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/ ISEE -Journal of Explosives : En o-ineering . • Vof. 13 No.7 July/August 1996 : Back to Basics: : Electric Initiation Part 4 A blaster, using electric initia­ sense require that whenever an tion, must be aware of certain electrical storm approaches, hazards specific to this type of blasting operations must be sus­ system. Since electric detonators pended. Not only must opera­ are designed to fire when electri­ tions cease, but the blasting crew cal energy is applied to them, any must evacuate the area and clear extraneous source of electric cur­ the site of personnel and equip­ rent represents a potential prema­ ment, just as if detonation were ture initiation source. Therefore, imminent. There are devices such things as lightning, radio available which are capable of transmitters, high voltage power detecting a storm's approach · lines or sources of static electrici­ before it poses a threat to the ty m~rst be avoided. Additionally, blasting crew, and emitting a any condition which would cause warning signal when a thunder­ by Larry Schneider a reduction in the applied current storm and lightning comes within may result in insufficient firing a certain range. current and create a misfire haz­ ard; any condition producing an High Voltage Power excessive current may result in Transmission Lines hangfires. High voltage power lines pre­ sent two distinct dangers to Lightnin9 blasters using electric initiation Lightning is considered to be systems. First, high voltage alter­ the greatest danger associated nating current in the power li~e with electric blasting. Nearly eve,ry has the potential to produce a ,blaster has either experienced this secondary current in the blasting personally, or knows of such circuit. This "induction" of electric instances where lightning has set current can take place in a blast­ off a blast prematurely. Lightning ing circuit without the blasting is not required to strike a blasting wires being in physical contact circuit directly; even a near miss with the energy source. In agdi­ can set off any elec­ tion, such high-voltage power tric detonators. lines may themselves suffer some (Note: lightning rep­ current leakage which can intro­ resents a hazard to ALL types of duce a "stray current" in the blasting, including non-electric ground. There is test equipment systems, while non-electric deto­ available that can be used to nators may not be as sensitive to measure any stray currents in the near misses, a direct hit by a light­ vicinity of a blasting site. Any sig­ ning bolt will almost certainly nificant source of extraneous cur­ cause them to initiate.) There are rent must be eliminated before also instances of lightning travel­ electric detonators are used. ing several miles underground A second and even more along pipes and cables, setting off direct danger to the blaster from electric detonators. high voltage lines is that of elec­ Safety regulations and common trocution. This can occur if the detonation throws the leg wires distances from the specific radio in contact with another conduc­ or lead lineup in the air and they sources. That is, if the frequency tor, or even with a conductive come into contact with overhead and the power of a radio trans­ portion of the ground. Certain high voltage lines. This type of mitter are known, this publication shales and clays are very good accident, which should be so will list a safe working distance. electrical conductors, especially easy to foresee and prevent, has A significant danger can exist when wet. Likewise, areas nevertheless occurred repeatedly when blasting near commercial around the boreholes where in past years. To avoid such an AM, FM, or TV stations because of Ammonium Nitrate-Fuel Oil has accident, the blaster should their large power output. Mobile been spilled, or any puddle of either make sure that the total radio transmitters in vehicles may water containing impurities will length of wires, (leg wires, con­ pose a danger because they may also conduct electricity. necting wire, plus firing line) is approach very near to the blast To prevent current leakage too short to hit the high voltage site. Radar is also dangerous from occurring or to minimize its lines should they be thrown in because of its highly directional impact, the following precautions the air; or the wires must be nature of its beam. can be taken: securely anchored to the ground On every new project using • All bare wire connections so that they would break before electric detonators, the blaster should be kept as far apart as they are thrown in the air and should survey the area for possi­ possible. The wire connections into any high voltage lines. If nei­ ble sources of radio energy. And should be bent in such a fash­ ther of these alternatives is feasi­ even after the blasting operation ion to keep the bare metal up ble, a non-electric system should is under way, he must remain out of dirt and water. be used. aware of the danger from radio • The number of electric detona­ sources. Precautions must be tors in any circuit should be Radio Frequency Energy taken to keep mobile transmitters kept below the recommended Transmission from any two­ away from the area, including maximum per series so that the way type of radio sends out posting the familiar road signs circuit will have power to spare energy in the form of electro­ warning everyone to "Turn Off in the event there is a small magnetic waves. In a very real Two-Way Radios." If a source of leakage of current. sense, the wires of a blasting cir­ radio frequency energy is found, • Only enough insulation should cuit can act as an antennna, pick­ and cannot be eliminated, or be removed from the wires as ing up radio frequency energy complying with the recommend­ necessary to make the connec­ and converting it to electric ener­ ed distance is not possible, then tions. gy in the blasting circuit. switching to non-electric initiation • The blaster should routinely The degree of danger that a is highly recommended. check the insulation on his fir­ radio transmitter poses depends ing cable for cuts or damage. on four criteria: (1) the frequen­ Current Leakage Any splices made to repair cy of the radio waves; (2) the A blasting machine provides breaks in a firing cable should output power (in watts) of the electrical energy in the form of be well insulated. radio transmitter; (3) the distance voltage and current applied to the from the transmitter to the blast­ cap circuit. If a part of this firing Arcing ing circuit; ( 4) the shape and the current is lost during transmission When an excessive current is orientation of the blasting circuit. through the firing cable, leg applied for an excessively long The Institute of Makers of wires, or connecting wires, it is time, an electric detonator will Explosives publishes an excellent described as current leakage. The likely malfunction by rupturing guide, entitled "Safety Guide to remaining electrical energy may before the base charge fires. If the Prevention of Radio be reduced to such a point that arcing occurs and the detonator Frequency Radiation Hazards in the current reaching the detona­ ruptures, the explosive in which the Use of Commercial tors in the circuit is below the the detonator is embedded may Detonators." Whenever any level needed to reliably fire them, catch fire. This is known as a question arises concerning safety resulting in a partial or complete "hangfire" where the explosive in, of blasting in the vicinity of a misfire. the borehole begins burning, and source of radio waves, the blaster This leakage or loss of current eventually may detonate. This should consult the IME publica­ often occurs when bare electrical detonation may take place a few tion. It provides tables listing safe connections are allowed to come seconds after the blast was fired or a few minutes later. Due to result in a partial misfire putting position to fire the shot, he alone this delayed detonation of hang­ unexploded charges in the muck should have control of the blast­ fires, no one should approach a pile. ing machine. Prior to connecting misfired charge for at least 15 Blasting machines must be the firing line to the blasting minutes after a shot is fired elec­ kept clean and in good working machine, he should make one trically. The best way to avert any order with fresh batteries in final check of resistanceand con­ danger from arcing is to use a order to provide their rated out­ tinuity of the blasting circuit. The condenser-discharge-type blast­ put. Except for replacing batter­ firing cable should not be con­ ing machine. These machines ies, a blaster should never nected to the blasting machine deliver their energy in a very attempt to work on his blasting until he is ready to detonate the brief time interval, too short for machine. If it is in need of repair, blast. Upon hearing the final arcing to occur. The use of bat­ it should be returned to the warning signal, the blaster teries, generators, or ordinary manufacturer or an authorized should wait the predetermined electric lines may produce a con­ service facility. The blaster time, connect the firing line, and tinuous high firing current and should never attempt to initiate detonate the shot. Immediately result in arcing. an electric blasting circuit with after the shot is fired, the lead anything other than a power line should be disconnected Firing the Blast source specifically designed. for from the blasting machine and After all the boreholes are electric blasting. The use of shorted.• loaded, the detonators are wired makeshift energy sources, such into a circuit to form an initiation as automobile or flashlight bat­ path.

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