The influence of several factors on pulsed electrical breakdown in rocks

Abstract

It has now become possible [1][2][3] to find methods of rock breaking by means of various forms of energyelectrical, thermal, electromagnetic, etc.The most promising method is the use of electrical energy without intermediate conversion to mechanical energy. The operative medium is an electric spark, and the solid body is fragmented by the high pressures in the discharge zone. The creation of fundamentally new rock-breaking equipment and the development of suitable techniques require a wide knowledge of the electrical properties of rocks. There is very little available information on the pulsed dielectric strengths of rocks [4], and we are only beginning to study them in connection with electrical rock breaking.In this article we give the results of experiments on pulsed electrical breakdown of certain types of rock over a wide range of voltage durations in a nonuniform field. We give data on the breakdown voltages of rocks in relation to their moisture contents and to whether they are impregnated with insulating liquids. The results of these experiments may prove useful in the design of rock-breaking equipment or in the design and construction of grounding electrodes in ledge rock.Our source of single high-voltage pulses was a multistage pulsed voltage generator (Russian "GIN") working at 480 kV with a discharge capacitance of 6250 pF. The pulse voltages were recorded by an electronic oscillograph with a capacitative voltage divider. Combined calibration of the oscillograph and divider with respect to the voltage amplitude was effected with the aid of sphere gaps (standard wave 1.5/40 gsec, positive pulses). We used plane-parallel specimens of sandstone, fine-and coarse-grained marble, granite, plagioclase granite, fine siRstone, quartz porphyry, and coal, 50 x 50 x 10 mm in size.To eliminate overlapping of the specimen surfaces with these dimensions, the experiment was performed in transformer oil of strength 35 kV in a standard breakdown tester. To eliminate end effects [5] during pulsed breakdown, a conical 45-degree hole was drilled at one end of the rock specimen and the other end was polished. The thickness of the specimens at the point of breakdown was taken as 10 mm and was measured with an IZV-1 indicator gauge.The moisture contents of the rock specimens were estimated from the weight losses of specimens dried in an oven at t = 150~ (the weights of the wet specimens were measured after immersion in technically pure water with a conductivity of o = 0.15 9 10 -3 mho/cm for 10 days). The effect of soaking in transformer oil on the rock's breakdown voltage was investigated on specimens of sandstone and coarse-grained marble (the specimens were immersed in transformer oil for 24 h). The volt-second characteristics of the dry rocks were recorded for specimens covered with a film of paraffin. This avoided penetration of transformer oil into the pores of the rock, and did not affect its breakdown voltage.The electrodes were a brass flat, 120 mm in diameter, and a bross rod, 8 mm in diameter, on to which was pre...