down positions. With contact member 48 of relay is in its down closed. position, a circuit is thus completed. from the 13+ wire 22, contact member 48', wire 88, the fault lamp H, and wire 89 to the 3- wire 43, thereby energizing the lamp I? and giving a visual indication of fault in the magnet coil or control circuit. With contact member 41 in its down closed position, a circuit is thus completed from the 13+ wire 22 by way of wire 51, contact member 41, wire 98, audible warning signal l8, and wire 9 I to the B- wire 43, thereby giving an audible warning of a. fault in the magnet 0011 circuit.

If the fault in the magnet coil circuit endures only momentarily, it is undesirable to have this fault indicated. A feature of my invention is to prevent the drop-out of relay it (which effect a fault indication) unless the fault on the control circuit is prolonged and continued. I obtain this result through the arrangement now to be ole scribed.

When the current energizing winding 8! of. relay i3 is unable to maintain the relay l3 picked" up by reason of a fault on the control circuit. relay 13 drops out causing contact member 54 to move to its down closed position and contact member 65 to move to its open position. The opening of contact member 65 interrupts the mentioned circuit from the B+ wire 22 for energizing said relay [5, but relay [5 does not immediately dropout due to the fact that condenser 52 discharges through. the winding of relay [5, as previously mentioned, and. thereby maintains relay is energized for a short time interval sufiicient to enable any momentary fault to be removed. However, even though the momentary fault on the control circuit is removed, the winding 81 of relay I3 is unable to cause pick-u of relay [3 when the winding 8| again becomes energized. As previously mentioned, the winding 81 of relay I3 is ineffective, when energized, to cause the relay 13 to pick-up, although, after the relay picked-up, winding ill is effective to maintain the relay picked-up. In order to cause the relay l3 to be picked-up upon termination of a momentary fault, the arrangement including condenser 10 is provided, according to my invention. With contact 64 of relay I3 in its lower closed position, condenser 10 discharges current through the lower winding 6|] of relay [3 by way of the loop circuit consisting of condenser l0, wire 59, contact member 64 of relay l3 in its lower closed position, wire 92, resistor 93, wire 94, wire 59, the lower winding Bil of relay I3, wire 6|, resistor 62, wire 63, wire 43, and wire ll, thus causing relay I 3 to be picked-up. As soon as the relay I3 is picked-up by the current discharging from the condenser 1'0 through the just described loop circuit, the contact member 54 of relay 1?, which is correspondingly shifted to its upper closed position, interrupts said loop circuit and re-estab lishes the charging circuit previously traced, for condenser 10.

Thus, if the fault is prolonged, relay #3 will continue to be alternately picked-up and dropped-out due to the completion of the corn denser discharge circuit for picking up relay It as soon as said relay drops out due to the fault.

The discharging time of condenser it is regu lated by the variable resistor 93. The resistor controls the charging time of said condenser iii and serves to so limit the amount of recharge the said condenser each time the contact member 54 of relay I3 is picked-up during the intermit tent operation of relay [3, that after the expira tion of a certain uniform time the condenser will be discharged sufficiently so as to be unable to effect the pick-up of relay i3, thereby termi-- nating continuous alternate pick-up and drop out of relay [3.

When the charge of the condenser it is ultimately dispensed after a certain length of time during which relay I3 continues to pick-up and drop-out alternately, the relay E3 is caused to drop-out and remain dropped-out to effect a fault indication as previously described, until said fault is corrected. However, if the fault is momentary and corrects itself before condenser iii is sufiiciently discharged, the upper winding iii of relay 13 becomes effective to maintain the said relay picked-up as previously described, and thus the relay 13 will stay picked-up, and the con" denser Hl will be fully recharged. Consequently the fault lamp ll will not be energized as it was when the relay 53 remained deenergized, previously described, because relay l5 will'not be caused to drop-out.

By means of the novel arrangement which I have provided, as above described, a fault indication is thus given only if the fault is prolonged and not if the fault endures only momentarily.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. For use in checking equipment used to check the integrity and lack of integrity of an electrical circuit, the combination of a first means operatively responsive to the existence of integrity or lack of integrity of said circuit While said circuit is deenergized, a second means operatively re-- sponsive to the existence of integrity or lack of integrity of said circuit while said circuit is energized, means responsive to energization and deenergization of said circuit to selectively render either said second means or said first means effective, respectively, means operative to cause said second means when effective to be responsive to the lack. of integrity of said circuit only when said lack of integrity endures longer than a 'certain uniform interval of time, and signal means common to both said first means and said second means and controlled alternatively thera by, depending on whether the said circuit deenergized or energized, for indicating the existence of integrity or lack of integrity of said circuit.

2. For use in a checking equipment used to check the integrity and lack of integrity of an electrical circuit, the combination of a relay having two separate windings, one of said windings being effective when energized to cause pick-up of said relay and to maintain said relay pickedup, the other winding being effective only when energized by a current exceeding a certain value for maintaining the said relay picked-up, a first means for energizing said one winding when said circuit is deenergized and for deenergizing said one winding when said circuit is energized, sec ond means for energizing said other winding above said certain value only so long as the integrity of said circuit is not impaired for reducing the energization of said other winding to below said certain value upon the impairment of the integrity of said circuit while circuit is energized, third means for intermittently energizing said one winding over a certain uniform interval of time while said circuit is energized to thereby cause intermittent pick-up and drop-out of said relay while the current energizing said other winding does not exceed said certain value, means responsive to drop-out of said relay only after the expiration of said certain uniform interval of time while said circuit is energized and while the current energizing said other winding does not exceed said certain value, and signal. means controlled by the last said means for indicating the existence of integrity or lack of in tegrity of said. circuit.

3. For use in a checking equipment used to check the integrity and lack of integrity of an electrical circuit, the combination of a relay having two separate windings, one of said windings being effective when energized to cause piclnup of said relay and to maintain said relay picked-up, the other winding being effective only when energized by a current exceeding a certain value for maintaining the said relay picked-up, a first means for energizing said one winding when said circuit is deenergized and for deenergizing said one winding when said circuit is energized, sec- 0nd means for energizing said other winding above said certain value only so long as the integrity of said circuit is not impaired and for reducing the energisation of said other winding to below said certain value upon the impairment of the integrity of said circuit while said circuit is energized, a condenser, means for first charging said condenser and then discharging said condenser repeatedly through said one winding for intermittently energizing said one winding over a certain uniform interval of time while said circuit is energized to thereby cause intermittent pick-up and drop-out of said relay while the current energizing said other winding does not eX- ceed said certain value, means responsive to drop out of said relay occurring only after the expiration of said certain uniform interval of time while said circuit is energized and while the current energizing said other winding does not exceed said certain value, and signal means controlled 9 by the last said means for indicating the existence of integrity or lack of integrity of said circuit.

4. For use in a checking equipment used to check the integrity and lack of integrity of an electrical circuit, the combination of a relay having two separate windings, one of said windings being efiective when energized to cause pick-up of said relay and to maintain said relay pickedup, the other winding being effective only when energized by a current exceeding a certain value for maintaining the said relay picked-up, a f rst means for energizing said one windin when said circuit is deenergized and for deenergizing said one winding when said circuit is energized, second means for energizing said other winding above said certain value only so long as the integrity of said circuit is not impaired and for reducing the energization of said other winding to below said certain value upon the impairment of the integrity of said circuit while said circuit is energized, a condenser, means for first charging said condenser and then discharging said condenser repeatedly through said winding for intermittently energizing said one winding while said circuit is energized to thereby cause intermittent pick-up and drop-out of said relay while the current energizing said other winding does not exceed said certain value, means for limiting the degree of recharge of said condenser during each successive recharge interval thereby to effect depletion of the charge on said condenser at the expiration of a certain uniform interval of time, means responsive to drop-out of said relay occurring after the expiration of said certain uniform interval of time as a result of depletion of the charge on said condenser, and signal means controlled by the last said means for indicating the existence of integrity or lack of integrity of said circuit.

5. For use in a checking equipment used to check the integrity and lack of integrity of a control circuit, the combination of a relay having two separate windings, means effective when said circuit is deenergized for causing one of the windings of said relay to be energized and effective when said circuit is energized for causing said one winding of said relay to be deenergized, said one winding of said relay being effective when energized to cause pick-up of said relay and to maintain said relay picked-up, the other winding of said relay being effective only when energized by a current exceeding a certain value for maintaining the said relay picked-up, first means for energizing said other winding of said relay above said certain value only while said circuit is energized and the integrity of said circuit is not impaired, and for reducing the energization of said other winding to below said certain value upon the impairment of the integrity of said circuit while said circuit is energized, second means for intermittently energizing said one winding of said relay over a certain uniform interval of time while said circuit is energized to thereby cause intermittent pick-up and drop-out of said relay while the said current for energizing said other winding does not exceed said certain value, and for deenergizing said one winding of said relay after said certain uniform time interval when said circuit is energized and said current for energizing said other winding of said relay does not exceed said certain value, and signal means controlled by said relay for indicating the existence of integrity or lack of integrity of said circuit.

6. For use in a checking equipment used to 10, c i check th t grit an lack of integrity O n c cal ci t he com nat on of a I 1 9 hall? ing two separate windings, means effective when said ui is deene ized or cau i one of the windings of said relay to be energized, and eiiece tive when said circuit is energized for causing s W n ng c ai rela to be de n d said one winding of said relay being effective when energized to cause pick-up'oi said relay and to maintain said relay picked-up, the other winding of said relay being effective only when energized by a current exceeding a certain value for maintaini-ng the said relaypicked-up, first means for energizing said other winding of said relay above said certain value only while said circuit is energized and the integrity of said circuit is not im; paired and for reducing the energization of said other winding to below said certain value upon the impairment of integrity of said circuit while said circuit is energized, second means for intermittently energizing said one winding of said relay over a certain uniform interval of time while said circuit is energized to thereby cause pick-up and drop-out of said relay while the said current for energizing said other winding of said relay does not exceed said certain value due to lack of integrity of said circuit for said certain uniform time interval, and for deenergizing said one winding of said relay after said certain uniform time interval when said circuit is energized and said current for energizing said other winding of said relay does not exceed said certain value. electro-responsive means controlled by said relay, and operatively responsive only after the elapse of a second certain uniform interval of time following drop-out of said relay to indicate a lack of integrity of said circuit.

7. For use in a checking equipment used to check the integrity and lack of integrity of an electrical circuit, the combination of a relay having two separate windings, means effective when said circuit is deenergized for causing one of the windings of said relay to be energized, and effective when said circuit is energized for causing said one winding of said relay to be deenergized, said one winding of said relay being effective when energized to cause pick-up of said relay and to maintain said relay picked-up, the other winding of said relay being effective only when energized by a current exceeding a certain value for maintaining the said relay picked-up, first means for energizing said other winding of said relay above said certain value only while said circuit is energized and the integrity of said circuit is not impaired and for reducing the energization of said other winding to below said certain value upon the impairment of integrity of said circuit while said circuit is energized, second means for intermittently energizing said one winding of said relay over a certain uniform interval of time while said circuit is energized to thereby cause pick-up and drop-out of said relay while the said current for energizing said other winding of said relay does not exceed said certain value due to lack of integrity of said circuit for said certain uniform time interval, and for deenergizing said one winding of said relay after said certain uniform time'interval when said circuit is energized and said current for energizing said other winding of said relay does not exceed said certain value, electro-responsive means operatively unresponsive to intermittent drop-out of said relay occurring during the said certain uniform interval of time and operatively responsive only to drop-out of said relay occurring 11 at the expiration of said certain uniform interval of time, and signal means controlled by said electro-responsive means to indicate integrity and lack of integrity of said circuit.

8. Circuit checking equipment comprising electro-responsive means operatively responsive to reduction of the normal current in the circuit resulting from impairment of the integrity of the circuit, means effective upon such operative response of said electro-responsive means for causing intermittent restoration of said electroresponsive means to the normal condition thereof over a certain period of time, so long as the impairment of the integrity of the circuit continues, relay means controlled by said electroresponsive means, means for preventing a change in the position of said relay means under the control of said electro-responsive means until after said certain period of time has expired, and signal means controlled by said relay means for indicating integrity and lack of integrity of the circuit,

WILLIAM J. LAWRENCE,

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,291,861 Harlow Jan. 21, 1919 1,349,961 Hopkins H Aug. 17, 1920 2,052,057 Reid Aug. 25, 1936 2,116,462 Hopkins, et a1. May 3, 1938 2,409,840 Derr et a1. Oct. 22, 1946 2,556,363 Lord et a1. June 12, 1951

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