Chapter 17: Safety Fuse Assemblies

Learning Objectives

• Describe the components of a safety fuse assembly.
• Discuss burning speed and the factors that affect it.
• Explain storage and handling requirements for safety fuse.
• Describe procedures for safely igniting a safety fuse assembly.
• Describe acceptable lighting devices.
• Explain how to check that a fuse is actually burning.
• Discuss the hazards of safety fuse assemblies and the precautions to take when using them.

Overview

A safety fuse assembly is made up of a length of safety fuse with:

• A detonator attached to one end
• A removable plastic cover on the other end

Components

Safety Fuse

Safety fuse consists of a special black powder core in a spirally wrapped cover of textiles and waterproof materials. The cover protects against contamination and abrasive damage. It also allows the fuse to carry flame to the detonator at a uniform speed.

The shortest safety fuse assembly allowed is 1 m (3.3 ft.) long. In some cases (e.g., with longer fuse assemblies), it may be necessary to trim the end of the fuse.

Damaged or deteriorated safety fuse must not be used, as it can result in a misfire.

Detonator

The detonator is an initiating device capable of detonating most explosives. It has an aluminum shell about 6 mm (¼ in.) in diameter and 48 mm (1⅞ in.) long, with a composite charge pressed into the base end.

The composite charge is made up of a primary charge and a high-explosive base charge. The primary charge is heat- and flame-sensitive and composed of lead azide and lead styphnate. The high explosive is PETN (pentaerythritol tetranitrate).

The ignition spit from the safety fuse ignites the primary charge. The primary charge then detonates the base charge.

Static Shunt (Staple)

All detonators used in safety fuse assemblies contain a static shunt. A metal staple is embedded in the detonator. The staple touches the shell and penetrates through the fuse to the powder core. The staple drains off static electricity to help prevent premature detonation.

The static shunt does not provide complete protection from static and other electrical hazards. For this reason, a safety fuse assembly should not be used if excessive amounts of static or other extraneous electricity are known or suspected to be present.

Burning Speed

The typical burning speed of safety fuse is 131 seconds per metre (40 seconds per foot) at sea level. The actual speed may vary by plus or minus 10 percent.

Factors Affecting Burning Speed

The burning speed may be affected by the following:

• High altitude - Exerts less external pressure on the powder granules and causes the fuse to burn slower.
• Altitudes below sea level - Causes the fuse to burn faster due to greater external pressure.
• Confinement - Causes the fuse to burn slightly faster. Confinement can result from fuse that is underwater or buried under mud.
• Moisture - Absorbed by the powder and causes the fuse to burn slower. Moisture absorption usually occurs over a long period of storage. For this reason, it's important to use the oldest product first. In some cases, moisture absorption means the full length of the fuse does not burn, and the result is a misfire.
• Chemicals, oil, and solvents - Can destroy the outer cover and contaminate the powder. This causes the fuse to burn slower or fail to burn (misfire).
• Kinks and bends - Can cause the fuse to burn faster, but are more likely to make it burn slower or fail to burn (misfire).

If a safety fuse assembly comes in contact with itself or another assembly, cross initiation (lighting) may occur.

Storage and Handling

To maintain the specified burning speed, the safety fuse must be stored in a cool, dry, well-ventilated magazine. Rotate stock by using old fuse first.

When handling safety fuse, avoid bending, pinching, or twisting it. These actions can damage the covering. In cold weather, warm the fuse slightly before using it to avoid cracking the waterproofing.

Safety fuse assemblies should not be used to prime except in an avalanche control operation.

Igniting the Safety Fuse Assembly

Igniting a safety fuse assembly is a critical operation. Difficulties in lighting one assembly while others are burning have resulted in accidents and serious injuries. For this reason, the safety fuse and the lighting device must be in good condition.

Safety fuse is easy to ignite if the lighting device has an intense (hot) flame and the fuse end is clean and dry.

If a fuse end is moist or contaminated, about 25 mm (1 in.) should be cut from the end of the fuse before attempting to ignite it. This is known as trimming the fuse.

The minimum length of safety fuse assembly that can be sold is 1 m (3.3 ft.). The safety fuse must not be trimmed to a length shorter than 90 cm (3 ft.).

A blaster should use two safety fuse assemblies when initiating a blast. This greatly reduces the chance of misfires.

Acceptable Lighting Devices

A safety fuse assembly must only be ignited using an acceptable device. Smouldering or open-flame devices such as cigarettes, cigarette lighters, and propane torches are not acceptable for lighting a fuse assembly. Such devices are unreliable and could endanger people in the danger area. In addition, smouldering materials and open flames are prohibited on the blast site.

The following types of devices are acceptable for lighting safety fuse assemblies.

Pull-Wire Lighters

A pull-wire lighter is a flame-producing device used with a single fuse assembly. It is effective where high winds could hamper other methods of ignition. A pull-wire lighter is the preferred device for lighting a safety fuse assembly.

To prepare the assembly, remove the plastic cap from the end of the fuse. Make sure the inside of the pull-wire lighter and the exposed end of the fuse are clean and dry.

To attach the pull-wire lighter, gently insert the fuse about 50 mm (2 in.) into the open end of the tube. Internal teeth will hold it in place. Do not apply excessive force or twist the fuse. Serious damage or premature ignition could result.

To activate the pull-wire lighter, hold the tube securely and pull the handle firmly. This will produce an internal flame jet and ignite the fuse.

Safety Matches

Safety matches only ignite when struck against a specially prepared surface. They are permitted if only one fuse assembly is to be ignited in one operation. Safety matches produce a flame of medium intensity and short duration.

Determining Ignition

A blaster must be able to determine that a fuse is actually burning. After lighting the fuse, take about five seconds to check for the following:

• Visible grey smoke
• The acrid smell of smoke
• Discoloured fuse
• Fuse droop (i.e., the fuse becomes very flexible and limp)
• Ignition spit (sparks that shoot out the end of a fuse)

As the fuse burns, the outer covering darkens. But the powder core in the fuse burns ahead of the discoloration.

Everyone but the blaster of record should leave the area before the safety fuse is lit. As soon as a blaster knows or suspects that a fuse is burning, they must leave the area and go to a safe place before the first charge detonates.

Hazards and Precautions

The main causes of premature detonation are mishandling and exposure to heat. Impact or shock can compress the explosive charge in the detonator to the extent that it will detonate. Temperatures in excess of 66°C (150°F) can ignite the sensitive detonator. Accidental detonation may also result from improper ignition techniques.

Do not handle detonators roughly, tamper with them, or expose them to heat sources. The human body can generate temperatures that will affect the sensitive composition of a detonator. For this reason, do not carry them in clothing, and avoid holding them in a closed hand.

Note: With the exception of avalanche control, safety fuse assemblies should be used as a last resort. There are much safer options available (e.g., shock tube assemblies used with remote firing systems).

Take care when handling fuse-lighting devices. Mishandling could cause them to ignite prematurely or make them less efficient. When lighting fuses, keep the device away from detonators and heat-sensitive explosives.

In some cases, blasters did not realize a fuse was burning, or they waited too long to leave the blast site after lighting the first fuse. Some of these incidents have resulted in serious or fatal injuries. When more than one safety fuse assembly is ignited, the blast site must be evacuated well in advance of the expected detonation of the first charge (i.e., at least two minutes before).

Returning to the blast site too soon has also resulted in many incidents. If a misfire is known or suspected to have occurred, wait the full 30 minutes (or the manufacturer's recommended wait time) after the last charge was expected to detonate.

Mislight

When a blaster cannot confirm that a safety fuse assembly has been lit, or if an assembly is in an unknown state, the blaster should assume that it is lit. Re-lighting a safety fuse assembly is prohibited. The blaster and all workers should exit the danger area. If the detonator does not initiate the explosives, the blaster can then assume there is a misfire.