What is Misfire, and What is Safety in Surface Blasting

Waiting Periods, Risk Factors, and Industry Practice

Practical Explosives Training School (PETS)  |  Petr Explosives Group (PEG)

1. Why Waiting Periods Matter

A misfire is any explosive charge, or portion of a blast, that fails to detonate as planned. Under guidance from the Institute of Makers of Explosives (IME) and applicable international blasting standards, all misfires must be treated as live explosives until a qualified blasting professional has formally declared otherwise.

A misfire is not always a complete failure. Conditions that create ongoing hazards include:

• A detonator that functions late or after an unpredictable delay
• Burning safety fuse that continues underground, unseen by surface personnel
• Damaged initiation systems capable of reigniting
• Explosives that remain in an active chemical reaction internally
• Delayed shock tube transmission through damaged or kinked tubing
• Incomplete communication in electronic detonator systems
• Reactive ground that elevates instability in the loaded hole

Mandatory waiting periods exist to protect personnel from delayed detonations, flyrock, toxic fumes, sympathetic initiation, accidental initiation during re-entry, static electricity, lightning-induced currents, and equipment interaction with unexploded charges.

PRINCIPLE  When in doubt, wait longer. IME Safety Library publications are unambiguous: no person should approach a blast area until the minimum waiting period has elapsed, the area has been secured, and the blaster-in-charge has issued formal re-entry authorization.

2. Typical Minimum Industry Waiting Times

The following table presents regulatory minimums alongside conservative industry practice. The minimums represent the floor, not the target. Site-specific hazards routinely justify significantly longer exclusion periods.

3. Calculating a Conservative Waiting Period

Waiting time should never be arbitrary. A structured approach that uses operational risk factors yields a defensible, appropriately conservative waiting period.

3.1 Basic Conservative Formula

T = B + E + G + W + C

Where:

• T = Total recommended waiting time
• B = Base waiting time for the initiation system in use
• E = Environmental adjustment (visibility, access conditions)
• G = Ground condition adjustment (moisture, reactivity, conductivity)
• W = Weather adjustment (lightning, precipitation, wind)
• C = Circuit complexity adjustment (number of holes, series/parallel wiring)

3.2 Example: Electric Blast in Wet Ground During a Thunderstorm

3.3 Safety Fuse Burn Time Formula

For safety fuse systems, theoretical burn time is calculated as:

Expected Burn Time = Fuse Length (m) × Burn Rate (sec/m)

Example: a 1.5 m fuse at a nominal burn rate of 120 sec/m yields an expected ignition time of 180 seconds. However, IME guidance recognizes that wet, damaged, or improperly stored fuse burns unpredictably. In practice, waiting periods for fuse systems must extend well beyond theoretical calculations, and conservative operations mandate a minimum 1-2-hour wait as a baseline.

4. Electric Detonator Systems

4.1 Minimum Guidance and Conservative Practice

OSHA and IME guidance require a minimum 30-minute wait following a failed electric shot. Many mines and contractors extend this to a full 60 minutes, citing hazards including:

• Stray and induced electrical currents
• Damaged or partially severed circuits retaining continuity
• Partial firing leaves energized detonators in the pattern
• Stored electrical energy in capacitive circuits
• Radio frequency interference from nearby communications equipment

4.2 Additional Precautions

Prior to any re-entry, the blasting machine must be disconnected, all wires shunted, circuits grounded, and radio transmission prohibited within the blast area. Where practical, continuity should be verified remotely before any personnel approach.

4.3 Good and Dangerous Practice

5. Non-Electric Shock Tube Systems (NONEL)

Shock tube systems carry a lower electrical hazard than electric systems, but they introduce their own misfire risks. Cut or kinked tubing may continue transmitting signal; downhole reactions may be delayed; connector blocks may partially function; and damaged downlines may remain active despite surface silence. Bootleg explosives left in the hole following partial initiation remain sensitive and dangerous.

Many operations now mandate a 60-minute wait before personnel enter the blast area following a suspected shock tube misfire. IME guidance requires that all shock tube remnants be treated as potentially live until the blaster-in-charge has physically verified the area is safe.

6. Electronic Detonator Systems

Electronic detonators require system-level verification before re-entry can be considered. Depending on the platform, this includes manufacturer diagnostics, blast logger review, software confirmation of firing status, capacitor discharge verification, and communication integrity checks. Systems may report ambiguous states such as “not fired,” “unknown status,” or “communication failure” — all of which must be treated as unresolved misfires.

Conservative operations isolate the area, conduct remote diagnostics, enforce a 30 to 60-minute minimum wait, and prohibit manual inspection until system verification is complete.

INCIDENT NOTE  Multiple international mining operations (2018–2023) reported electronic detonator communication failures in which blast crews initially believed all holes had fired. Follow-up drone inspections identified unexploded charges remaining in the pattern. These incidents reinforced the importance of remote inspection technology, blast mapping, electronic tracking systems, and strict re-entry authorization protocols.

7. Safety Fuse Systems

Safety fuse systems are among the highest-risk initiation methods for delayed firing. Burn rates vary with moisture content, manufacturing tolerances, fuse damage, degree of confinement, temperature, and storage conditions. Conservative operations enforce a minimum waiting period of one to two hours, and some historically required an end-of-shift waiting policy before any approach to a suspected fuse misfire.

IME publications strongly discourage any handling of suspected live fuse systems. The uncertainty inherent in fuse burn rates makes theoretical calculations unreliable as a basis for re-entry decisions.

8. Historical Misfire Incidents

8.1 Mining

At the Sunshine Mine, Idaho (1972), confusion involving smoke, fire, and concurrent blasting operations contributed to delayed evacuation and multiple fatalities. While not a classic misfire event, the incident demonstrated the critical importance of strict blast-area control and conservative re-entry procedures in underground environments.

In South African deep-level gold mines during the mid-twentieth century, several documented incidents involved delayed detonations caused by damaged electrical circuits in wet conditions. Miners who returned to the face too early after assumed misfires sustained injuries from late-firing charges.

Australian open-pit mines documented incidents in which ANFO columns remained live following partial initiation. Equipment operators subsequently drilled into unexploded explosives days after the blast, causing secondary detonations and fatalities. These incidents drove the adoption of blast-hole tracking, GPS mapping, exclusion-marking systems, and post-blast drone surveys across the Australian mining industry.

More recently, investigations in North America and Australia identified multiple accidents in which heavy equipment operators entered blast areas before formal clearance, resulting in detonation of unexploded toe charges during excavation. Contributing factors consistently included poor communication, inadequate blast documentation, production pressure, and failure to apply IME misfire procedures.

8.2 Military

Large quantities of World War I artillery shells failed to detonate on impact. Many remain buried across European battlefields and continue to be discovered today; some have detonated during recovery operations, killing disposal personnel and civilians.

In Vietnam, Laos, and Cambodia, millions of unexploded bombs and cluster munitions remained after the conflict. Delayed explosions from decades-old ordnance continue to kill and injure civilians, particularly farmers and scrap collectors.

Military training ranges worldwide regularly encounter unexploded ordnance from exercises. Numerous accidents involving personnel approaching apparently inert rounds led to the development of strict EOD procedures, remote inspection tools, robotic disposal systems, and mandatory exclusion distances. Modern military explosive safety doctrine closely parallels IME guidance for commercial blasting.

8.3 Avalanche Mitigation

Avalanche control teams at Alta Ski Area, Utah, documented delayed explosive responses due to deep-snow burial and damage to the fuse system. Near-miss incidents prompted the introduction of strict waiting periods and mandatory visual confirmation procedures before re-entry into the area.

Swiss mountain operations have documented artillery shells that failed to detonate during avalanche-control missions and were later discovered after snowmelt, posing hazards to hikers and maintenance crews. In British Columbia, avalanche crews encountered misfired charges buried in highway snowpacks; GPS tracking of deployed charges and conservative exclusion zones subsequently became standard practice.

9. Special Conditions

9.1 Lightning

Lightning conditions are among the strictest hazard scenarios in surface blasting. When lightning occurs near loaded holes, blasting operations should be suspended and all personnel evacuated. Hazards include induced currents in electric circuits, static discharge, accidental initiation, and electromagnetic interference with electronic systems. Many operations require a minimum 30-minute wait after the last lightning strike before any return to a loaded blast area. IME guidance strongly recommends suspending blasting operations entirely whenever thunderstorms approach the site.

9.2 Reactive and Hot Ground

Reactive or hot ground — including sulfide-rich formations, geothermal zones, and chemically active ore bodies — represents an extreme hazard for loaded blast holes. Spontaneous heating, premature detonation, explosive degradation, and toxic gas generation are all documented risks. Conservative management requires continuous temperature monitoring, remote inspection, extended exclusion zones, and re-entry only with explicit authorization from the blasting supervisor. Standard waiting periods are insufficient; site-specific protocols based on ongoing monitoring data are required.

9.3 Fume Clearance

Even after a waiting period has elapsed, re-entry may remain unsafe due to toxic blast gases. NOx, carbon monoxide, and ammonia reaction products can accumulate in and around the blast area, particularly in confined or low-wind conditions. Re-entry should occur only after the dust has cleared, fume concentrations have been confirmed to be acceptable by air monitoring, visibility has been restored, and ventilation systems have been verified as operational. No waiting period alone satisfies the fume clearance requirement.

10. PETS / PEG Conservative Guidance

11. Recommended and Prohibited Practices

11.1 Recommended Practices

• Secure and isolate the blast area immediately following any suspected misfire
• Maintain accurate blast records, timing logs, and hole-by-hole firing confirmation
• Deploy remote inspection methods — drone surveys, blast mapping, electronic tracking — wherever practicable
• Clearly mark all suspected misfire locations with durable, visible indicators
• Notify all affected personnel and supervisors without delay
• Follow IME guidance and manufacturer instructions precisely
• Re-prime and re-fire misfires only under the direct supervision of a qualified blasting professional

11.2 Prohibited Practices

• Digging or drilling directly into a suspected misfire location
• Pulling on the detonator wires or the shock tube following a misfire
• Allowing production pressure to override safety protocols or waiting periods
• Assuming a charge is inert because no detonation was observed
• Leaving misfires undocumented or unmarked
• Using metal tools in proximity to exposed or suspected explosive charges

Final Safety Principle

Production can always wait. A delayed detonation gives no second warning. The safest blaster is the one willing to wait longer than everyone else.