Chapter 16: Misfires

Learning Objectives

• Describe misfires and the hazards they can create.
• Discuss the common causes of misfires in charges, cartridge explosives, and initiation systems.
• Discuss misfire wait times by type of initiation system or event.
• Describe how misfires can present differently depending on the type of initiation system.
• Describe the signs of partial misfires, and identify one reason for them.
• Describe procedures for handling misfires safely.
• Explain how to troubleshoot misfires by type of initiation system.

Overview

A misfire occurs when explosives fail to explode, in whole or part, as planned. Misfires are dangerous and increase the potential for an accident. They can cause personal injury, equipment damage, and lost production.

Misfires must be handled with care by certified, experienced blasters. They must be dealt with as soon as possible, yet at a safe and suitable time. If in doubt, contact the explosives manufacturer's representative for expert assistance.

Regardless of precautions taken, misfires can still occur. For this reason, blasters are expected to know the causes of misfires, recognize their indicators, and understand the safe work procedures for handling them.

Causes of Misfires

If blasting procedures are carried out properly, the chances of a misfire are greatly reduced. However, for a variety of reasons, misfires do occur. Some of the more common reasons are listed in the sections that follow.

Issues with Explosives

Misfires in charges can result from the following:

• Detonator becomes detached from a charge.
• Charges in the blast hole become separated by gravel, other materials, or ground movement.
• There is too much space between individual charges.
• Detonators are not hooked into the blasting circuit.
• Cartridges are primed incorrectly.

Misfires in cartridge explosives can result from the following:

• Product has deteriorated or reached expiration date.
• Product is frozen or below manufacturer's recommended temperature.
• Cartridges become separated due to water in the hole.

Issues with Initiation Systems

Misfires in all types of systems can result from the following:

• Incorrect use of initiation system
• Damage to initiation system
• Improper use of delay detonators or detonating connectors

Safety Fuse Assemblies

Misfires in safety fuse assemblies can result from the following:

• Detonator has deteriorated.
• Detonator is damaged.
• Powder core is moist or contaminated.
• Fuse is cut or kinked.
• Fuse or assembly is low quality.
• Safety fuse is damaged.

Electric Initiation Systems

Misfires in electric initiation systems can result from the following:

• Unbalanced circuits (multiple series)
• Improper electrical connections
• Insufficient electric current
• Damage to electric detonator leg wires
• Damage to shot line
• Blasting wire shorted out or connected improperly
• Current leakage in blasting circuit
• Blasting machine damaged or defective
• Blasting machine used improperly

Shock Tube Assemblies

Misfires in shock tube assemblies can result from the following:

• Tubing is damaged.
• A hole or a row is not connected.
• Moisture has compromised the explosive powder core of the tubing.

Electronic Initiation Systems

Misfires in electronic initiation systems can result from the following:

• Current leakage
• Communications interference
• Loss of connection during programming
• Loss of connection after successful programming and in the holding window prior to sending or receiving blast commands
• Dynamic pressure (a high-pressure pulse that results from detonation of adjacent blast holes)
• Incorrect procedures or operations

Other Causes

Other reasons for misfires include the following:

• Explosives have been exposed to extreme temperatures.
• The charge is primed inadequately.
• The blasting circuit or tie-in has not been examined.
• A charge or part of a charge has been "dead pressed" by the detonation of an adjacent charge.
• A charge or part of a charge has been "cut off" due to excessive ground movement (such as a fracture, fault, or joint).
• Loading was improper.
• Coarse stemming has cut the detonator wires or tube.

Misfire Wait Times

In the case of a known or suspected misfire, no one can enter or move around the blast site until the blaster has examined it. The blaster must also wait until the applicable waiting period has passed before examining the blast site. The duration of the waiting period depends on the method of initiation or type of event, as shown in the following table.

Misfire Wait Times by Type of Initiation System or Event

Burning Charge

Burning explosives are highly sensitive and can detonate on their own at any moment. Keep everyone away from a burning charge for the minimum waiting time (one hour after the last visible smoke or as stated in the manufacturer's instructions, whichever is longer). Make no attempt to extinguish burning explosives.

Misfire Indicators by Type of Initiation System

Misfires can present differently depending on the type of initiation system. The following sections describe misfire indicators in several common initiation systems.

Safety Fuse

Safety fuse has the greatest delay potential. It may burn more slowly for a number of reasons, such as lower atmospheric pressure at higher altitudes. Temperature and humidity also affect the atmospheric pressure and can cause moist powder or damaged fuse.

With safety fuse, a misfire has occurred if the explosives fail to explode after the determined appropriate time based on the length of the fuse. Under the Occupational Health and Safety Regulation, blasters must wait 30 minutes (or longer if stated in the manufacturer's recommendations) after the estimated time of detonation to return to the blast site. Manufacturers that provide safety fuse in Canada recommend a minimum wait time of 60 minutes.

Electric

When firing electrically, a misfire has occurred if the explosives fail to explode after attempting to fire the shot. After the attempt, the blaster must do the following:

• Disconnect the firing cable (shot line) or lead wires from the power source.
• Shunt the wires.
• Wait 15 minutes (or longer if stated in the manufacturer's recommendations).

After the wait, the blaster should inspect the cable and connections and repair any obvious break in the cable.

In some cases, a break in the cable or a poor connection can be repaired quickly, and a successful detonation can occur on the second attempt. This situation does not count as a misfire.

Hangfires

A hangfire is an unplanned delay in the electric detonation of a charge. This delay can occur in any part of the system that is contaminated, damaged, or otherwise defective.

A hangfire may occur when an explosive in a hole begins to burn rather than explode. The fire may eventually cause detonation if it reaches the detonator's base charge. Detonation can take a few minutes or seconds after firing the blasting machine. Due to the potential for a hangfire, no one should ever approach a misfired electric blast for at least 15 minutes.

Electronic

When firing electronically, a misfire has occurred if the explosives fail to explode after attempting to fire the shot. The blaster must wait 30 minutes (or longer if stated in the manufacturer's recommendations) before returning to the blast site.

Shock Tube

When firing with shock tube, a misfire has occurred if explosives fail to detonate after initiating the blast. After the attempt and waiting 15 minutes (or longer if stated in the manufacturer's recommendations), the blaster should first determine whether the powder in the lead-in line has burned.

It's easy to tell when a shock tube misfire has occurred because the surface connectors on both the assembly and the delays will remain intact if the shock tube has not fired. In most cases, hooking up missed holes with fresh surface connectors and lead-in line will fire the shot.

Partial Misfires and Cutoffs

In addition to charges failing to fire at all, misfires can also involve only part of a charge going off.

Partial misfires are the most dangerous kind, as there may be little, if any, evidence to suggest that a misfire has occurred. However, the following signs should arouse suspicion:

• A change in the sound (noise) of a blast
• A change in fragmentation (e.g., oversize boulders in the muck pile)
• Finding detonating cord or undetonated surface connector blocks (tattletales)
• Finding explosives in the muck pile
• Less displacement than expected
• Too much displacement on one side
• High bottom or humps and valleys
• Smoke rising from the pile (could indicate burning explosives)
• Orange- or yellow-tinted fumes (could indicate oxides of nitrogen, which are products of improper detonation)

Cutoffs

One reason for a partial misfire is a cutoff in the hole. A cutoff is a type of misfire that involves a break in the initiation system. Cutoffs are usually caused by interference such as shifting ground or flying material.

Procedures for Handling Misfires Safely

After the required wait time has passed, blasters can take the following steps to deal with misfires.

Keep the Danger Area Clear of People and Assess the Hazard

Once the waiting time is over, the blaster of record may approach the misfires or suspected misfires to assess the hazard. As the "all clear" has not yet been sounded, guards must continue to stop traffic and keep people out of the danger area.

Based on the blaster of record's assessment, the boundaries of the danger area may need to be adjusted. Everyone except the blaster and the equipment operator must be removed from the danger area. Any misfired holes that cannot be dealt with immediately must be clearly marked, and the danger area cordoned off and guarded.

Identify the Locations of Misfires

Any hole containing a misfired charge should be identified by one of the following:

• Spray painting the collar of the hole
• Placing a wooden marker
• Stuffing a coloured rag in the open end of the hole
• Placing a plastic cone in or near the collar of the hole
• Tying coloured "survey" flagging to a stake near the collar of the hole
• Other effective means

Remove Broken Material by Hand Before Excavating

The blaster must direct the hand removal of as much broken material as possible before metallic tools or equipment is used. Sparks from metallic tools or equipment can cause accidental detonation.

Do not use metallic equipment to remove broken material unless the following apply:

• A blaster directs the use of the equipment.
• Lighting in the area is adequate.
• Precautions are taken to prevent injury in the event of accidental detonation.

When using an excavator, everyone except for the blaster and the operator must be removed from the danger area. Operators must follow the direction of a blaster and use caution when excavating blast rock. They should understand that a misfire may be handled only by, or under the direction of, a blaster. Adequate safeguards must be provided to protect the blaster and the operator in the event of an accidental detonation.

Specific misfire procedures should be developed for the explosives being used. These procedures must be contained in the blast plan. For advice on developing these procedures, contact the manufacturer or its representative.

Follow Restrictions on Removing Explosives from Holes

Until a misfire has been successfully detonated with a fresh primer, the unexploded charge that contains a primer must not be removed. No attempt to remove it may be made, and no other work may take place within the danger area.

Extracting or attempting to extract a primer or explosive of the nitroglycerine type from a loaded hole is prohibited.

A blaster or a person authorized by the blaster may remove ammonium nitrate, water-gel, or emulsion explosives from a blast hole only if all of the following apply:

• The blast hole does not contain a detonator.
• The removal procedure is carried out with caution using:
  • Moderate air and/or water pressure.
  • A blow pipe made of non-metallic materials.
• Stemming material is removed carefully.

Destroy Misfired Charges

Once a misfired charge has been located and identified, it must be destroyed before other work begins. Misfired charges are usually destroyed by detonation using a fresh primer.

Destruction of misfires should be treated like any other blast. Warning signals should be sounded and the danger area guarded.

Drill to Re-Fire a Misfire

One way of destroying a misfire is to drill, load, and fire a hole parallel and close to the misfired hole. The purpose of the additional hole and charge is to expose the misfire so it can be destroyed with a fresh primer.

Drilling an additional hole to re-fire the charge is permitted, but only if the following apply:

• The angle of the misfired hole is accurately determined.
• The blaster of record who placed the misfired charge directs the angle and depth of the hole being drilled.
• The hole being drilled is at least 600 mm (2 ft.) from any part of the misfired charge.

These precautions help prevent the possibility of drilling into any undetonated explosives.

Ensure Sufficient Burden Before Re-Firing

If the method for treating the misfire is re-firing, the blaster must ensure that there is sufficient burden before proceeding.

Whenever re-firing is attempted, the blaster must establish that it is safe, or even possible, to do so. If only some holes have misfired, this could mean that there is a greatly reduced burden for the hole(s) to be re-fired. With reduced burden, the rock could fly many times the normal distance, and the air blast would be greatly increased. In some cases, it may be possible to increase the burden and cushion the blast by using methods such as dumping additional material in front of the face.

How to Troubleshoot Misfires by Type of Initiation System

The following sections describe how to troubleshoot misfires depending on the type of initiation system.

Electric

If several blast holes (or the complete round) have misfired, and a visual inspection has not revealed the fault, it will be necessary to test the circuit.

Take the following steps before looking for the problem:

• Disconnect the firing line from the power source and shunt it.
• Put the power source under positive control.

Next, disconnect the firing line from the rest of the circuit and re-test it. Then check the leg wires for detonator continuity using an approved testing device (e.g., a blasting galvanometer or blasting multimeter). If there is continuity, it may be possible to fire the missed charge by hooking up the wires again.

If continuity is not present, then use another technique. (See "Locating a break in the circuit" in Chapter 20.) Never pull on the wires leading to an electric detonator.

If a fault is not found, divide the circuit in half and test each half. Continue to divide the defective circuit in half until the defective detonator is found. (Note that there may be more than one.)

Testing must be done from a place of safety. Blasting meters are designed so that the current is insufficient to set off a detonator. However, one can never be too careful with a misfire.

Shock Tube

If a misfire happens with shock tube, the problem could be with the surface delays or a particular hole. With surface delays, the blaster should be able to replace surface connector blocks or reconnect them to re-fire. If a hole has not fired, a surface connector block will still be intact. These shock tubes can be reconnected and fired. The blaster must ensure that the shock tubes have not been damaged by flying material.

Another test can be performed to verify that a tube has fired. Cut a short piece (15 cm or 6 in.) of the tube. Blow through the tube as if it were a straw, while pointing the other end of the tube toward the palm of your hand. If a small amount of white or greyish powder appears on your hand, the tube or detonator is live. The tube can then be reconnected and blasted.

Safety Fuse

Never attempt to re-light any safety fuse. Re-fire with a fresh primer and safety fuse assembly.

Mislight

A mislight is a failure or perceived failure of a pull-wire lighter to ignite a safety fuse assembly, particularly in an avalanche control setting. A mislight is not considered a misfire.