US3452350A - Explosion suppression system including explosion simulation and testing apparatus - Google Patents

Description

June 24, 1969 c. F. ROCKWELL 3,452,350 EXPLOSION SUPPRESSION SYSTEM INCLUDING EXPLOSION SIMULATION AND TESTING APPARATUS Filed April 1966 Sheet I of 3 J z w? N g C 0 P a u. s aa w I) 5?.1 all! .4 Tm i -l PE 0 9- 0 "Q N 5 04? um U a g 5- E "'P U U W4! P 5 cs m o D 0 z z 2 I a 5 w E a 2% n 6 U y t; U m k 2 w 33 09 P g .IO F 2 w m '||||l|', A O O 52 2 r S 2 2 g 9 "5o 30) INVENTOR.

CHARLES F- ROCKWELL BY ,M m

1 ATTORNEYS June 24, 1969 c. F. ROCKWELL 3,452,350

EXPLOSION SUPPRESSION SYSTEM INCLUDING EXPLOSION SIMULATION AND TFS'I TNG APPARATUS Filed April 5. 1966 Sheet 2 of s an m 0 J -D 43 Z a N Q 2 a 5 k h S2 P a m 0 11..

JJ w- N L|: .N 4: e g o 4:

u z O z u. w o o a 8 .1 o O a o o g 2 E2 0 1 (Q P P P: .1: (fl 3 .u m N u E i o w w cnu Z Z I JNVENTOR. CHARLES F. ROCKWELL ATTORNEYS June 24, 1969 c F. ROCKWELL 3,452,350

EXPLOSION SUPPRES SION SYSTEM INCLUDING EXPLOSION SIMULATION AND TESTING APPARATUS Filed April 5, 1966 Sheet of 3 FIG, 3

U) I a I I hgllp I 5' $3 PJ l- N m H .J a m U 7 MW I E I a 1 I w v--- I I- CHARLES Rama BY I L? Q LHLC KUZL ATTORNEYS 3,452,350 EXPLOSION SUPPRESSION SYSTEM INCLUDING EXPLUSION SIMULATION AND TESTING APPARATUS Charles F. Rockwell, Sherborn, Mass., assignor to Fenwal, Incorporated, Ashland, Mass, a corporation of Massachusetts Filed Apr. 5, 1966, Ser. No. 540,318 Int. Cl. G08b 29/00 US. Cl. 340-410 4 Claims My invention relates to explosion suppression, and particularly to a novel self-testing explosion suppression system including apparatus for simulating an explosion and testing the suppression system without firing the suppressors.

Explosion suppressors have been developed for preventing an explosive rise of pressure following. the ignition of a combustible mixture of gases in an enclosed or partially enclosed container. These suppressors comprise a frangible container of inerting fluid, such as dibromotetrafluoroethane, and an electrically detonated charge for bursting the container and expelling the inerting fluid into the burning mixture before the combustion can result in a destructive rise in pressure.

An incipient explosion in a space may be detected by radiation responsive means, such as a photovoltaic cell, responding to radiation from a flame front to actuate the suppressor before a dangerous rise in pressure can occur. Illumination of the photocell by radiation characteristic of a flame actuates a circuit that supplies current to the firing circuit of the suppressor. When the suppressor is fired, its actuating circuit is broken.

In my co-pending US. application Ser. No. 540,318 filed on the same day as the present application, for Control and Indication System for Explosion Suppressors and assigned to my assignee, I have disclosed a control circuit for explosion suppressors in which a switch is included in the circuit for supplying current to the suppressors that is opened as soon as the suppressor ignition circuit is opened. By that arrangement, electrical voltage does not appear across the terminals of the suppressor after a suppressor has been actuated. The apparatus of my copending application also includes indicating means for visually indicating the actuation of an explosion suppressor, so that it will be replaced, and not relied upon for protection, after it has been used. Although that apparatus does provide a clear indication after the fact of the operation of the suppressor, it would be highly desirable to be able to ascertain the oeprativeness of the apparatus at any time without actuating the indicator and suppressors, so that a protection system including an open circuit, a short circuit, or a defective component, would not be relied upon for protection. It is the object of my invention to enable the complete check of an explosion suppression system, without more than a temporary interruption of its normal stand-by operating condition, and without reducing the safety and reliability of the system.

Briefly, the apparatus of my invention is built upon the basic combination of a source of voltage, an electronic switch, and a current-interrupted circuit connected in series, together with a condition-responsive signal generator for controlling the switch to close it in response to a predetermined condition and send current through the current-interrupted circuit to perform a desired function, such as the ignition of an explosion suppressor or the like. In accordance with my invention, I include in this series circuit a second switch that is normally closed. Connected in parallel with the second switch is a capacitor and a current limiter connected in series. I further provide sequencing circuits operating in response to a test command signal to open the second switch and then supply a nited States Patent ice simulated command signal to the signal generator to cause the first switch to be closed. The capacitor will thereby be charged through the current limiter. The sequencing circuits next open the first switch. Finally, the second switch is again closed, to discharge the capacitor through an indicating circuit that will respond only if the limited current has been passed by the basic circuit and the second switch is operating properly. By this arrangement, both circuit continuity and proper sequence of operation of all circuit components are checked. As will appear, the apparatus is automatically restored to its original state following the completion of a successful test.

The apparatus of my invention will best be understood in the light of the following detailed description, together with the accompanying drawings, of a preferred embodiment thereof.

In the drawings:

FIG. 1 is a schematic block and wiring diagram of a control and testing circuit for a current-interrupted element comprising the condition-simulating and circuit testing apparatus of my invention;

FIG. 2 comprises a timing chart illustrating the operation of the circuit of FIG. 1; and

FIG. 3 is a schematic wiring diagram of an explosion control system comprising a specific embodiment of the apparatus of FIG. 1.

In FIG. 1, I have shown a circuit arranged to supply a pulse of current to a current-interrupted actuating bridge wire W, as for an indicator, explosion suppressor, or the like, in response to a command signal supplied to a signal generator SG. The basic actuating circuit extends from ground through a suitable source of voltage, here shown as a battery B, across the terminals of a normally open switch S1, through the bridge wire W, and through a diode D1 and a normally closed switch S2 to ground.

The switch S1 is arranged to be closed by a switch control SCI, either by an output signal produced by the signal generator SG, in response to an applied command signal or a simulated command signal. The command signal may be any condition which it is desired to detect, such as the radiation from a flame front. The simulated command comprises a pulse of the same nature as the command signal, produced by the sequencing circuits SQ in response to a test command signal such as the depression of a pushbutton.

The sequencing circuits SQ also supply an operating control signal to a switch control SC2 for opening and closing the switch S2, supply a test reference voltage to a current limiter CL, and supply an indicate enable level to a test indicator TK in response to the test command level. An inhibit signal is applied to the sequencing circuit SQ from the junction of the bridge Wire W and the diode D1. The inhibit signal comprises a ground level current sink preventing operation of the sequencing circuits SQ to produce a simulated command signal while the switch S2 is closed. As will appear, that circuit prevents the operation of the test circuits to supply current to the bridge wire W if there are any shorts to ground in the firing circuit.

A capacitor C1 and a current limiter CL are connected in series, and the series combination is connected in parallel with the switch S2. The current limiter CL may simply comprise a resistor, but in the preferred embodiment also includes means controlled by the test reference signal supplied by the sequencing circuits SQ for limiting the voltage across the capacitor C1 in the test mode of operation.

As shown, a diode D2 is connected in series with a test indicator TK between ground and the junction of the capacitor C1 and the current limiter CL. When supplied with an indicate enable signal from the sequencing cir- 3 cuit SQ, with the switch S1 open, the test indicator will be operated by the discharge of the capacitor C1 through the diode D2 if the capacitor has been charged.

The operation of the apparatus of FIG. l is illustrated in FIG. 2. In FIG. 2, signals are indicated as 01f or on, switches are indicated as open or closed, and voltages are indicated as at volts, for ground potential, or above ground. Sequential times during the operation are indicated by sequentially indexed symbols ti, as indicated in FIG. 2. At 10, the switch S2 is closed, the switch S1 is open, the voltage Vc across the capacitor C1 is 0, the indicator TK is 011", the signal levels test command and simulated command are ofi, and the level inhibit is on. At some time t1, the test command level is produced and will stay on until some later time t10 at the end of the test.

At a very short time t2 later than 11, the switch S2 is opened, and the previously present inhibit signal goes off. At a predetermined time 13 later than [2-, the sequencing circuits Sq will produce a simulated command signal pulse applied to the signal generator 56.

At a time 14 slightly past t3, the signal generator SG will produce an output signal closing the switch S1 to permit a limited current flow from the battery B through the switch S1, the bridge wire W and the diode D1, charging the capacitor C1 through the current limiter CL. The current limiter CL is selected to produce a current that will not interrupt the bridge wire W. As indicated in FIG. 2, the time t5 at which charging begins is substantially t4.

Comparing FIGS. 1 and 2, as the capacitor C1 charges the voltage Vc will rise. At some time t6 after [3, determined by the operation of the sequencing circuits SQ, the simulated command signal will go off". The on time of this signal is selected simply to be long enough to operate the signal generator SG and short enough to be Well ahead of the time I8 when the switch S2 is closed. The time t7 when the switch S1 is opened may be caused to occur by timing or voltage measuring means controlled by the voltage across the capacitor C1, or by the closure of the switch S1, in the broader aspects of my invention. However, the simplest and preferred manner of performing this function is to employ an electronic switch as the switch S1 which is cut oif simply by the decrease of voltage across its load terminals as the capacitor C1 becomes charged to some desired value.

At a later time 18, determined by the sequencing circuits SQ, the switch S2 will be again closed, and the capacitor C1 will be discharged through the test indicator TK and the diode D2. Operation of the test indicator TK to produce its indication will indicate both that the circuit is intact from ground through the battery, the switch S1, the bridge wire W and the diode D1, and that the switch S2 is functioning properly. Should there be a ground in the firing circuit, or if the switch S2 has failed to open, the inhibt signal will be produced, and even if it is not produced and current is caused to flow through the wire W, breaking it, the capacitor C1 will not be charged and it will be realized that service is needed.

As soon as the switch S2 is again closed, at t8, the inhibit level will be produced to prevent further operation of the sequencing circuits SQ. The apparatus will remain in that condition until the time I10 when the test command signal is removed, causing the indicator to go back to its ofi state.

FIG. 3 shows a specific embodiment of my invention adapted for use in an explosion suppression system, as for fuel tank protection in aircraft and the like. A typical installation is designed to protect a set of fuel tanks in the wing of an aircraft having vents communicating with a surge tank, the latter being vented through a vent conduit leading to a port on the wing.

The apparatus of FIG. 3 comprises a pair of explosion supporters XS1 and X52 of conventional design, adapted to be mounted in spaced relation in the wall of a surge tank or other container to be protected. Preferably, the apparatus includes an indicator TK, of any suitable electrically operated, visual type, adapted for mounting in an exposed place in which it will be readily observed, as in the outer surface of an aircraft. The signal generator SG comprises a photocell PC, such as a silicon solar cell or the like, to be mounted in position to respond to radiation from a flame front corresponding to an incipient explosion. For example, in the aircraft protection system just described, the photocell PC would preferably be mounted in the wall of the vent conduit.

The explosion suppressors XS1 and XS2 may be of any conventional type, and may comprise, for example, a frangible steel container of an inerting fluid, such as dibromotetrafluoroethane or the like, and an electrically detonated charge for bursting the container. The electrically detonated charge may comprise an electric blasting cap or the like. The indicator K may be of the conventional type actuated by a charge ignited by current, in which the force of the explosion is used to provide the desired indication, as by casting a quantity of bright colored powder from a concealed position to a position where it is visible. Such an indicator also serves as a fuse in the circuit, providing an open circuit across its terminals after actuation to give additional insurance against voltage appearing across the open terminals of the suppressor after their actuation. If another form of indicator, or no indication, is provided, a fuse is preferably included in the circuit.

The firing circuit for indicator K should be substantially the same electrically as those of the suppressors XS1 and X82, and include a bridge wire W1 inserted in the charge for setting it ofif when current is applied. As shown, the suppressors XS1 and X52 are provided with similar bridge wires W2 and W3 connected in series with the bridge wire W1 of the indicator K. The firing circuit for the suppressors and indicator extends from the positive terminal B+ of the power supply, not shown, such as a 28 volt D.C. source, in series through the switch S1, the bridge wires W1, W2 and W3, the diode D1, and the switch S2 to ground in the same manner as described in connection with FIG. 1.

The switch S1 comprises a controlled rectifier SCR1 having its anode and cathode connected in the firing path. The path between the anode and cathode is made conductive by gate current supplied to the gate terminal of the controlled rectifier SCR1 with respect to its cathode. Preferably, a capacitor C2 is connected between the anode of the controlled rectifier SCR1 and ground to protect the controlled rectifier against false operation by voltage transients in the supply. If desired, the controlled rectifier SCR1 may be replaced by another form of electronic switch, such as a power transistor, silicon controlled switch, or the like, without departing from the scope of my invention in its broader aspects.

The control unit SC1 for the switch S1 comprises a tunnel diode TD connected between the gate terminal and the cathode of the silicon controlled rectifier SCR1. The tunnel diode TD will pass current applied to it in the forward direction below a background level which it is desired to ignore, and will snap through its valley region to switch current into the control gate of the silicon controlled recitfier SCR1 and gate the latter into conduction when the current supplied to the tunnel diode is above the predetermined background level.

A control circuit for the tunnel diode TD comprises the signal generator SG including a photovoltaic cell PC. The photocell PC is connected across the input terminals of a conventional DC amplifier A, to produce a current signal when the photocell PC is irradiated.

The output terminals of the amplifier A are connected between ground and a second terminal connected to the anode of the tunnel diode TD. The cathode of the tunnel diode TD is returned to ground through three alternate parallel paths. The first comprises the firing circuit for the indicator and suppressors, and extends through the bridge wires W1, W2, and W3 of the indicator and suppressors in series, through the diode D1, and through the switch S2 to ground. As shown, the switch S2 comprises a controlled rectifier SCR2 having its anode and cathode connected in the series firing path.

A second path from the cathode of the tunnel diode TD also includes the indicator and suppressors and the diode D1 in series. The second path extends from the cathode of the diode D1 through a storage capacitor C1, and through the current limiter CL to ground.

A third path extends from the cathode of the tunnel diode TD through a timing network comprising part of the sequencing circuits SQ. This network includes a series path including an isolating diode D3 and a resistor R1. The resistor R1 extends to ground through a first path comprising a capacitor C7, and through a second path to ground through an isolating diode D4 and a capacitor C3 connected in parallel with a voltage divider comprising a resistor R2 and a resistor R3 in a series.

The gate terminal of the controlled rectifier SCR2 is returned to ground, and the cathode of the controlled rectifier SCR2, through a switch control circuit SC2 comprising an npn transistor Q1 having its collector connected to the gate terminal of the controlled rectifier SCR2 and its emitter connected to ground. The collector of the transistor Q1 is returned to the positive terminal B+ of the power supply through a resistor R4. The base of the transistor Q1 is connected to the junction of the resistors R2 and R3 in the sequencing circuits SQ.

The current limiter CL comprises an isolating diode D5 having its anode connected to one terminal of the capacitor C-1, and its cathode connected to ground through a resistor R5. A Zener diode Z1 has its anode connected to the junction of the diode D5 and the resistor R5, and its cathode connected to the test command input terminal. The resistor R5 serves to limit the current flowing through the capacitor C1 during the test mode of operation, in a manner to appear, and the Zener diode Z1 serves to block the diode D5 until the voltage at the cathode of the diode D1 is greater than a value equal to the positive supply voltage less the Zener breakdown drop across the Zener diode Z1. The purpose of that arrangement is to prevent the buildup of charge on C1 unless the switch SCR'l completely c oses.

The indicator TK comprises a lamp L2 controlled by a controlled rectifier SCR3 and an npn transistor Q2. As shown, an energizing circuit for the lamp L2 extends from the input terminal a of the sequencing circuits SQ through the filament of the lamp L2, and through the anode-to-cathode path of the controlled rectifier SCR3 to ground. A relatively small capacitor C4 is connected in parallel with the anode and cathode of the controlled rectifier SCR3 to protect the controlled rectifier against transients that might jolt it into conduction. This capacitor is chosen to be too small to permit noticeable blinking of the lamp L2 when the test command level is first applied. As shown, the test command level may be produced by depression of a pushbutton B connected to the positive terminal B+ of the power supply.

The gate terminal of the controlled rectifier SCR3 is connected to the collector of the transistor Q2, and the emitter of the transistor Q2 is connected to ground and to the grounded cathode of the controlled rectifier SCR3. The collector of the transistor Q2 is returned to the test command terminal a through a resistor R6. The base of the transistor Q2 is returned to the positive terminal B+ of the power supply through a resistor R7, and is connected to ground through a path including the diode D2, the capacitor C1, and the controlled rectifier SCR2 in its conducting state. It will be apparent that with the controlled rectifier SCR2 non-conducting, the base emitter path of the transistor Q2 will be forward-biased by current through the transistor R7. When forward biased,

the transistor Q2 will conduct saturation current be tween its collector and emitter, holding the gate terminal of the controlled rectifier SCR3 substantially at ground potential and thereby preventing conduction through its anode-to-cathode path. The lamp L2 is accordingly de-energized under these conditions even though the test command level may be present.

The sequencing circuits SQ include a lamp L1 illuminated at times to simulate a radiation signal produced by a flame front. In practice, the lamp L1 is mounted adjacent the photocell PC in the signal generator SG, to illuminate the photocell when the lamp L1 is energized. An energizing circuit for the lamp L1 extends from the input terminal a of the sequencing circuits SQ through the emitter-to-collector path of an npn transistor Q3 and thence through the lamp L1 to ground.

An inhibiting path at times preventing the forwardbiasing of the transistor Q3 extends from the base of the transistor Q3 through an isolating diode D6 and through the diode D1 and the controlled rectifier SCR2 in its conducting state to ground. The purpose of the inhibiting circuit is to protect the suppressors against actuation when the test command level is produced and the controlled rectifier SCR2 is conducting or shorted. The circuit also protects against operation of the lamp L1 in the event that there is a ground in the suppressors.

The base of the transistor Q3 is biased with respect to its emitter by a timing network. This network includes a first voltage divider, extending from the input terminal a of the sequencing circuits SQ to ground, and comprising in series a resistor R8 and a Zener diode Z2. The breakdown voltage of the Zener diode Z2 is selected to be a desired level below the test command potential B+ convenient for biasing other circuit components in a manner to be described.

The cathode of the Zener diode Z2 is returned to ground through a path extending through a capacitor C5, a resistor R9, and thence through a capacitor C6 in parallel with a Zener diode Z3 to ground. The Zener diode Z3 is selected to produce a voltage drop adequate to bias the base of the transistor Q3 forward with respect to its emitter by a sufficient amount, such as six volts, to cause the lamp L1 to be lighted. As shown, the base of the transistor Q3 is connected to the anode of the Zener diode Z3. It will be apparent that when a test com mand signal level is produced, the base of the transistor Q3 will rise from ground to the Zener breakdown potential of the Zener diode Z3, and then return to ground by discharge of the capacitor C6 as the capacitor C5 continues to charge up to the breakdown potential of the Zener diode Z2. This action will produce a flash of light from the lamp L1, applying a simulated command pulse to the photoconductive cell PC.

A second timing network extends from the cathode of the Zener diode Z2 through the capacitor C5, a diode D7 and thence to ground through the capacitor C3 in parallel with the voltage-dividing resistors R2 and R3.

Having described the apparatus of FIG. 3, its operation will next be described. First, consider the state of the apparatus with the test pushbutton TB open and the photocell PC dark. The amplifier A will produce no output signal, or at most a background noise signal that will be passed by the tunnel diode TD without triggering the control rectifier SCRl. The capacitor C2 will be charged to B+. The transistor Q1 will be non-conducting; thus, gate current will be supplied to the controlled rectifier SCR2 to hold it in a conducting state. The capacitors C1, C3, C7, C5 and C6 will be essentially discharged.

Assuming it is now desired to test the apparatus of FIG. 3, the test command pushbutton TB is depressed, causing the level test command to appear, at a potential of B+, at the input terminal a of the sequencing circuits SQ. The Zener diode Z2 serves to protect the circuit components from over-voltage.

The capacitor C5 will commence to charge through a first path including the resistor R9 .and the capacitor C6, and a second path including the diode D7 and the capacitor C3. Comparing FIG. 2 with FIG. 3, at a time shortly after the test command level appears, the voltage across the resistor R3 will rise to a value gating the transistor Q1 on, bringing the gate terminal of the controlled rectifier SCR2 essentially to ground potential and thereby cutting it off. Thereafter, as the capacitor C6 is charged, the voltage will rise to the potential of the Zener diode Z3 and remain at that level briefiy while the lamp L1 is flashed. That will occur when the transistor Q3 is both forward-biased and its base is high enough above ground potential to supply sufiicient current to illuminate the lamp L1. As the capacitor C5 continues to charge, the capacitor C6 will discharge through the baseemitter path of the transistor Q3 and the lamp filament L1, and the lamp L1 will go out.

At a time t3 shortly after the lamp L1 has flashed, the photoconductive cell PC will produce an input current to the amplifier A to cause it to supply current to the tunnel diode. TD. This current will return to ground through the diode D3, the resistor R1, and the capacitor C7. Diode D4 will be in its blocked state because of the voltage across the capacitor C3. The silicon controlled rectifier SCR2 will be non-conducting at this time, and current cannot flow through the capacitor C1 because the current limiter CL will be in its blocked state at this time. In one practical embodiment of the invention, positive potential 13+ was 28 volts and the Zener diode Z1 had a breakdown voltage of 6 volts, resulting in 22 volts across the resistor R5 blocking the diode D5.

As soon as the tunnel diode TD is triggered on, the silicon controlled rectifier SCR1 will be gated into conduction and the current will flow from B+ through the bridge wires W1, W2 and W3 of the indicator K and suppressors X81 and X82, respectively. Current will fiow from the bridge wire W3 through the diode D1 and charge the capacitor C1 through the current limiter CL substantially as soon as the controlled rectifier SCR1 is gated on, because the battery voltage of 28 volts, less the drops across the indicator and suppressors and the forward gaps of the controlled rectifier SCR2 and the diodes D1 and D5, will exceed the voltage across the resistor R5. However, the current charging the capacitor C1 will be limited both by the resistor R5 and by the maximum voltage of 3 or 4 volts that can be applied across the capacitor C1.

As the capacitor C1 is charging, the capacitor C7 is also charging through the controlled rectifier SCR1, the diode D1 and the resistor R1. At a point during the charge of the capacitor C7 as the capacitor C3 discharges, the diode D4 will become forward-biased and the voltage across the resistors R2 and R3 will be held up as long as the controlled rectifier SCR1 is conducting to hold the transistor Q1 in conduction and thus block the controlled rectifier SCR2. As the capacitors C1, C7 and C3 continue to charge, the current through the controlled rectifier SCR1 will go so low that conduction will cease, and the controlled rectifier will not conduct again until again triggered.

When the controlled rectifier ceases to conduct, the capacitors C7 and C3 will discharge through the resistors R2 and R3 for a predetermined time, such as two or three seconds. At the end of that time the transistor Q1 will no longer be forward-biased, and its collector will rise to a positive potential with respect to ground, gating on the controlled rectifier SCR2. When the controlled rectifier SCR2 is in its conducting state, a negative potential is applied by the capacitor C1 through the diode D2 to the base of transistor Q2 with respect to its grounded emitter. That will momentarily cut off the transistor Q2 while the capacitor discharges through the leakage resistance of the emitter-base path and R7. When the tarnsistor Q2 is cut off, the gate terminal of the controlled rectifier SCR3 will rise in potential and draw gating cur- 8 rent with respect to its cathode, causing the anode-tocathode path of the controlled rectifier SCR3 to become conducting and allow the lamp L2 to be illuminated. The lamp L2 will remain illuminated, indicating proper operation of the apparatus, until the test pushbutton TB is released. The apparatus will then be in its standby state, with the controlled rectifier SCR1 cut ofii and the controlled rectifier SCR2 gated on.

It will be apparent that the capacitor C1 can be charged during the initial portion of the testing cycle only through the actuating bridge wires of the series-connected indicator and suppressors. The limitation on the voltage appearing at the base of the transistor Q3, for example, to 6 volts, makes it impossible for the voltage at that point to charge the capacitor C1, because the diode D5 and the current limiter C1 is blocked by the Zener diode Z1 until a much higher potential of, for example, 28 volts, is applied. If the suppressor firing circuit is not intact, the capacitor C1 will not be charged. At the same time, the controlled rectifier SCR2 is tested by the fact that the capacitor C1 will not charge if the anode-tocathode path of the controlled rectifier is shorted to ground or fails to open electrically. A short circuit to ground in the suppressor and indicator line will cause the operation of the lamp L1 to be inhibited, also preventing the charging of the capacitor C1. Finally, when a charge is stored on the capacitor C1 the conducting state of the controlled rectifier SCR2 is tested by requiring it to be in a conducting state before the lamp L2 can be turned on.

Operation of the apparatus of FIG. 3 in its explosion suppressing mode of operation will not be affected by the testing and simulating circuits. If the photocell PC is illuminated by radiation signalling an incipient explosion, the tunnel diode TD will trigger the controlled rectifier SCR1 into conduction and supply a pulse of actuating current to the bridge wires of the indicator and suppressors, and thence to ground through diode D1 and the controlled rectifier SCR2. The controlled rectifier SCR1 will be gated ofi .as soon as the bridge wires W1, W2 and W3 are broken, preventing any application of voltage to the terminals of these devices after their actuation. In order to avoid interference with this desirable feature, the resistance to ground from the cathode is made sufficiently high that the controlled rectifier SCR1 cannot be gated into steady conduction, because the high resistance to ground from the cathode of the controlled rectifier SCR1 limits its current to a value too low to permit conduction to be sustained. By that arrangement, retriggering of the controlled rectifier SCR1 is prevented after the suppressors have been operated.

While I have described my invention with respect to the details of a preferred embodiment thereof, many changes and variations will become apparent to those skilled in the art upon reading my description, and such can obviously be made without departing from the scope of my invention. In particular, while I have described a specific embodiment particularly adapted for use as a fuel tank protection system, it is contemplated that the apparatus of my invention in its broader aspects can be applied to the non-destructive testing of other circuits including current-interrupted circuit elements.

Having thus described my invention, what I claim is:

1. Condition simulating and circuit testing means for a control system; said control system comprising first signal generating means for producing an electrical signal in response to a predetermined input signal and a series circuit comprising a source of voltage, a first switch closed by said electrical signal, and control means comprising a conductor connected between a pair of terminals in said circuit, said conductor conducting current below a predetermined value and being broken by current above said predetermined value, in which said source supplies current sufiicient to interrupt said conductor when said first switch is closed; said simulating and testing means comprising a second normally closed switch in said series circuit, a capacitor connected in parallel with said second switch, current limiting means connected in series with said capacitor to limit the flow of current therethrough to a value below said predetermined value, system test signal generating means for producing a test signal, means responsive to said test signal for opening said second switch, first time delay means responsive to said test signal for producing a simulating signal a predetermined time after said second switch is opened, means responsive to said simulating signal for applying an input signal to said first signal generating means to thereby cause said first switch to be closed to charge said capacitor through said conductor with a current below said predetermined value, means for opening said first switch when said capacitor is charged to a predetermined voltage, signal generating means connected in series with said capacitor and said second switch for producing an output signal indicating proper operation of said system when said second switch is closed and discharge current flows from said capacitor, and second time delay means responsive to the opening of said first switch for closing said second switch a predetermined time after said first switch is opened to restore the system to normal operation,

2. The apparatus of claim 1, in which said first signal generating means comprises radiation detecting means for producing a current signal in response to radiation from a flame, and said conductor comprises the bridge wire of an electrically actuated explosion suppressor.

3. Apparatus for the non-destructive testing of an electrical circuit of the kind comprising a circuit element interrupted by the passage of current above a predetermined value and connected in series with a source of direct voltage and a normally open first switch, in which closing the first switch will cause the flow of current above said predetermined value in the circuit element, said ap paratus comprising a second normally closed switch connected in series with said circuit element, a capacitor and current limiting means connected in a first series combination, said series combination being connected in parallel with said second switch, a rectifier and a voltage indicator connected in series with said capacitor in a second series combination, said second series combination being connected in parallel with said second switch, said rectifier being poled to oppose the flow of current from said source, said voltage indicator comprising means for producing an output signal when said capacitor is charged by said source and said second switch is closed, means for producing a test command signal, means controlled by said test command signal for opening said second switch, first time delay means responsive to said command signal for closing said first switch after said second switch is opened to charge said capacitor through said circuit element and said current limiting means, said current limiting means holding the charging current below said predetermined value, means responsive to the charging of said capacitor for opening said first switch when a predetermined charge has been stored in the capacitor, second time delay means enabled by the closing of said first switch and responsive to the opening of said first switch to close said second switch a predetermined time after said first switch is opened and thereby discharge said capacitor through said rectifier and said voltage indicator, whereby said indicator produces an output signal only when the circuit to be tested is operative.

4. An explosion suppression system for protecting a container at least partially enclosing a space that may contain a combustible gas against an explosive rise of pressure following ignition of gas, comprising an explosion suppressor mounted in the container and having a bridge wire and an explosive charge ignited by current above a predetermined value flowing in the bridge wire and breaking the wire, a radiation detector mounted in position for radiation from a flame front propagating in the container to produce a first output signal, a first normally open electronic switch, a second normally closed electronic switch, a source of direct voltage connected in series with said switches and said bridge wire, means responsive to said first output signal for closing said first switch, a capacitor and current limiting means connected in a first series combination, said first combination being connected in parallel with said second switch, a rectifier and a voltage indicator connected in series with said capacitor in a second series combination, said second combination being connected in parallel with said second switch, said rectifier being poled to oppose the flow of current from said source, said voltage indicator comprising means for producing a second output signal when said capacitor is charged by said source and said second switch is closed, means for producing a test command signal, means controlled by said command signal for opening said second switch, flame simulating means responsive to an applied signal to irradiate said radiation detector and cause it to produce an output signal, first time delay means responsive tosaid command signal to apply a signal to said flame simulating means to cause said first switch to close a predetermined time after said second switch is opened to charge said capacitor through said bridge wire and said current limiting means, said current limiting means holding the charging current below said predetermined value, means responsive to the charging of said capacitor for opening said first switch when a predetermined charge has been stored in the capacitor, second time delay means enabled by the closing of said first switch and responsive to the opening of said first switch to close said second switch a predetermined time after said first switch is opened and thereby discharge said capacitor through said rectifier and said voltage indicator, whereby said indicator produces an output signal only when the circuit to be tested is operative, and all components of the suppressor firing circuit are tested for proper operation.

References Cited UNITED STATES PATENTS THOMAS B. HABECKER, Primary Examiner.

U.S. Cl. X.R.

Claims (1)

1.CONDITION SIMULATING AND CIRCUIT TESTING MEANS FOR A CONTROL SYSTEM; SAID CONTROL SYSTEM COMPRISING FIRST SIGNAL GENERATING MEANS FOR PRODUCING AN ELECTRICAL SIGNAL IN RESPONSE TO A PREDETERMINED INPUT SIGNAL AND A SERIES CIRCUIT COMPRISING A SOURCE OF VOLTAGE, A FIRST SWITCH CLOSED BY SAID ELECTRICAL SIGNAL, AND CONTROL MEANS COMPRISING A CONDUCTOR CONNECTED BETWEEN A PAIR OF TERMINALS IN SAID CIRCUIT, SAID CONDUCTOR CONDUCTING CURRENT BELOW A PREDETERMINED VALUE AND BEING BROKEN BY CURRENT ABOVE SAID PREDETERMINED VALUE, IN WHICH SAID SOURCE SUPPLIES CURRENT SUFFICIENT TO INTERRUPT SAID CONDUCTOR WHEN SAID FIRST SWITCH IS CLOSED; SAID SIMULATING AND TESTING MEANS COMPRISING A SECOND NORMALLY CLOSED SWITCH IN SAID SERIES CIRCUIT, A CAPACITOR CONNECTED IN PARALLEL WITH SAID SECOND SWITCH, CURRENT LIMITING MEANS CONNECTED IN SERIES WITH SAID CAPACITOR TO LIMIT THE FLOW OF CURRENT THERETHROUGH TO A VALVE BELOW SAID PREDETERMINED VALUE, SYSTEM TEST SIGNAL GENERATING MEANS FOR PRODUCING A TEST SIGNAL, MEANS RESPONSIVE TO SAID TEST SIGNAL FOR OPENING SAID SECOND SWITCH, FIRST TIME DELAY MEANS RESPONSIVE TO SAIDD TEST SIGNAL FOR PRODUCING A SIMULATING SIGNAL A PREDETERMINED TIME AFTER SAID SECOND SWITCH IS OPENED, MEANS RESPONSIVE TO SAID SIMULATING FOR APPLYING AN INPUT SIGNAL TO

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