U.S. patent number 3,804,018 [Application Number 05/298,155] was granted by the patent office on 1974-04-16 for initiator and blasting cap.
This patent grant is currently assigned to ICI America Inc.. Invention is credited to Florian Bernard Janoski.
United States Patent |
3,804,018 |
Janoski |
April 16, 1974 |
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.
Inventors: |
Janoski; Florian Bernard
(Allentown, PA) |
Assignee: |
ICI America Inc. (Wilmington,
DE)
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Family
ID: |
26720313 |
Appl.
No.: |
05/298,155 |
Filed: |
October 16, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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43342 |
Jun 4, 1970 |
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Current U.S.
Class: |
102/202.4;
102/202.1; 102/202.11; 102/202.3 |
Current CPC
Class: |
F42B
3/18 (20130101) |
Current International
Class: |
F42B
3/18 (20060101); F42B 3/00 (20060101); F42b
003/18 () |
Field of
Search: |
;102/28R,28M
;333/79 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pendegrass; Verlin R.
Parent Case Text
This application is a continuation-in-part of an application Ser.
No. 43,342, filed June 4, 1970, and now abandoned.
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.
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.
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