explosion, it is only necessary for one of the blasting wires to come in contact with the object carrying the current, such as a pipe line, while the other is in contact with the ground.

None of these currents should be picked up by the lead wires as a premature explosion can occur when the miner connects the lead wires to the detonators. To pick up such a current and cause the

FIGURE 21.-Electric blasting; miner closing the switch of the stope circuit to connect it to the main blasting circuit.

The hazard of stray currents in electric blasting can be very greatly reduced, if not entirely eliminated, in most mines (1) by reducing or preventing the stray currents and (2) by safeguarding blasting cir

cuits so that stray currents will not reach the detonators. The first can be accomplished by secure bonding of the rails used by trolley locomotives and by grounding electric equipment such as fans and

FIGURE 22.-Electric blasting; miners removing their checks from section board as they leave area.

hoists, as well as pipe lines, armored cables, and other possible conductors. Lead wires and detonator leg wires should be short-circuited up to the time of connecting the detonator and lead wires. Testing the lead wires with a meter or with an electric igniter without a cap will give assurance against danger from stray currents, except that of a possible surge occurring while the connection is being made. Lead wires should be thoroughly insulated and as a further precau

FIGURE 23.-Electric blasting; after seeing that all men are out, foreman closes firing switch on main circuit.

tion should be divided into sections by switches or removable lengths of wire. The ends of such sections should be short-circuited.

One of the mines on the Vermillion range in the Lake Superior iron district uses all the safeguards that have been mentioned and also has an air-operated relay switch installed close to the face to be blasted. This switch is open while the leg wires are being connected and is closed by the miner on his way from the face by means of an air valve located a safe distance from the face.17

17 Jones, Wm., Eliminating the Hazards of Stray Currents in Electric Blasting: Nat. Safety Council, Lake Superior Section, 1943, 8 pp.

Mines of the Tennessee Copper Co. are in ore of a heavy sulfur and iron content. Electric blasting was installed in these mines to replace the use of fuse and caps in 1940.18 Current for blasting is taken from 220-volt alternating-current lines or from the 250-volt direct-current trolley wire. Blasting machines are used where these currents are not available. The firing cable is a No. 16 duplex reinforced cord to within 50 feet of the face and No. 18 cotton-covered copper lead wires from there to the shots. In power-circuit blasting an open, doublepole, double-throw switch is placed a safe distance from the face; the lead wires are short-circuited in the down position and connected to the power contactor in the up position. The firing circuits from the working stopes are connected to similar switches on common switchboards on the levels. When the switches on the common board are closed, the series-connected stope rounds (limited to 24 detonators each) are parallel-connected to the main firing circuit. Raise and drift rounds are similarly connected. The main firing circuits are connected to a magnetic switch where connection is made to the power circuit. The magnetic switches are actuated by separate circuits from points near the mine shaft at a designated time. Because of the high conductivity of the ore body and of moist broken ore in contact with rails and other metallic equipment, the firing cables are carefully insulated from both the direct-current wire and any contact to the ground until it is desired to blast. No direct current is used for stope or development blasting.

Blasting on grizzlies is done at any time during the shift. If more than one shot is fired, the detonators are connected in series, then by lead wires to the firing cable. The firing cable is installed on insulators and provided with a safety switch; the terminals of the cable are connected to a pointed wooden paddle hung on an insulated board a safe distance from the grizzly. To fire the blast, the pointed end of the paddle is inserted into a contact receptacle ("hot box") connected to the power source. The power circuit is fused and connected in series with an 8-ohm resister if alternating current is used; when direct current is used it is limited by a 25-ohm resister. Thirtyampere fuses were found to be impractical in direct-current circuits because the frequent grounds from air and water sprays and other sources caused too many short circuits.

The hazards from stray currents at this mine are greater than ordinarily found; but they have been reduced by the methods described and by careful bonding of the rails, and connections from them to ore bodies, pipe lines, and steel structures.

Tests made throughout the mine showed the existence of fluctuating stray direct currents ranging from one-tenth to 26 volts. The minimum current necessary to explode an ordinary electric detonator is approximately 0.35 ampere, and the minimum voltage required is 0.75 volt. Tests on the alternating-current circuits indicated that the insulation was effective in preventing dangerous stray currents from this source.

Occasional misfired shots were attributed to grounds in leg and lead wires because of insulation faults that shunted out some detonators in the series connection. The misfires are minimized by the use of detonators with plastic leg-wire insulation, by the exercise of care in

18 Clay, R. G., and Seaman, C. F., Electric Blasting of the Tennessee Copper Co.: Am. Inst. Min. and Met. Eng., Min. Technol., September 1942, 9 pp.

making connections, and by testing the blasting circuit with a circuit tester for continuity and for leakage to ground.

Other methods that may be used to reduce the dangers of stray currents are the substitution of Diesel locomotives for trolley-locomotive haulage or the installation of large return conductors bonded at frequent intervals to the rails, pipe lines, and current-conducting ore bodies.

STATIC ELECTRICITY

In addition to stray currents from leaking power circuits, static electricity must be considered a possible hazard in electrical blasting. One form that is not man-made and cannot be controlled is that caused by lightning. The only protection against such currents where electric blasting is being used at surface operations, or in mines where conductors extend from the blasting lines to the surface, is to refrain from loading holes or attempting to blast during a thunderstorm. In such circumstances, if holes are already loaded, men should be withdrawn from the vicinity until the danger from lightning is past. In deeper mines the short-circuiting of blasting wires, an effective grounding system for all electrical installations, pipe lines, rails, or any metallic medium that might carry an electrical current, and testing of blasting lines before connections are made will provide protection against current surges from lightning.

An example of the disasters that may be caused by the detonation of explosives charges by lightning occurred in 1918 at an open-cut iron mine in the Lake Superior district. Coyote holes and connecting drifts in a 40-foot bank had been loaded with black blasting powder and dynamite, and electric detonators were used. While a crew was placing stemming in the coyote holes, an electrical storm and heavy rain passed over the mine. Lightning struck somewhere in the operation, setting off several of the charges; the blast caused the bank to cave, killing and burying 18 of the 19 men in the crew.

In 1937 a similar accident happened in a deep shaft mine where a crew of 17 men was preparing a number of holes for electric blasting. Lightning struck near the head frame on the surface, and an electric charge passed through a bell line to the working place 4,000 feet below. Eleven holes were detonated, killing 8 and injuring 4 of

the crew.

In some circumstances machinery will build up static charges when in operation, but if an effective grounding system is provided, these static charges will be released before they become dangerous.

PRECAUTIONS IN USING FUSE

If the precautions in storing fuse that have been discussed are followed, it should reach the distributing station in good condition. Care must be exercised to prevent damage to fuse through all stages of blasting to reduce the chances of misfires, delayed shots, or even premature detonations. Any kinking of fuse while it is being brought to the working place or during loading may cause a break in the powder train that will cause a misfire or a delay in firing; cracking of the covering of the fuse may admit moisture or allow side spitting, which can ignite the explosive charge. This latter condition may explain of some of the reported accidents from "fast fuse." If the end of a length of fuse is wet, frayed in cutting, or not cut straight across before being inserted in a detonator, a misfire is likely.

Fuse should not be uncoiled while cold, as it may crack and admit