TAILIEUCHUNG - Systematic Drilling and Blasting for Surface Excavations Part 5

Tham khảo tài liệu 'systematic drilling and blasting for surface excavations part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | EM 1110-2-3800 1 Mar 72 2 Advantages of electric blasting caps include safety in handling variety of delay periods available and choice of exact time of detonation. Noise and potential public relations problems are reduced by initiating the charge in the borehole with a blasting cap instead of using trunk lines and down lines of detonating fuse. Care should be used to avoid stray induced electric currents or those caused by lightning or radio frequency energy see para 1-3 . Manufacturer s data should be consulted for current requirements. Because of variation from brand to brand mixing brands of caps in a round should not be done. 3 A delay element of explosive is placed between the bridge wire and the primer charge in a delay electric cap. The delay element is accurately calibrated to give a specified time lapse between the application of electric current and the detonation. Two series of delays are available short or millisecond delays with delay increments of 25 msec in the lower range and 50 msec in the upper range and longer delays often called slow delays with delay increments of and 1 sec. Where maximum fragmentation is desired millisecond delays are used to produce good breakage and reduce airblast and ground vibrations. Slow delays are primarily used underground whère they provide time for rock movement between delays. Longer delays are likely to result in coarser fragmentation than that obtained with millisecond delays. 4 The cap and fuse is another system of initiating explosives. A fuse cap is a small tube closed at one end which contains a heatsensitive primer charge plus a base charge such as PETN. The cap has an open space above the primer charge into which the safety fuse is inserted Fig. 3-7 . The safety fuse consists of a core of potassium nitrate black powder enclosed in a covering of textile and waterproofing compound. The several types of fuses vary in water resistance and flexibility. Most burn at 40 seconds per foot spf but some burn at .

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