Safety electric primers for mines, protected against lgnition by extraneous electric currents

摘要

842,043. Electric blasting fuzes. MONTECATINI SOC. GENERALE PER L'INDUSTRIA MINERARIA E CHIMICA. Nov. 8, 1956 [Nov. 10, 1955], No. 34150/56. Class 9(1). An electric primer 1 for blasting charges is provided with a safety device to prevent accidental ignition by stray currents, which device when set forms a short circuit for the primer leads and can be operated by remote control to break the short-circuit. The leads 8 and a conductor 9 electrically connected to the primer casing are connected to contact blades 4 mounted on a carrier 5 surrounding a slidable core 2 held in position by a spring 10. The core has a flared head contacting the blades 4 to form a shortcircuit between the leads and the primer casing. Immediately before firing the primer an electro-magnet 3 is energized through a circuit independent of the firing circuit and moves the core 2 so that the short-circuit is destroyed. The carrier 5 together with the core 2 and electro-magnet 3 are sealed in a casing 7 by plugs 6 through which extend the leads. In a modification the blades 4 are electrically connected by the head of a pin which is urged out of contact by a spring but maintained in the short-circuiting position by fuzible insulating material, e.g. sulphur, in which a heating coil is embedded. The coil is in a circuit independent of the firing circuit and on passage of a current fuses the insulating material, whereupon the pin moves to destroy the shortcircuit. In a third form, the short-circuit pin is moved by pneumatic means which may be a tube led through a closure plug 6 to a compressed air supply or a vacuum chamber.