US3640223A

US3640223A - Electric blasting cap having increased safety against unintentional initiation

Info

Publication number: US3640223A
Application number: US764069A
Authority: US
Inventors: Per O I Olsson
Original assignee: Nitro Nobel AB
Current assignee: Nitro Nobel AB
Priority date: 1967-10-06
Filing date: 1968-10-01
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08
Also published as: ES358763A1; SE396472B

Abstract

The present invention concerns an electric blasting cap having increased safety against unintentional initiation, said blasting cap presenting a bridge wire and in contact with said bridge wire a fuse head composition which latter is surrounded by a protective tubing of dielectric material.

Description

United States Patent Olsson Feb.8, 1972 [541 ELECTRIC BLASTING CAP HAVING INCREASED SAFETY AGAINST UNINTENTIONAL INITIATION [72] inventor: Per 0. I. Olsson, Gyttorp, Sweden [73] Assignee: Nltro Nobel AB, Gyttorp, Sweden [22] Filed: Oct. 1, 1968 211 App]. No.: 764,069

[30] Foreign Application Priority Data Oct. 6, 1967 Sweden ..l3698l67 [52] U.S.Cl ..l02/28 [51] Int Cl [58] FieldofSearch ..102/28, 28M

[56] References Cited UNITED STATES PATENTS 2,995,086 8/1961 Scott ..l02/28 M Ball 102/28 M Miller 102/28 M Primary ExaminerVerlin R. Pendegiass AttorneyFred C. Philpitt [57] ABSTRACT The present invention concerns an electric blasting cap having increased safety against unintentional initiation, said blasting cap presenting a bridge wire and in contact with said bridge wire a fuse head composition which latter is surrounded by a protective tubing of dielectric material.

2 Claims, 1 Drawing Figure ELECTRIC BLASTING CAP HAVING INCREASED SAFETY AGAINST UNINTENTIONAL INITIATION Blasting caps of this type are previously known (vide Swedish Pat. specification No. 143,680). The protection against static electricity is achieved by surrounding the fuse head composition, but not the exposed parts of the fuse head along its whole length by an open-ended tube made of a soft dielectric material which engages around the fuse head composition.

A disadvantage of the known blasting cap is stated in the Pat. application on page 3, left column, lines 9-16.

The known embodiment of the blasting cap does thus not provide for complete safety since charges of static electricity can cause considerably higher potential differences.

The object of the present invention is to render the known electric blasting cap still safer than has hitherto been possible.

It has now been surprisingly found that this object can be I realized if the firing impulse of the fuse head is increased to a value higher than the values normally applied, which are in the order of some few millijoules per ohm. More particularly, it has been found that this result can be achieved for impulse values which lie above 10 millijoules per ohm, preferably between 50 and 150 millijoules per ohm.

The value of the firing impulse is determined by the dimensions of the bridge wire and the material from which it is made, and to a certain degree by the fuse head composition. Setting of the firing impulse to a certain desired value is therefore easily accomplished by one having normal skill in the art.

It is true that electric blasting caps having a firing impulse value exceeding the stated limit are known to the art but these caps are not provided with the protective tube. Such blasting caps, however, have been found insufficiently safe against unintentional initiation due to static electricity. The conclusion must be drawn from this that an increase in the firing impulse does not generally afford any decisive improvement in the safety of an electric blasting cap against initiation due to discharges of static electricity. Consequently, the result gained by combining the teachings of the above Swedish specification Tests have been made to support these assertions, the results of which are given below.

lt can be said against the technical utility of the present invention that the necessary firing impulse value demands an increase in the requirement of energy, when the blasting caps are fired by means of a current impulse through the lines, since requisite pulse energy per blasting cap in this instance is the product of its resistance and firing impulse value. However, it has been found that even an increase in the firing impulse value to some tens of millijoules per ohm affords safety against high discharges of static electricity which can be conceived to take place in practice. Firing apparatus constructed to operate at such a magnitude are technically fully acceptable.

According to a preferred embodiment of the invention the firing impulse value therefore lies between 50 and 150 millijoules per ohm.

According to a further embodiment of the invention the protective tube is not attached to the fuse head in accordance with the teachings of the above patent specification, but freely inserted in the blasting cap. This design of the protective tube is technically advantageous as the tube can be mounted more easily when mass-producing the blasting cap and as the risk of transferring substances deleterious to the fuse head composition from the tube to this composition decreases. Neither need the choice of material be restricted to soft materials as in the above Patent specification, but that a rigid material may be used, which is economically more advantageous.

The invention will be more closely described in the following with reference to accompanying drawing which shows a diagrammatic view of a blasting cap.

According to the drawing the blasting cap comprises a metal casing l, in which is located an explosive charge 2, a charge 3 of a primer and a pyrotechnical delay charge 4 in a thick walled tube 5. The casing 1 presents at the top a crimped portion 6 which engages a sealing plug 7 through which two insulating wires 8, 9 are passed. Connected to the two wires is a fuse'head element which comprises two legs 10, ll, fixed relative to one another by means of a strap 12. The ends of the legs are connected with a bridge wire 13 which is embedded in a fuse head composition 14. The composition 14, but not the exposed portions of the fuse head, is surrounded along its whole length by a protective tube 15 which is freely inserted in the casing of the blasting cap.

to prove the technical efi'ect of the invention laboratory tests were made in which charged capacitors were discharged, firstly through blasting caps designed according to the invention and secondly through blasting caps in which either the protective tube or the increase in firing impulse had been omitted. When effecting the tests the metal casing of the blasting caps were connected to the one terminal of the capacitor whereas the other terminal, subsequent to being charged to certain voltage values, was brought into contact with'one of the conducting lines of the blasting cap. Spark breakdown was thus obtained in the blasting cap so that the capacitor discharged, despite the fact that the two conducting lines were separated by the insulating sealing plug of the blasting cap. The following test results were obtained with a capacitor having acapacitance of 1,000 pF discharged to voltage 10 kv.,

which values can be conceived to appear during practical applications:

Type of blasting cap Relative detonating frequency 1. according to Swedish Pat. specification 143,680 with a firing impulse value of approx. 3.5 millijoules p'er ohm Approx. 50 k 2. according to the invention with a firing impulse value of approx. lOO millijoules per ohm O b For the sake of comparison a number of tests were made with blasting caps having a firing impulse value of approximately millijoule per ohm, all of which lacked protective tubes around the fuse head. A relative frequency of initiation which lay between 1 and 10 percent was obtained hereby.

The strong reduction in firing frequency (from 50-0 percent) achieved according to the invention must,.against the background of the result obtained on comparison tests, be considered surprising to one having normal skill in the art.

What I claim is:

I. An electric blasting cap having increased safety against unintentional firing whichcomprises:

a. a metal casing,

b. an explosive charge, primer and pyrotechnical delay charge positioned adjacent one end of said casing,

c. a sealing plug positioned adjacent the other end of said casing,

d. two insulated conductor wires passing through said sealing plug,

e. an intermediate space within said casing located between 1. said sealing plug and 2. said explosive charge, primer and pyrotechnical delay charge, space,

a fuse head assembly partially filling said intennediate space said fuse head assembly including a fuse head composition and a bridge wire embedded therein,

g. bare electric conductors in said intermediate space, said bare electric conductors interconnecting said two insulated conductor wires that pass through the sealing plug with the bridge wire embedded in the fuse head composition,

h. said electric blasting cap presenting a firing impulse of at least 10 millijoules per ohm, and

i. a protective tube of dielectric material freely positioned space.

within said intermediate space and spaced annular-1y from 2. An electric blasting cap as described in claim 1 wherein said fuse head assembly, the firing impulse value is in the range of from about 50 to 150 j. the longitudinal dimension of said protective tube being millijoules P less than the longitudinal dimension of said intermediate 5

Claims

1. An electric blasting cap having increased safety against unintentional firing which comprises: a. a metal casing, b. an explosive charge, primer and pyrotechnical delay charge positioned adjacent one end of said casing, c. a sealing plug positioned adjaCent the other end of said casing, d. two insulated conductor wires passing through said sealing plug, e. an intermediate space within said casing located between 1. said sealing plug and 2. said explosive charge, primer and pyrotechnical delay charge, space, f. a fuse head assembly partially filling said intermediate space, said fuse head assembly including a fuse head composition and a bridge wire embedded therein, g. bare electric conductors in said intermediate space, said bare electric conductors interconnecting said two insulated conductor wires that pass through the sealing plug with the bridge wire embedded in the fuse head composition, h. said electric blasting cap presenting a firing impulse of at least 10 millijoules per ohm, and i. a protective tube of dielectric material freely positioned within said intermediate space and spaced annularly from said fuse head assembly, j. the longitudinal dimension of said protective tube being less than the longitudinal dimension of said intermediate space.

2. said explosive charge, primer and pyrotechnical delay charge, space, f. a fuse head assembly partially filling said intermediate space, said fuse head assembly including a fuse head composition and a bridge wire embedded therein, g. bare electric conductors in said intermediate space, said bare electric conductors interconnecting said two insulated conductor wires that pass through the sealing plug with the bridge wire embedded in the fuse head composition, h. said electric blasting cap presenting a firing impulse of at least 10 millijoules per ohm, and i. a protective tube of dielectric material freely positioned within said intermediate space and spaced annularly from said fuse head assembly, j. the longitudinal dimension of said protective tube being less than the longitudinal dimension of said intermediate space.

2. An electric blasting cap as described in claim 1 wherein the firing impulse value is in the range of from about 50 to 150 millijoules per ohm.