# Chapter 16: Misfires A misfire can be defined as the failure of an explosive charge to detonate in the planned manner. It can be otherwise termed an explosive malfunction. This can be a premature detonation, a hangfire (detonation long after the planned time), partial detonation of an individual charge or several charges, or a complete failure to detonate at all. The most effective prevention of misfires is the careful and precise design, evaluation, and preparation of each blast. Every selected material must begin with consideration of the geologic conditions and then proceed to the design and execution of the blast. These processes include the explosives and initiation systems selected and used. The most effective prevention of misfires is the careful and precise design, evaluation, and preparation of each blast. Due to the hazards created by misfires it's recommended that every company develop a misfire policy. The factors shown in Table 16.1, when not considered properly in the blast design, are known to produce misfires: ## Table 16.1 – Misfire causes. (After Konya, Walter, 1995) | Misfire Causes | |----------------| | Ground shift during blast | | Improper priming | | Inadequate water resistance | | Desensitization | | Improper detonator | | Inadequate booster | | Improper timing | | Failure to initiate detonating cord | | Cutoffs from rock movement | | Contaminated explosives | | Out of date explosives | | Deteriorated explosives | | Product quality control | | Faulty products | | Handling or storage damage | | Explosive incompatibility | | Product use error | | Impact desensitization | | Temperature extremes | | Improper loading | | Loss of cap sensitivity | | Loss of booster sensitivity | | Loss of explosive sensitivity | | Excessive decking | | Dynamite frozen/unfrozen | | Inadequate confinement | --- ## GEOLOGIC FACTORS The evaluation of the geologic conditions in the area to be blasted is a primary concern. Dipping layers of rock or unsupported ground are subject to shifting during the progression of a blast. This ground shift can move ahead of the initiation system to cut off down lines or portions of the powder column. If possible, the timing of the shot should progress towards the strength of the geology (down dip or away from unsupported ground). Blasting near weak geologic structures or rock can cause ground shifting resulting in a misfire. As ground shift is most likely at the top of the hole, double priming is strongly recommended in all blasting conditions where feasible. A bottom and top primer will allow initiation from both ends of the powder column to compensate for any initiation system cut off or column shift during progress of the blast detonation. Bottom hole cut off is more likely than many may assume. There have been occurrences of bottom hole cut off with top priming only. Seams of clay, weathered rock, or even open gaps may allow explosives pressure to travel from hole to hole. This pressure can cause column cut offs, low order detonation, or premature detonation. --- ## BLAST DESIGN FACTORS A basic concern in conjunction with geologic considerations is the timing of the shot. Too much time in the above conditions will allow the ground movement to disrupt the explosives columns before they can be detonated. Under these conditions, design the timing to arm as much of the shot as possible before initiation begins. Arming of the whole shot before the first borehole fires is the ideal condition. Measurement of back break or surface and/or rock shifting of previous blasts is a good measure of how far ahead the timing must be for safety. This distance will translate into how far the timing has to precede the actual detonation. Designing a blast with borehole-to-borehole delay on the surface only, (detonating cord with MS connector with no in-hole delay) can greatly increase the likelihood of misfires due to cut off down lines and should be used with caution and understanding. To be done successfully an understanding of local geology is important, and very short delays are necessary to keep borehole down lines from being disrupted or broken due to the heaving or shifting resulting from the detonation of boreholes. Designed times can be so slow that they allow ground movement that may shift un-fired explosives, or unburden succeeding boreholes. Without good reasons, the use of very slow delay times during blasting should be avoided. Fast times may generate back pressure and excessive ground shift behind the shot progression. These pressures can also cause desensitization of explosives in the shot or even premature detonation. The selection of the initiation system, if done incorrectly, will generate misfires. Electrics in electronics in high stray current or radio interference areas is a definite problem. No in-hole delay in a heave or shifting ground will result in many misfires unless timing is carefully considered. The combination of burden, spacing, hole depth, stemming, and explosives selection can have an impact on misfire generation. These design factors should remain within recommended guidelines or ratios of burden. Changes from these recommendations should be thoroughly considered. Variations from the norm may result in ground shift, premature detonation, explosives sensitivity, or other misfire causes. As drilling is an important part of the total blasting process, drilling the blast to design parameters is critical. Incorrect angles, depths, or locations can create misfire hazards. --- ## EXPLOSIVES PRODUCT FACTORS Selecting improper explosives for a blast can contribute to the occurrence of a misfire. It is important to select explosives with adequate water resistance and sensitivity. The quality and condition of the explosive itself can also result in malfunction or misfire. An incorrectly chosen initiation system can contribute to the occurrence of a misfire in some applications. ### Explosives Water Resistance The use of unprotected ANFO in wet ground will result in unfired explosives in the muck pile or face. In some cases, the primer will fire, but the ANFO column, if sufficiently wet, will not initiate at low order detonate (leaving ANFO in the muck pile or face). ### Explosives Sensitivity The use of explosives with improper sensitivity (ease of initiation or chemical reactivity), or sensitiveness (propagation reliability) may result in a misfire. The use of impact sensitive high explosives products in high pressure excavations may cause instantaneous propagation of some or all of the charges despite the designed timing. Blasting agents that are not resistant to shock pressure may fail to detonate under the same conditions or detonate at low order. Bulk and packaged explosives formulated or manufactured for large borehole diameters may not detonate completely or properly in smaller holes. The use of inadequate primer assemblies (too small or too low in shock pressure) can result in low order detonation of the main charge or complete failure. Location of the primer assembly in the wet or muddy borehole bottom can also result in low order detonation or failure. ### Explosives Quality The quality, composition, and delivery (as in bulk pumped explosives) can result in misfired explosives. Many modern explosives, especially water based products, are very sensitive to incorrect manufacture and delivery. Loss of gelling or emulsifying chemistry will seriously affect the products' ability to detonate properly or at all. This changed chemistry will not only affect detonation sensitivity but also water resistance. The lack of or wrong sensitizing process (microballoons or gas bubbles) may cause low order detonation or complete failure. Commercial explosives have maximum "shelf life" in storage, and "sleep time" in the blast hole. These durations are readily available from the manufacturer and must be followed. The emulsion and water gel types are often formulated to remain usable for specific time durations from the time of manufacture and will lose sensitivity or integrity beyond those specifications. Explosive manufacturers have established quality control criteria for the evaluation of their products during storage and use. These may include specific gravity (density), viscosity, color, texture, and appearance. The quality control values should be understood and applied by the blaster in the field. The blaster on the shot is the final inspector in the manufacture and use of explosives systems. ### Loading Procedures The most common cause of malfire potential during loading are damage to the initiation system downlines and/or explosives column separation. Decoupled column charges that have been separated during loading in caving or squeezing holes must be re-primed to maintain continuity of detonation. --- ## INITIATION SYSTEM FACTORS By far the most common cause of initiation system misfires are human error in application of the products. The training and understanding of the particular initiation system is another critical factor in preventing misfires. System limitations, operation particulars of the system, and potential failure modes must be thoroughly understood before the system is put into use. Blasters and crews should have extensive training in the application and use of the system being used. Any questions about these particulars should be addressed by the manufacturer or a company representative. As errors in manufacturing, shipping, and storage have occurred, the blaster should inspect all initiation products for quality and appearance. The incorrect application, deployment, connection, or operation of the initiation system will have a critical effect on their proper function. ### Detonators The selection of the proper strength detonator is critical to the proper initiation of detonator sensitive explosive products. Explosives suppliers manufacture and market many different detonators with varying strengths of base charge. These different strength detonators are meant to initiate products such as shock tube, detonating cord, cast primers, NG dynamites, crushed ANFO, etc. It is important that low strength detonators not be applied to a product that requires a high strength detonator. In extreme pressure situations (close spacing, wet or pressure transmitting ground, tight shooting), the physical strength of the detonator shell and its internal components is critical to proper functioning. This may be especially true of the new electronic detonators. I-steel cast primers or other protection for the detonator may be required. In some very extreme situations such as wet and clayey ground, it has been shown that shock tube in a nearby blast hole can initiate from transmitted pressure. This will then initiate the detonator attached to it. In "blast beside" situations, this potential must be considered. Pyrotechnic detonators are subject to timing changes due to aging. Time and changes in temperature and humidity will alter the designed times, and in the case of very long delays, they may fail completely. ### Priming Inadequate or improper priming is often the cause of supposed misfired conditions. The attempted use of small, or less energetic primers has resulted in poor breakage or unfired explosives left in the blast. When using cartridged explosives in wet or muddy boreholes, multiple re-priming is often necessary to prevent column cut off or unfired cartridges. The placing of a single primer in the wet and muddy bottom of the borehole can and will result in low order detonation and potential failure of the column of explosives above it. Any single priming of explosives columns longer than several feet may result in undetonated explosives left in the blast. The column length may be as short as 1.5 meters (5 feet), but any column longer than 3.0 meters (10 feet) can be a problem. This is due to ground shift during blast detonation, which is primarily limited to the top of the powder column, but can occur in the bottom if only top priming is used. ### Electronic Systems The recent introduction and widening use of electronic detonation systems has provided the blaster with a tool that can greatly improve the safety, performance, and productivity of blasting operations. As these systems combine electronic, electrics, software, and hardware components, they have also added quite a lot of complexity to blast initiation systems. They require a much higher level of training and understanding to be used in a safe and productive manner. The blaster has to understand how these different electrical, electronic, software, and hardware components operate in conjunction with each other to manage the blast at it is designed, deployed, tested, programmed, armed, and fired. For instance, the fact that all these systems operate from an internal capacitor (energy source) after the fire command is given means that discharge characteristics of these capacitors have to be considered if there are suspected unfired detonators after a blast. Each system has its methods of operation, components, and procedures that can be very different. Current leakage is perhaps the biggest potential problem that can cause electronic misfires. Electronic systems may be sensitive to communications interference from outside sources and these potentials have to be considered. ### Nonelectric Systems Shock tube systems are very sensitive to improper connection of components in the detonation chain. Care must be taken to closely follow manufacturers' recommendations and procedures. All initiation products will come packaged with instructions and warnings for use. When mixing shock tube with detonating cord, the interface between the two systems must be very carefully made up and protected. Detonating cord's energy can damage and shut down shock tube lines, and the initiation of detonating cord by nonelectric or electric detonators can be very problematic if not done properly. Wildlife damage of down lines left to sleep overnight is not uncommon. Any nonelectric systems left overnight should be re-checked before initiation. When using nonelectric lead-in-lines to initiate a blast, it is important to keep contamination by dirt or fluids from all splices, and from the end of the tubing in the initiating device. Several feet should be cut off the ends to be spliced or connected to the initiating device. After cutting, care should be taken to keep the cut ends protected from contamination. Initiating devices must be cleaned periodically to remove contamination from initiation reactions (see, (Air, Jet, etc.). Any contamination in the tube will shut down the reaction. Even that blows into the tube by the initiating device will likely stop the reaction. No splices should ever be used within a blast tie-in, only as a lead line to that where the effect of a misfire will be minimal. ### Electric Systems Conventional electric systems require a fairly high level of understanding about electricity and its circuits. The effect of current leakage from stranded insulation or poor connections in a blast has contributed to misfire problems. Batteries that have not been maintained or completely charged are also a cause of electric misfires. Unbalanced circuits have been the cause of numerous problems — especially underground in multiple loading operations. Sequential electric systems carry with them the same danger of current leakage due to conductor insulation leakage and circuit balancing. An especially common hazard is abraded sequential cables that have been used too long or not maintained properly. These are constantly abrading the conductors together in the bundle as they are deployed and recovered for future use. Battery condition and charge are also an issue. ### Detonating Cord Systems For detonating cord, understanding cord-to-cord sensitivity and knot tying procedures are very important. As the core load of the detonating cord decreases, the violence, noise, and disruptive effect is decreased. However, its reliability, durability, pickup sensitivity, and donation strength to other cords is decreased. Improperly tied or loose knots can fail to transfer the initiation from one section of cord to another. Shock tube to detonating cord and cord to shock tube connection will cause misfires if not completely understood and applied correctly. The great difference between detonating cord and shock tube in speed and violence can cause problems if not fully understood. The use of detonating cord with MS connectors in loose or shifting ground is a poor practice as a result of poor tie-ins, delays that are too long from hole to hole, or excessive violence due to the stemming disruption by the detonating cord down line, the shot can stop during progression. Detonating cord surface and down lines can be damaged by wildlife if left over night on patterns. Foxes, coyotes, mice, etc. have been known to chew on these lines and actually sever them. Any detonating cord shot left to sleep overnight should be thoroughly checked before initiation. Wet ends of detonating cord may not pick up initiation from donor detonators or detonating cord. Any detonating cord with suspected wet cord must be replaced with dry core load cord, or end primed with a detonator to assure proper initiation. ### Cap and Fuse System Fuse and cap systems have declined in use dramatically in the past decades. Those using this method of timing or initiation must be aware of the safety and misfire potential of incorrect application and use. The fuse itself is based on a black powder core that is very susceptible to wetting and extinguishing of the burning front by water or even small amounts of moisture. In addition, other liquids such as petroleum-based fluids will alter the burning rate or cause complete failure. Pre-cut and made up fuse assemblies can have the pre-cut ends contaminated by contact with floors or other unclean surfaces. Contaminated cutters or crimpers will also increase the potential. These contaminations will cause misfires. Improperly cut ends (not at a 90° angle to the axis) will result in poor coupling to the fuse cap or fuse connector. Incomplete coupling will not effectively transfer the ignition from connector to fuse or fuse to cap. Caps may fail to detonate if contaminated with moisture or other materials. Moisture can collect in the cap shell or fuse coverings during temperature cycling from cold to warm environments. All safety fuse should have a minimum of 25 millimeters (1 inch) cut off from both ends before use. Misfires and accidents have also been caused when small particles of contaminants have become lodged between the ignition charge of a fuse cap and the end of a safety fuse. All foreign material must be kept out of the fuse cap and off the ends of the safety fuse. If foreign material is present in a fuse cap, the cap should be disposed of in an approved manner. > **Caution** > > Do not attempt to remove dirt or other material from a blasting cap. To guard against contaminants, safety fuse and caps should be stored in approved, clean, dry, well-ventilated areas or in magazines that have as low humidity as possible. Blasting caps and safety fuse should always be stored at normal temperatures. Storage temperatures over 60°C (140°F) can melt some waterproofing materials in the covering thus reducing the water resistance of the fuse. At temperatures below 7°C (45°F) the fuse covering can become stiff and brittle. If stored at cold temperatures, the fuse should be warmed before being bent or flexed. Bending cold fuse can crack its covering and expose the powder column to contamination or moisture. --- ## INITIATION SYSTEM TIE-IN A very common factor in the prevention of initiation system misfires is the blaster's follow up of the blast tie-in with some form of final inspection. With electric or electronic systems, the check can be made electrically or electronically, but should be thorough. With nonelectric systems, a visual check of all connections as a separate operation after tie-in is the most important step in misfire prevention. These steps are a confirmation that the blast has been connected in the planned manner and that all communications/connections should work as designed. It is designed for underground or bench, a final visual check is necessary as lighting conditions can easily result in boreholes not being tied into the round. Electrical continuity checks with a "blasters" or multimeter, blasting galvanometer, or blasting ohmmeter cannot exactly account for the number of detonators as the round — only that that there is continuity in the circuit. --- ## WAITING TIMES Most regulations have recommended waiting times before personnel can return to the blast site after a misfire. The waiting time should be longer than that required by the manufacturer or as work regulations require. These time periods must be followed to keep personnel safe. The Institute of Makers of Explosives (IME) recommends 30 minutes for fuse and cap and 15 minutes for electric, nonelectric, or electronic systems. --- ## INITIATION SYSTEM DEVELOPMENT As bulk explosives blasting technology has progressed, so too have the initiation systems. We have progressed from cap and fuse to electric, to sequential electric to shock tube to electronic. With this product evolution has come complexity and the need for additional training, skills, and understanding on the part of the blasters. --- ## REFERENCES Konya, Calvin J., Edward J. Walter. 1991. *Surface Blast Design*. Prentice Hall Englewood Cliffs, NJ. --- ## ADDITIONAL RESOURCES Atlas Powder Company. *Explosives and Rock Blasting*. Dallas, TX. International Society of Explosives Engineers. 1998. *ISEE Blasters Handbook* TM, 17th Edition. Cleveland, OH.