Patent GB-555969-A (GB)
Title: Improvements in means for safeguarding electric igniters of blasting detonators against accidental firing
IPC: F42B3/185
Filed: 1941-10-31
Abstract: 555,969. Protective arrangements. ROLFES, Ltd., and ROLFES, H. J. Oct. 31, 1941, No. 14059. [Class 38 (iv)] [Also in Group XXI] A " coherer mass," comprising a metal powder which offers a complete or substantially complete resistance to voltages of the magnitude used for intentionally firing an electric igniter but which becomes locally conductive at voltages substantially higher than said firing voltages and caused by atmospheric electricity, is used to safeguard an electric igniter of a blasting detonator from atmospheric electricity. The mass is earthed and in contact with each lead or extensions thereof. The metal powder is preferably finely divided aluminium flakes, e.g. those used in the paint trade, but may be powdered silver, nickel, iron, brass or copper. The powder may be mixed with oil, liquefied paraffin wax, rubber or rubberlike material to form a plug. In Fig. 1 the leads 1a, 1b, which terminate in a bridge-wire 8 which is heated to incandescence on firing, are bared at 13, 14, the bared parts being surrounded by a coherer plug 15 in contact with the copper or other conductive shell 4 of the detonator. Teeth 17, 18 on the parts 13, 14 promote discharge of a high voltage charge from the leads 1a, 1b through the plug to the shell 4 and thence to earth. The plug may form part of a wax primer and make direct contact with the rock or side of the borehole. Two plugs may safeguard one detonator. The bared parts of the leads may be separated by a paper sheet or other insulator disposed in the plug. The teeth 17, 18 may be on the interior of the casing containing the plug, or may be on electrically-conductive members embedded in the plug between the leads. In a modification the leads 50, 51, Fig. 5, are tapped by insulated conductors 53, 54 the bared ends of which are embedded in a coherer mass 57 contained in an earthed metal casing 58 which may be connected to the detonator 52 by a conductor 59 and a sleeve 60. A group of seriesconnected detonators or of parallel-connected detonators may be protected by a similar exterior plug. The tap-wires from the two leads may lead into two separate coherer masses. A rigid coherer plug assembly, Figs. 8 and 9, may comprise two strips 63, 64 of metal foil adhesively secured to the opposite sides of an I-shaped piece of cardboard 65 which is a neat fit in a casing 66. A copper wire 71 is soldered at one end to the casing and at its other end to a copper sleeve 72 to be connected to the detonator casing as in Fig. 5. Means for obviating short circuiting between leads and casing after simultaneous high potential discharge on both leads comprises an insulating casing 73 containing the coherer mass and closed at the top by sealing means 73a, the bared parts 75, 76 of the leads being disposed parallel to interior conductors 79, 80 integral with or connected to exterior conductors 81, 82. A coherer mass may be shaken, after its resistance has been reduced by a high voltage charge, to restore its resistance to its normal value. Specification 556,071 is referred to.