US 3524408 A
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United States Patent Inventor Edward G. Pierson Grand Island, New York Appl, No. 699,450 Filed Jan. 22, 1968 Patented Aug. 18, 1970 Assignee Conax Corporation New York, New York a Corp. of Delaware by mesne assignments.
ELECTROSTATIC DISCHARGE DISSIPATOR FOR A HEATER BRIDGEWIRE CIRCUIT OF AN ELECTRO-EXPLOSIVE DEVICE 16 Claims, 4 Drawing Figs.
US. Cl 102/28, 102/702 Int. Cl F42b 3/18 Field of Search 102/28, 70.2
 References Cited UNITED STATES PATENTS 1,807,708 6/1931 Ruhlemann 102/702 2,818,020 12/1957 Burklund 102/28 2,934,015 4/1960 l-lerdman.... 102/702 2,996,991 8/1961 Menzel 102/702 3,001,477 9/1961 Ruehlemann 102/702 3,293,527 12/1966 Julich 102/702 3,320,889 5/1967 Holtz l02/28X Primary Examiner Verlin R. Pendegrass A ttorney Sommer, Weber and Gastel ABSTRACT: The heater bridgewire circuit of an electro-explosive device is rendered safe against unintentional firing resulting from electrostatic discharge by operatively associating with such circuit an electrostatic discharge dissipator of a gaseous electrical conductable type which includes electrodes arranged in a predetermined environment of an ionizable gas so that one electrode is electrically connected to a lead wire for the heater bridgewire and another electrode is electrically grounded.
Patented Au 18, 1970 3,524,408
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Edward G. Pierson ATTORNEYS ELECTROSTATIC DISCHARGE DISSIPATOR FOR A HEATER BRIDGEWIRE CIRCUIT OF AN ELECTRO- EXPLOSIVE DEVICE BACKGROUND OF THE INVENTION In the field of electro-explosive devices such as are used in commercial blasting, military applications and latterly space exploration, lack of measures to prevent inadvertent electrostatic discharge has resulted in unintentional firings and in some cases even fatalities to personnel.
A typical electro-explosive device includes a pair of lead wires connected to a heater bridgewire embedded in a body of explosive material. With the electrostatic discharge problem, the initiating phenomenon consists of an are which occurs between one bridgewire circuit and the outer case of the device or between bridgewire circuits when redundant circuitry is employed. Since the heater bridgewire is located in the initiating explosive material, it is presumed that any electrostatic discharge between circuits. or between circuit and case, involves a positive ion bombardment of the ignitable material in the path of the are.
If the discharge voltage is sufficiently high to initiate con' duction, only a very small milliampere current of fractional watt second energy'is required to produce ignition of the explosive material. The human body is capable of building up a static charge of 810,000 ergs equivalent to a 500 mmf capacitor charged to 18,000 volts.
The average electro-explosive device can be inadvertently fired if carried about while holding onto the lead wires or the case and at the same time contacting a grounded conductor such as a pipe or radiator, or contacting even an ungrounded conductive mass of comparatively large size with the case or lead wires not in direct contact with the person's body. The fact that the lead wires may be twisted or fastened together does not alter the susceptibility of the device to electrostatic discharge firing.
Spark gaps of millimeter size either hermetically sealed or open to atmosphere have been used in an attempt to restrict the maximum potential that can be applied and maintained between the bridgewire circuit and the case of an electro-explosive device. But the use of such spark gaps has not been fully satisfactory. This is because in an uncontrolled vacuum or gaseous atmosphere, the gap breakdown voltage value can be very unpredictable unless the gap is built with extreme precision. Moreover, ionizing potentials may change with each discharge even though the gap length remains constant.
SUMMARY OF THE INVENTION It is accordingly the primary object ofthe present invention to provide means which are predictable and reliable in performance dissipating an electrostatic discharge in an electroexplosive device and thus protect said device against accidental firing by spark ignition.
This is achieved through the use of gaseous electrical conductable means including two or more electrodes arranged in a controlled or predetermined environment of an ionizable gas and operatively associated with circuits to provide one or more preferential alternate electrostatic discharge paths rather than those that would discharge through the explosive material of the electro-explosive device. The gaseous electrical conductable means may be incorporated directly into the structure of the electroexplosive device or may be a separate component directly insertable by in-line series connections between the electrical connector of the device and the electrical connector on the firing circuit cable.
Another advantage of the present invention is that it dissipates an electrostatic discharge other than through the explosive material in a simple manner which adds very little increase to the cost of the electro-explosive device.
Other advantages of the present invention will be apparent from the ensuing description of preferred embodiments considered in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal central sectional view, partly schematic, through an electro-explosive device equipped with electrostatic discharge dissipator means embodying one form of the present invention.
FIG. 2 is a fully schematic view similar to FIG. 1 and showing another form of the present invention.
FIG. 3 is another fully schematic view similar to FIG. 2 and showing still another form of the present invention.
FIG. 4 is still another fully schematic view of protecting a redundant heater bridgewire circuit against electrostatic discharge and showing yet another form of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The arrangement shown in FIG. I employing commercially available components provides excellent protection for a heater bridgewire circuit in an electro-explosive device against accidental firing from electrostatic discharge.
As there shown, the numeral 10 represents a conductive metal case of any suitable configuration for housing the electro-explosive device. This case is shown as having an enlarged cavity 11 at one end and a smaller cavity 12 at the other end. The wall portion of case 10 between its cavities l1 and 12 is shown as having two holes 13 and I4.
Shown arranged in cavity 12 is a squib or primer 15 containing an explosive material 16 of well known composition and in which a heater bridgewire I7 is embedded. A lead wire 18 is electrically connected to one end of bridgewire I7 and another lead wire 19 is electrically connected to its other end. These lead wires 18 and 19 are shown as extending exteriorly of the device to provide terminals 20 and 21, respectively. Explosive material 16 is intended to be ignited by bridgewire 17 when a low voltage firing signal is applied to terminals 20 and 21 to heat the bridgewire.
Insulation between the aforementioned firing circuit and case is provided by a ceramic insulator 22 arranged in the base of case cavity 12 and through which lead wires 18 and 19 extend, and also by glass seals 23 and 24 arranged in case holes 13 and 14, respectively, and through which lead wires 18 and 19, respectively, also extend.
In accordance with the present invention, gaseous electrical conductable means are shown incorporated in the electro-explosive device to provide an electrostatic discharge path alternate to spark discharge through explosive material 16. While a predetermined environment of any suitable ionizable gas may be employed as the gaseous electrical conductable means, it is preferred to use a confined body of an inert noble gas such as helium, neon, argon, krypton and xenon. An inert noble gas is preferred because of its relatively low ionization potential. Conveniently neon glow lamps are commercially avialable for this purpose. Accordingly, a neon glow lamp 25 is shown operatively associated with the firing circuit including lead wires 18 and 19 and heater bridgewire 17. This lamp and the associated circuitry are embedded in suitable insulative potting represented by the numeral 27. Epoxy insulation is a preferred potting material.
Neon glow lamp 25 includes an envelope 28 filled with neon gas and a pair of metal electrodes 29 and 30. A conductor or wire 34 electrically connects electrode 29to lead wire 18. A conductor or wire 37 electrically connects electrode 30 to case 10. Preferably as shown in FIG. l, conductor or wire 37 includes in series a resistor 38 of a non-inductive type for limiting current flow. Case 10 is shown grounded at 39.
A typical commercially available neon glow lamp suitable for use as lamp 25 has a breakdown potential of 70 to volts D.C. depending on ambient temperature and exclusion of light conditions. Such a lamp is rated for current operation of 2 milliamperes using a current limiting resistor at higher currents. sputtering of the'metal electrodes occurs and failure of the lamp can occur in milliseconds. When a 1500 mmf capacitor charged to 9000 volts is discharged into such a glow lamp with a 10 ohm series resistor. a peak current of I50 am peres has been recorded without damage to the lamp. This is probably explained by reason of the extremely low RC time constant of .075 micro s'econd. Well spaced repetitive pulses from capacitors with live times this energy can be dissipated in such a small lamp. With voltages below the lamp ionization potential. there is no current How to ground. Measurements of the insulation to ground using voltages up to 50 DC. read in excess of 200 megohms.
Unlike a spark gap, bleed-offof unwanted high voltage electrostatic charges by gaseous ionization can be reliably depended upon to initiate at 70 to 135 volts. The normal intended firing signal would be in the to 28 volt range. where no bleed-off occurs.
Some of the electrostatic energy could be dissipated as heat in the bridgewire 17 if the discharge were applied to a single pin of the circuit and the lamp 25 were connected to ground by a path that included the bridgewire. The use of a ohm nominal non-inductive resistor 38 in the circuit is intended to assure distribution of the applied static energy so that it would be impossible for more than 109? ofthe total discharge energy to be dissipated directly in the heater bridgewire 17. Although resistor 38 is not essential, it is preferred because it permits much greater total electrostatic discharge dissipation \vithout ignition.
The electrical circuit represented in FIG. 2 is similar to that depicted in FIG. 1 except for omission ol the resistor 38 and inclusion of a second net .i glow lamp 26. This lamp 26 includes an envelope 3| filled with neon gas and a pair of metal electrodes 32 and 33. In FIG. 2. the neon glow lamp corresponding to lamp is designated 25' having an envelope 28' and electrodes 29'and 30', and the bridgewire l7'shown embedded in explosive material l6'has lead wires I8'and 19 between which lamps 25'and 26 are arranged. Electrode 29' is electrically connected to lead wire 18 by conductor or wire 34'. Electrode 33 is electrically connected to lead wire 19' by conductor or wire 35. A conductor or wire 36 electrically connects electrodes 30' and 32 together. Ground conductor or wire 37'is shown as electrically connecting conductor or wire 36. externally of envelopes 28' and 31. to ground 39. It is pointed out that conductor or wire 37'has no resistor therein comparable to resistor 38 shown in FIG. l.
The electrical circuit represented in FIG. 3 is similar to that dipicted in FIG. 2 except that only a single neon glow lamp 40 having three electrodes 4|. 42 and 43 is employed A conductor or wire 44 electrically connects electrode 41 to lead wire 18''. and a conductor or wire 45 electrically connects electrode 43 to the other lead wire l9". Heater bridgewire l7 extends between lead wires [8" and I9" and is embedded in explosive material 16''. A conductor or wire 46 electrically connects electrode 42 to ground 39".
The electrical circuits depicted in FIG. 4 contemplate the use of neon glow lamp means to protect against electrostatic discharge in a redundant heater bridgewire electro-explosive device. The numeral 47 represents a heater bridgewire having lead wires A and B. and the numeral 48 represents a second heater bridgewire having lead wires C and D. Both bridgewires 47 and 48 are shown embedded in the same body ot'explosivc material 16a. A three-electrode neon glow lamp 54 is shown as having two of its electrodes 55 and 56 electrically connected by conductors or wires 57 and 58. respectively. to lead wires C and A. respectively. lts third electrode 59 is electri cally connected by conductor or wire 60 to ground 61. In this manner. neon glow lamp 54 protects against static discharge between circuit AB and ground. between circuit C-D and ground. and between circuit A-B and circuit C-D.
I. In an electro-explosive device having a circuit including a pair of lead wires connected by a heater bridgewire embedded in a body olexplosive material and to which lead wires a firing signal is applied to heat said bridgewire for exploding said material. the im rovement of an electrostatic dischar e dissipator independent of the means which applies sai firing signal which comprises gaseous electrical conduct-able means including electrodes arranged in a predetermined environment of an ionizable gas. first conductor means electrically connecting one of said electrodes to one of said lead wires. and second conductor means electrically grounding another of said electrodes.
2. An electro-explosive device according to Claim 1 wherein said second conductor means includes a resistor.
3. An electro-explosive device according to Claim 2 wherein said resistor is non-inductive.
4. An electro-explosive device according to Claim 1 wherein said gas is inert noble gas.
5. An electro-explosive device according to Claim 4 wherein said second conductor means includes a resistor.
6. An electro-explosive device according to Claim 5 wherein said resistor is non-inductive.
7. An electro-explosive device according to Claim 1 which further comprises a third conductor means electrically connecting still another of said electrodes to the other of said leads.
8. An electro-explosive device according to Claim 7 wherein said gas is inert noble gas.
9. An electro-explosive device according to Claim 1 wherein said gaseous electrical conductable means includes two separate confined bodies of ionizable gas and a pair of electrodes arranged in each such gas body. said first conductor means electrically connects one electrode in one gas body to one oi said leads. said second conductor means electrically grounds the other electrode in said one gas body and also electrically grounds. one electrode in the other gas body. and further comprises third conductor means electrically connecting the other electrode in said other gas body to the other of said leads.
10. An electro-explosive device according to Claim 9 wherein said second conductor means includes a resistor.
11. An electro-explosive device according to Claim 10 wherein said resistor is noninductive.
12. An electro-explosive device according to Claim 9 wherein said gas is inert noble gas.
13. An electro-explosive device according to Claim 12 wherein said second conductor means includes a resistor.
14. An electro-explosive device according to Claim 13 wherein said resistor is non-inductive.
15. In an electro-cxplosive device having a first heater bridgewire circuit including a first pair of lead wires con nected to a first heater bridgewire embedded in a body of explosive material and to which first pair of lead wires a firing signal is applied to heat said first bridgewire for exploding said material. and a second heater bridgewire circuit including a second pair of lead wires connected to a second heater bridgewire embedded in said body ofexplosivc material and to which second pair of lead wires a firing signal is applied to heat said second bridgewire for exploding said material. the improvement of an electrostatic discharge dissipator independent of the means which apply said firing signals which comprises gaseous electrical conductable means including electrodes arranged in a predetermined environment of an ionizable gas. first conductor means electrically connecting one of said electrodes to one lead wire of said first circuit. second conductor means electrically connecting another of said electrodes to one lead wire of said second circuit. and third conductor means electrically grounding still another one of said electrodes. I
An electro-explosive device according to Claim I5 wherein said gas is an inert noble gas.