# Chapter Seven - Accident Prevention ## Premature Firing of Electrical Blasts Certain precautions are necessary when conducting electrical blasting operations to guard against premature firing: 1. Adequate measures must be taken to avoid accidental application of the firing current before everyone has reached a safe location 2. Entry of extraneous electricity into the blasting circuit must be prevented ### IME Safety Standards The Institute of Makers of Explosives (IME) has established the maximum "safe" current permitted to flow through an electric blasting cap without hazard of initiation as: - **One-fifth of the minimum firing current: 0.05 amperes (50 milliamperes)** - This provides a current safety factor of five or an energy factor of 25 > **Electric blasting must not be conducted in areas where extraneous currents are greater than 0.05 amperes (50 milliamperes).** ### Six Types of Extraneous Electricity 1. Lightning 2. Stray Currents 3. Galvanic Action 4. Static Electricity 5. Radio Frequency Energy 6. Transmission Lines --- ## Stray Current Electric current flowing through power lines to electrical equipment from a battery, generator, welder, or transformer will always return to that source by all available paths. ### Current Return Paths 1. Additional conductors insulated from ground (electrical cables) 2. Conductors not insulated from ground for electric haulage (rails) 3. The earth itself ### Testing for Stray Current Testing can be done by driving brass rods in the ground: 1. Place rods in central location of area to be tested 2. If ground is not damp, add water to surface area before testing 3. Run connecting wire from one stake to the other, cut in middle, place voltmeter at that location **Testing Procedure:** 1. Plug jacks into pin marked "volts" 2. Turn selector knob to 30 ACV setting for first test 3. If AC stray current is present, its potential will be indicated directly on the corresponding scale - If DC current present, it will show on 30 ACV scale at approximately double its actual amount - Turn knob to 30 DCV scale for correct potential - If no reading, reverse position of pin jacks - If still no reading, stray current is of AC origin 4. For more accurate reading, turn to 1.5 ACV or 1.5 DCV position > **Remember: The reading on your voltmeter must not exceed 0.05 amperes (50 milliamperes).** --- ## Galvanic Action While the danger of firing electric blasting caps by voltages generated by galvanic action is slight, it should not be disregarded. **Galvanism**: Electricity produced by chemical action ### Known Hazards - In some base metal mines, simple iron-copper ore cells generate sufficient voltage to explode a single cap - The use of shunted caps will provide protection against this hazard - In seismic prospecting, aluminum loading poles in alkaline drilling mud contained in steel casing can generate sufficient potential to fire a single electric blasting cap > **Only wooden or non-metallic loading poles should be used in seismic work.** --- ## Static Electricity Certain conditions may cause dangerous charges of static electricity to build up. Grounding of these charges through an electric blasting cap may result in a spark of sufficient intensity to fire the cap. ### Hazard Conditions This hazard may be encountered when: - Atmospheric static is present - The workman handling the electric blasting cap accumulates a high static charge while well insulated from the ground If the worker drops the wires to earth while holding the cap shell, the static charge will go to ground through the leg wires and, if sufficient intensity, will fire the cap. ### Factors Affecting Static Buildup Capacity to accumulate static varies greatly between individuals and is affected by: - Type of clothing worn - Size of person - Dryness of skin - Environmental conditions > **Shunting and insulating the ends of cap wires offers no assurance since the voltages are so high they can break down the leg wire insulation itself.** ### Special Hazards **Seismic Prospecting**: Particularly hazardous because work is often carried out in areas subject to dust and snow storms with low humidity. **Static Generation**: Since static electricity can be generated by particle movement, especially under dry conditions: - Leg wires of electric blasting caps should always be uncoiled along the ground rather than thrown out in the air - Any brief suspension in a highly charged atmosphere may create sufficient internal static discharge within a cap to detonate it **Electrical blasting operations should be discontinued during severe dust or snow storms.** ### Protective Measures 1. Ground all moving equipment in the vicinity of blasting operations (through resistance of less than 1 ohm) 2. Insulate all connections by taping 3. Keep lead wires clear of rails, pipes, or other electrical wiring 4. Always use shunted caps (even though protection against static is limited) 5. Remove shunts only at the last possible moment ### Pneumatic Loading and Static Pneumatic placement is the most efficient means of loading free-running ANFO blasting agents, but may generate static electricity as the mixture is blown into boreholes. **Control Measures:** 1. Use Lo-Static Conductive Loading Hose to provide path for charges to drain to ground 2. Ground the loader by: - Placing in direct contact with wet surface - Attaching ground wire to scaling bar inserted in borehole, rockbolt, or pool of water 3. **Do not attach ground wire to pipes, track, or other fittings** that could conduct stray electrical currents ### Priming Procedures for ANFO **Bottom Priming**: May be used with Anodet Delays, High Strength blasting caps, or safety fuse with conventional explosive primers **Collar Priming**: Recommended with electric blasting caps, with caps placed in boreholes after pneumatic loading is complete **If Bottom Priming with Electric Caps** (requires permission): - Use good conductive loading system - Keep shunts on cap leg wires during loading - Load only under conditions of relatively high humidity - Use sound electrical blasting practices > **Plastic borehole liners must not be used in boreholes bulk loaded with ANFO. Neither should pneumatic loading be carried out under extremely dry operating conditions.** --- ## Radio Frequency Energy Radio-Frequency (RF) transmitters (AM, FM radio, television, radar) create powerful electromagnetic fields decreasing in intensity with distance from the transmitter antenna. Tests have demonstrated that electric blasting cap wires may pick up enough electric energy from such fields to cause caps to explode. ### Hazard Levels by Transmitter Type | Type | Hazard Level | Reason | |------|--------------|--------| | Commercial AM broadcast (0.535-1.605 MHz) | **Most hazardous** | High power, low frequency means little RF energy loss in lead wire | | FM and TV transmitters | Unlikely to create hazard | High frequency currents rapidly attenuated; high tower antennas reduce field at ground level | | Mobile radio | Potential hazard | Low power but can be brought directly into blasting area | ### Citizens Band (CB) Radios CB radios present an unusual problem because: 1. Millions of units in use by general public 2. Operating frequency is worst-case for typical electric blasting circuits 3. Some operators use illegal linear amplifiers to increase transmission range **Safe Distances** (for FCC approved units): - Double sideband: 4 watts maximum output power - Single sideband: 12 watts peak envelope power > **Safe distances cannot be specified for illegal units as they do not operate within established FCC limits.** ### Regulations U.S. Federal regulations require posting signs within 1,000 feet of construction sites warning that two-way radios should be turned off because of blasting. **Recommendations:** - All CB operators should obey posted signs and turn off units - For two-way radios used in blasting operations, maintain minimum separation specified for particular transceiver (frequency and power) For specific tables on extraneous electricity, refer to IME Publication No. 20 - "Safety Guide for Prevention of Radio Frequency Radiation Hazards in the Use of Commercial Electric Detonators." --- ## Transmission Lines There is a possibility that an electrical blast may be detonated prematurely when the blasting circuit is located near a transmission line. ### Hazards Involved 1. **Stray Currents**: Caused by accidental grounding of power line 2. **Lightning Discharge**: Power lines can carry lightning discharge over long distances (up to 16 kilometres) due to excellent grounding systems 3. **Capacitive Discharge**: Electric field of high voltage transmission line can charge an isolated blasting circuit suspended above ground to several kilovolts, causing spark to ground through blasting cap 4. **Induced Currents**: AC currents in transmission lines create magnetic field around the lines ### Wires Thrown Over Power Lines The firing line or other blasting circuit wires could be thrown up into contact with high-voltage phase wires by the force of the blast, resulting in a short circuit through the blasting circuit wires. > **Several blasters have been injured or killed by these short circuits.** ### Operating Procedures for Blasting Near Transmission Lines - Survey for stray and coupled currents before any blasting is started - Keep all blasting circuit wires close to, yet insulated from, earth (minimizes capacitance coupling and stray current) - Use duplex wire or lay out circuit so large closed wire loops are not formed (minimizes inductive coupling) - Run firing line perpendicular to, rather than parallel with, power line - Extend area of concern for lightning storms out to 8-16 kilometres - Anchor blasting circuit wiring to ground using wood stakes or large rocks - Protect the shotfirer from electrical shock by: 1. Not running firing line underneath or parallel to power line 2. Employing positive means of cutting firing line from blasting circuit immediately after blast activation --- ## Drilling Into Explosives This is one of the most common types of blasting accident and one of the most difficult to eliminate since there is always a possibility of undetonated charge remaining after a blast. > **On March 1989 in Halifax, N.S., two men were killed and a third was injured when their drill came into contact with loaded holes.** ### Prevention The hazard can be minimized if drilling, loading, and firing are carried out properly. However, this hazard will still be present regardless of precautions under certain conditions. **Before Drilling:** 1. Examine working face and surface area for evidence of unexploded explosive 2. Carefully wash down face with water 3. Flush out all bootlegs and cut-off holes 4. Check for explosive in cracks or crevices in rock **Handling Misfired or Cut-Off Holes:** - **Do not attempt to recover any part of charge by withdrawing** - **Do not use any tool, especially metal ones, to scrape or prod explosive from borehole** - Blast misfire or cut-off holes before any further operations using fresh primer - Misfired holes containing ANFO must be washed out completely before reblasting with high explosives - Never light old fuse protruding from a hole (no way of knowing fuse length) ### Drilling Regulations - Drill holes shall be of sufficient size to admit free insertion of explosive without ramming, pounding, or undue pressure - Equipment shall not be brought into partially or fully loaded blasting area - Drilling holes containing explosives shall be clearly identified - Only the Powderman or designated helper shall be allowed in area containing such holes - Drilling shall not be carried out in loaded area unless under direct supervision of Powderman --- ## Unsafe Loading Practices Most accidents in this category result from unnecessary pounding or tamping of high explosives, or through use of metal tamping or loading devices. ### Safe Loading Rules 1. Use only wooden tamping rods (or approved non-metallic materials) containing no exposed metal parts - Brass, copper, aluminum, or "non-sparking" metals **cannot be safely used** because most explosives can be detonated by impact or friction between metal surfaces, or between metal and rock 2. **Never drop a cartridge on top of a primed cartridge or primed booster** - When using large diameter cartridges, use a lowering hook 3. Check all holes for proper depth and obstructions before loading 4. Keep noise level down in loading area so you can: - Hear cartridges hit bottom - Hear if rocks have fallen in and become lodged 5. If rock falls in hole: - First check if rock has broken circuit or shock tube - Put another primed cartridge down hole first, then continue loading - **DO NOT TRY TO GET THE ROCK OUT OF THE HOLE** 6. **Never remove paper shells from explosive cartridges** (greatly increases friction hazard) 7. **Never slit primed cartridges** 8. Place only single detonator in any one stick of explosive 9. **Never tamp a primed cartridge** 10. Prevent detonator from coming out of cartridge 11. **Do not load borehole if hot from previous blasting, drilling, or any other cause** --- ## Delaying Too Long at the Face Great care must be taken in blast design. Delaying too long at the face can result in severe problems: ### Problem 1: Excessive Flyrock Too much delay between holes could expose a loaded hole to open air, causing excessive flyrock when detonated. ### Problem 2: Excessive Airblast Air overpressure (airborne shock wave from detonation) may be caused by burden movement or release of expanding gas into air. Results in complaints from local residents (rattling dishes, windows, property damage). ### Historical Note When safety fuse was used frequently, numerous blasting accidents were caused by delaying too long in lighting fuse or at the face after fuses were lighted. These premature blasts resulted from lack of knowledge of burning speed. When Thermalite Igniter Cord is used and fundamental rules are followed, these accidents are eliminated. --- ## Inadequate Guarding ### New Brunswick Regulations The Powderman in charge of firing explosive charges or blasting shall, before firing, ensure that: 1. Sufficient audible warning is given to all persons in the immediate vicinity 2. All persons in the vicinity have moved to a safe distance 3. All roads and approaches to the blast area are guarded or barricaded to prevent anyone from entering 4. All machinery and equipment is clear of the effects of the blast ### Communication The posting of signs is very important, but the area must be guarded just the same. > **Two-way radios used properly at safe distances are the best way for the blaster to communicate with guards.** If a guard spots something, they can alert the blaster quickly and shut down the blast. Otherwise, shouting might not be heard. --- ## Taking Insufficient Cover Serious and even fatal accidents have been caused by people being struck by flying material. > **Many blasters become too sure of themselves, saying "it will never come this far." These are the people we sometimes read about having blasting accidents.** ### Blaster Responsibilities - Ensure safe distance is maintained while blasting - Always be able to see the shot at all times - Have something to hide behind if needed - **Never stand or allow anyone to stand in front of the blast** ### Proper Stemming - Use good stemming material, preferably 1/2" to 1" chips - Especially important when water is encountered (drill cuttings will not tamp well) - When using borehole liners, chips must be used (liners tend to shoot stemming material upward) --- ## Returning Too Soon After Blasting > **Always assume toxic fumes are present from all blasting or burning of explosive material and stay away until they have dissipated.** ### Toxic Fumes of Concern - Carbon monoxide - Hydrogen sulphide - Nitrogen oxides - (Many other fumes may be present but are not considered poisonous) ### Safety Precautions - Extreme care must be taken when working in confined quarters - Watch out for **orange smoke** after blasting (contains poisonous fumes) - Critical when blasting in buildings, underground, and other confined areas - Before mucking is allowed to start, blaster must comb the area to ensure all explosives have detonated and no misfires remain --- ## Insufficient Ventilation After Blasting Ventilation is very important because of the presence of toxic gases. ### Causes of Toxic Gas Emission - Improper priming - Lack of confinement - Insufficient water resistance ### Surface Blasting Postblast fumes rarely cause problems because they disperse rapidly into atmosphere. However, sufficient time must be allowed for fumes to disperse before personnel return. ### Underground Work It must be assumed that all explosives generate some toxic fumes (most common: carbon monoxide and oxides of nitrogen). Therefore: - Adequate ventilation is essential - Allow sufficient time for fumes to clear > **ANFO and emulsions lack the characteristic postblast smell of dynamite and can give miners a false sense of security.** If there is any doubt, on-site measurement should be made by personnel with adequate protection against noxious fumes. ### Fume Class 1 Specifications According to IME regulations approved by U.S. Bureau of Mines: A 1-1/4" x 8" (or smaller) cartridge of explosive must produce not more than 0.16 cubic feet of noxious gases when detonated under satisfactory conditions. ### Precautions and Facts for All Explosive Users 1. Toxic fumes are liberated by any commercial explosive or blasting agent when detonated 2. Improperly detonated or burning charges produce much greater quantities of toxic gases 3. Wooden spacers substantially increase carbon monoxide production 4. **A clear atmosphere is not necessarily safe** - carbon monoxide is colourless, tasteless, odourless, and non-irritating 5. Even in well-ventilated places, ensure no "pockets or dead ends" contain harmful concentrations of carbon monoxide 6. Water sprays and wetting down the muck are **not effective** in reducing carbon monoxide concentration 7. **Adequate positive ventilation is the only certain method** of dissipating toxic gases. No worker should enter underground working place after blasting until properly ventilated. --- ## Handling of Misfires If all recommendations for transporting, storing, handling, loading, and firing explosives are strictly followed, failure of a charge to explode will be extremely rare. > **All misfires are potential accidents and every precaution should be taken to prevent them.** There are so many different conditions involved that detailed instructions covering all situations are virtually impossible. A misfired charge of dynamite is very hazardous and should be handled only by a thoroughly experienced person. A misfire of blasting agents and detonating cord, while undesirable, poses a much less serious problem. ### Surface Blast Misfires With failure of a charge in surface blast: - Considerable portion of explosives load is likely to be thrown into muck pile by action of other holes - All visible explosive materials should be retrieved (power shovel operation may detonate them) - Danger is greatly aggravated by presence of detonator in unfired explosives - Misfire is frequently not discovered until dynamite is uncovered or accidental explosion occurs ### Disposal Methods **If misfired hole is discovered immediately after blast:** 1. Wash out stemming with water jet and rubber/plastic pipe 2. Reprime and fire (unless hazard from flying missiles) 3. If borehole is offset or disturbed, mark location and recover explosives as material is dug out **Removal of explosives with auger or mechanical means is very hazardous** (generally contains grit): - If explosive is granular, it can be washed out and desensitized with water jet - Otherwise, reprime, cover with several feet of loose sand or screenings, and fire ### Large-Diameter, Deep Hole Misfires Disposal of failures in large-diameter, deep holes loaded with cap-sensitive blasting materials poses a much more serious problem: - Often extremely dangerous from missile standpoint to refire - Adjoining holes have probably dislodged most of burden in front - **Do not attempt disposal without expert advice** - In extreme cases, expose and remove unfired explosives by progressively drilling and shooting lightly loaded small drill holes ### Drilling Alongside Misfires > **Drilling a hole alongside and firing an auxiliary charge to explode the failed one by propagation is a very hazardous procedure.** The chance of drilling into the misfired charge (which may have been disturbed or offset) makes this extremely dangerous. Keen observation and careful investigation will usually disclose the cause of misfire, generally resulting from violation of recommendations. --- ## Accidental Initiation by Flame, Spark, etc. Explosives, blasting caps, electric blasting caps, detonating relays, nonel, etc., should not be subjected to flame, sparks, or excessive heat. ### Precautions 1. Open flames must not be permitted within safe distance of all loading operations 2. **No smoking during loading operations** 3. When cutting detonating cords: - Use a sharp knife - Cut with a quick motion to avoid scraping action - Cut only one strand at a time 4. Ensure non-electric systems don't get hit by falling rocks, drill steel, scaling bars, or mobile equipment --- ## Lightning When electrical potential between two clouds or between cloud and earth reaches sufficient high volume (about 25,000 volts per inch), the air becomes ionized along a narrow path, resulting in a lightning flash. Lightning strikes are static discharges of gigantic proportions. The extremely large amounts of electrical energy released by the powerful electric and magnetic fields represent a hazard to any explosive material. > **A warning is included in every case of explosive materials: Keep away all explosive products during the approach and progress of an electrical storm, irrespective of the type of explosives or initiation systems used.** ### Safety Measures - Discontinue all explosive loading operations - All personnel around blast area should retreat to position of safety until storm has passed ### Inductive Coupling Information collected on lightning-related premature detonation indicates that in most instances, extraneous electricity was introduced into the blasting circuit by inductive coupling (process by which electric energy is transferred from primary to secondary windings in a transformer). **Prevention:** - Open all closed loops in blasting circuits (prevents inductive coupling) - Install lightning gaps in all permanent firing line systems - When firing off permanent firing line, use lightning gap of at least 15 feet separation > **It is very important to abandon all blasting operations upon the approach of an electrical storm. This is a major safety precaution and must always be observed. ALL explosive materials, electric initiation systems, and nonelectric initiation systems are susceptible to premature initiation by lightning.**