Initiator And Blasting Cap

Abstract

An electric initiator and blasting cap exhibiting increased protection against activation by discharges or extraneous static electricity and by electromagnetic radiation in the radio frequency spectrum. The initiator contains an inductor, such as a coil, in series relationship and adjacent to its ignition means and may have alternative current path elements preferentially located to guard the bridgewire element.

Parent Case Text

This application is a continuation-in-part of an application Ser. No. 43,342, filed June 4, 1970, and now abandoned.

Description

This invention relates to an electric initiator and a blasting cap offering increased protection against initiation by extraneous electricity pulses of current exhibiting a high di/dt (rate of change of current with time) which may be applied to the initiator in any input mode. Such pulses may derive from extraneous static electricity or electromagnetic radiation in the radio frequency (RF) spectrum. Specifically, the invention relates to the use of an inductor such as a coil, preferentially positioned with respect to other circuit components, to impede and attenuate the passage of a current pulse or pulses having a high di/dt in the circuit of an initiator to such an extent that the initiator is not activated.

A hazard sometimes faced by persons who handle blasting caps, detonators, squibs, and other electro-explosive devices is the possible activation of these devices by the discharge of extraneous static electricity or by RF radiation. By "extraneous static electricity," is meant static electricity of the magnitude and duration which can be accumulated on human beings or equipment in the field, such as in an electric blasting operation, as opposed to static discharges produced by static machines or lightning; discharges of extraneous static electricity are of relatively short duration (typically less than about 10 microseconds) and of relatively high voltage (typically from about a few thousand volts to about 35,000 volts). The radio frequency energy usually encountered in the field and against which protection is afforded by the present invention includes that associated with, for example, commercial AM and FM broadcasting, amateur radio transmission, navigational broadcasting, and radar transmission.

It has been found in accordance with the present invention that selective placement of an appropriate inductor in series with an electric ignition means can prevent the firing of the electric ignition means by discharges of extraneous static electricity, or by radio frequency energy but will not prevent the electric ignition means from firing when a pulse from a blasting machine or other conventional firing means is used. The inductor is selected and so positioned that the inductive reactance produced is sufficient to provide an attenuation to RF energy and a blocking action to the short-duration electrical pulse associated with the discharge of extraneous static electricity. It has been found that the greater degree of protection is provided by using this inductor in conjunction with a voltage breakdown circuit element that will, on activation, provide an alternate path other than the bridgewire for the extraneous static electricity or RF energy to flow through. However, the inductive reactance must not be so great that normal firing current from an approved blasting machine or power line cannot fire the device.

The degree of protection provided against short current pulses depends largely upon the inductance of the circuit to which such a pulse is applied. Inductors having an inductance of less than about 2 microhenries, as may be obtained by a coil of 40 turns, do not provide sufficient protection to be of practical value. On the other hand, inductances of more than about 196 microhenries, as may be obtained by a coil of 250 turns, provide so much impedance that current pulses produced from some commercial blasting machines may also be blocked. A preferred range of inductance values which will provide a high degree of protection with assurance that firing may be obtained from current sources customarily employed for blasting operations is from about 4 to 20 microhenries. Such values are obtained with coils of from 55 to 80 turns.

The inductor employed according to this invention is preferably a coil of wire, but other inductors such as printed circuit, deposited inductors, or other inductive elements could also be used.

Electric initiators include a resistance wire, called a bridgewire, embedded or otherwise in close contact with a heat sensitive ignition composition. It is desirable to locate the inductor of the present invention adjacent the ignition composition so as to leave as small a portion of circuit as possible unprotected. On the other hand, because in performing its protective function, the inductor produces some heat, it should not be in contact with the ignition composition or close enough to provide substantial heating effect upon it.

Electric initiators in modern practice are generally used in circuits which may require substantial currents for activation or they may be used where relatively low firing voltage is available. Hence, it is desirable that they be made of as low resistance as practicable consistent with the bridgewire resistance necessary to provide ignition. An inductor coil does provide some added resistance to the initiator circuitry. However, by use of wire of adequate conductivity to provide a low coil resistance this effect may be made of little practical significance. Coil resistance values as high as 4 ohms still permit functioning of initiators for most purposes. Although no minimum resistance is required, coil resistance values below about 0.5 ohms do not seem practicable with copper wire. A preferred range of resistance is from about 0.8 to about 1.3 ohms.

For added protection in an electric blasting cap, it is preferable to provide alternative paths (i.e., paths other than through the electric ignition means) for the electrical charge resulting from the discharge of extraneous static electricity or from RF energy. Thus, the inductor will momentarily block a short-duration pulse and force it to take an alternative path thereby preventing the cap from firing. One alternative path is between the circuits to and from the electric ignition means and the inductor so that a charge may pass directly from one side of the circuit to another without passing through the ignition means. An example of such an alternative electrical path is a break-down plug between the two sides of the circuit. A break-down plug contains a material which normally has a high electrical resistance but which "breaks down" when a voltage is impressed across it, behaving as a very rapid-action switch. An example of such a plug is the plug described in U. S. Pat. No. 3,295,447.

A second alternative electrical path which may be preferably provided in a device such as a blasting cap or squib, which has an external shell lies between a legwire and the shell which encloses the inductor and electric ignition means. An example of such an alternative path is a bare (uninsulated) portion of a legwire which is positioned inside and near the shell so as to provide a short gap across which an arc may be formed in the event that the voltage pulse is high enough to ionize the gap medium (usually air) and to sustain a flow of charge across the gap. If an arc is formed, a break-down plug may or may not enter as a part of a shunting circuit element depending on whether or not the single legwire involved includes the bare portion of the wire which is one element of the gap.

Thus the present invention can protect against activation when static discharge or RF currents are impressed between the two legwires, between shunted legwires and the shell, or between either single legwire and the shell.

The accompanying drawings illustrate the presently preferred embodiment of my invention.

FIG. I is a side view in section of an electric blasting cap.

FIG. II is a side view in section of the electric match assembly used in the electric blasting cap of FIG. I.

In FIG. I, a metallic shell 1 sealed at one end contains a detonating explosive 2 which is held in place by means of an inner capsule 3. An electric match assembly 4 has been inserted into shell 1 and is held in position by rubber plugs 5 and 6 (which can also be a single molded plug) through which pass legwires 7 and 8; the rubber plugs 5 and 6 are held in place by friction between the plugs and the inner wall of the shell and by crimps 9 and 10 in the shell. Legwise 8 has been bared at 11 and plug 5 has been cut away in order to provide a spark gap between bare wire 11 and the adjacent portion of shell 1. Either legwire, 7 or 8, may be bared for this purpose. Heat flame, and any hot metallic particles resulting from any arc discharge between bare wire 11 and shell 1 are confined within the cut-away portion between the rubber plugs and are thereby kept from entering that portion of the shell containing the electric match assembly 4 and the detonating explosive 2.

Referring now to FIG. II as well as FIG. I, in electric match assembly 4 insulating tab 12 is sandwiched between metallic tabs 13 and 14 to which legwires 7 and 8 have, respectively, been soldered. A hole filled with a semi-conductive material forms a break-down plug 15. Opposite the legwires, bridgewire 16 is soldered between tab 13 and tab 17 which is below and on the same side as tab 14. The bridgewire is coated with a pyrotechnic composition 18 to form the electric match head. An insulating tape 19 is wrapped around tabs 12, 13, 14 and 17, and an insulated wire 20 is wound over to form a coil and is soldered to tabs 14 and 17. Thus, current can flow from legwire 8 to tab 14 through coil 20 to tab 17 through bridgewire 16 to tab 13 and through legwire 7 to form a circuit.

EXAMPLE -- PROTECTION FROM STATIC ELECTRICITY

An electric match with a break-down plug made by ICI America Inc., was modified by rounding the corners of the match tab and routing out a portion of the tab to produce a match as shown in FIG. I. A portion of the left side of one of the metallic tab faces was filed away as shown in FIG. II to result in the two tab faces 14 and 17 on the left side. The match was wrapped with insulating tape and No. 36 AWG enamel-coated copper wire was wound over the tape until there were 60 random turns to form a coil having an inductance of about 5 microhenries and a resistance of about 0.95 ohms. The ends of the coil were soldered to tab portions 14 and 17. The entire match assembly 4 was given a single electrical insulation coating dip so as to completely cover the assembly from and including the match head 18 to approximately one-sixteenth inch over the PVC insulation coating of the legwires 7 and 8 shown in FIG. II. The match assembly 4, along with rubber plugs 5 and 6, was placed into a standard gliding metal tube 1, containing a live detonable explosive 2, and capsule 3. The assembly was crimped and was then similar in detonation qualities to a live standard No. 6 electric blasting cap. Caps of this construction repeatedly withstood static discharges from a capacitance bank of 0.11 microfarad, charged to 15,000 volts (the maximum capacitance and voltage of the equipment available) which were introduced to the live cap via the legwire-to-legwire, shunted legwires-to-shell, and either single legwire-to-shell modes. The caps nevertheless could be and were fired by a pulse from a standard Trojan Warrior (40C) capacitor discharge blasting machine which was about 200 volts peak with a 0.0001 second time constant.

EXAMPLE -- PROTECTION FROM ELECTROMAGNETIC RADIATION

A blasting cap as shown in FIG. I and described in the previous Example, having a coil of about 5 microhenries inductance, was cut through the bottom portion to remove the end containing the explosive charge. The cap was then directly coupled and tuned to a transmitter in order to note the amount of radio frequency attenuation which the electric match system would provide. Five test frequencies were chosen spanning frequencies of concern to many electric blasting operations. The test frequencies, the power in and power out in milliwatts, and the attenuation in decibels are listed below:

Frequency Power In Power Out Attenuation (MHz) (mw) (mw) (db) 0.1 0.71 0.45 - 2.0 1.0 0.044 0.019 - 3.65 50 38.0 0.03 - 31.0 500 9.65 0.10 - 19.8 1200 4.00 0.08 - 17.0

Thus, electrical power reaching the bridgewire 16 was reduced by from 36 percent to over 99 percent.

Claims

What is claimed is:

1. An electric initiator comprising an electric ignition means including a bridgewire, an ignition composition in contact therewith, an electrical connection to said bridgewire and including, in series with said bridgewire and adjacent said ignition composition but separated therefrom, and inductor having an inductance of from about 2 to about 196 microhenries and a resistance no greater than about 4.0 ohms.

2. An electric initiator in accordance with claim 1 wherein the inductor is a coil of from about 40 to about 250 turns.

3. An electric initiator in accordance with claim 2 wherein the inductor is a coil of from about 55 to about 80 turns, has an inductance of from about 4 to about 20 microhenries and has a resistance of less than 1.3 ohms.

4. An electric blasting cap having a shell containing an electric initiator in accordance with claim 3 and a circuit including legwires leading to and from said initiator and further including a break-down plug between both sides of said circuit.

5. An electric blasting cap in accordance with claim 4 including a spark gap between said shell and one of said legwires.

6. An electric initiator comprising an insulating tab sandwiched between a conducting tab on one side and two separated conducting tabs on the other side, a bridgewire coated with an ignition composition connecting one of said two separated conducting tabs to said conducting tab on the opposite side of said insulating tab, and insulated wire wound from 40 to 250 turns around said insulating tab and joining said two separated conducting tabs to form a coil having an inductance of about 2 to 196 microhenries and a resistance less than 4.0 ohms.

7. The electric initiator of claim 6 wherein there is a break-down plug in said insulating tab.

8. The electric initiator of claim 6 wherein said coil has from about 55 to about 80 turns, has an inductance of about 4 to about 20 microhenries, and has a resistance of less than 1.3 ohms.