Chapter 18: Detonating Cord

Learning Objectives

Discuss typical uses of detonating cord and why shock tube assemblies are more commonly used.
Describe the components of detonating cord.
Describe how to safely handle and store detonating cord.
Explain what can cause accidental detonation.
Describe the process for cutting detonating cord.
Explain loading procedures, layout patterns, and hookup procedures.
Describe procedures and materials used to splice or connect detonating cords.
Describe initiation procedures for detonating cord.
List the safety procedures for working with detonating cord.

Overview

Detonating cord is an explosive product used to initiate other explosives directly or with a booster or primer. In applications such as pre-splitting or perimeter blasting, detonating cord is used as the main explosive charge. It can also be used with shock tube assemblies and surface delays to increase timing options.

Detonating cord was widely used in the past. However, shock tube assemblies are now more commonly used due to their timing options and lower noise potential. Today, detonating cord is typically used in certain types of pre-splitting or in remote mining operations where noise is not an issue.

Components

Detonating cord has a core of high-velocity explosive contained in a plastic sleeve wrapped in other materials.

The explosive core is usually made of PETN (pentaerythritol tetranitrate). When initiated by a high-strength detonator, detonating cord explodes with a velocity of about 7000 m (23,000 ft.) per second.

Detonating cord has several layers of covering materials. These typically include an inner braid of textile, a plastic sleeve, an exterior textile covering, and a wax finish. These materials protect the core from abrasion, moisture, and damaging substances.

These layers of covering materials are not necessarily destroyed during detonation. Instead, they may be ejected violently outwards. This can often cause cutoffs.

Types of Detonating Cord

There are several types of detonating cord. The strength of the explosive core load can vary between types, as can the quality of the outer covering and its flexibility. Thinner, more flexible detonating cords can be tied together and used in a wide variety of applications.

Explosive strength is usually expressed in terms of grams of PETN per metre (g/m) of cord, or grains of PETN per foot (gr./ft.).

Handling and Storing Detonating Cord

Detonating cord is relatively safe to handle and has good storage properties. However, it can detonate accidentally if it is struck by machinery, falling rock, or lightning. For this reason, detonating cord must be treated with respect.

Protect detonating cords and connectors from damage and abuse. Damaged cord could result in a misfire. Damage to a surface delay connector could result in accidental detonation or a misfire.

Causes of Damage

Detonating cord can be damaged by:

Scraping action on metal and rock surfaces
Contact with bulk loading hoses
Vehicles driving over it
Shovels used to place stemming
Coarse or jagged stemming material
Frozen chunks of stemming

A reel of detonating cord should not be thrown or allowed to come in contact with tools, rocks, or other sharp objects.

Storage Requirements

Detonating cords are kept or stored with other explosives. Surface delay connectors that contain sensitive "primary" explosives must be kept or stored with detonators, and not with detonating cord or other explosives.

Detonating cord reels must be kept in boxes when stored in magazines. Once the boxes are opened, reels must be marked and identified with the magazine number and the approximate amount of cord left.

Cutting Detonating Cord

Cut detonating cord with a knife or a cord cutter. Ensure the cutting edge is sharp and clean. A cord cutter must have a single cutting blade acting against a brass plate or a non-metallic surface. Do not cut detonating cord with scissors, plier-type cutters, or similar instruments.

After cutting detonating cord, the cord ends should be sealed with tape or a plastic sleeve. This keeps the explosive clean and dry. It also prevents the PETN (white powder) from spilling out of the cord.

All cutting tools should be cleaned to remove any PETN from the metal surfaces or moving parts. Contact between PETN and grit or metal may result in accidental detonation.

Loading, Layout, and Hookup

Loading Procedures

Detonating cord is commonly used on the surface to initiate multiple downhole detonators (e.g., shock tube assemblies) attached directly to the detonating cord. Using detonating cord to initiate primers in the blast hole is no longer a common practice.

If planning to use ANFO or emulsion in a hole containing detonating cord, consult the manufacturer's technical data sheets, as this may not be recommended. Detonating cord detonates significantly faster than most commercial explosives. This can cause problems such as misfires, dead pressing, and desensitized and unburnt explosives in the blast hole. Blasting agents must always be adequately primed as recommended by the manufacturer.

Layout Patterns

In the past, detonating cord was often used to initiate a blast pattern containing one or more rows of loaded holes. These patterns were initiated instantaneously or with delays.

Today, detonating cord is used primarily for the following:

Initiating downhole detonators
Initiating stem charges (small charges placed into stemming to help break cap rock, the harder top layer of rock)
Wall control using a single row (precision pre-splitting)

Depending on how it is used or its location in a blasting circuit, detonating cord is often referred to as the following:

Downline — a length of detonating cord that extends into a blast hole
Trunklines — the lengths of cord on the surface to which the downlines or downhole detonators connect

Hookup Procedure

Do not interconnect blast holes with detonating cord until:
- All holes are loaded.
- Immediately before the intended time of detonation.
That's because an accidental detonation will affect all interconnected charges. However, connecting charges within the hole at the time of loading is permitted.
Begin connections from the farthest part of the circuit and work toward the point of initiation.
Keep connections tight and secure, and avoid in-hole connections. If this is not possible, make a tight knot and secure the ends with electrician's tape.
Remove excess cord from the downlines after tying in, and destroy cut-off ends in a safe manner. This helps prevent cutting off the trunkline.
Keep all connections at right angles (90 degrees) to the path of detonation. An acute angle can cause a cutoff.
Ensure there are no loops, sharp angles, or kinks that direct the cord back toward the oncoming detonation path.
Ensure the circuit has no excessive slack and the cord is undamaged.
Keep the blast site clean so the detonating cord layout is readily visible.
Detonators used to initiate the main trunkline should not be connected until:
- All connections have been visually inspected.
- Everything is ready for the blast.

Connections

Detonating cords are spliced or connected together using knots, tape, or plastic connectors. A reel of detonating cord may contain a factory splice (an overlap connection secured by a string tie). This splice must not be used in a blasting circuit. When a splice is encountered, the string should be removed and a connection made using a standard knot.

Knots

Knotted connections are popular. The flexibility and wax surface of most detonating cords make knot-tying easy and reliable under most conditions and temperatures.

Common knots include:

The square or reef knot - Recommended for joining lengths of trunkline. Ends should be secured (with electrician's tape) or trimmed. This ensures that the ends do not lie across trunklines or downlines and cause cutoffs.
The double half hitch - A popular connection between downlines and trunklines. Pull the knot tight so the lines are in positive contact.
The clove hitch and double wrap half hitch - Used for connecting trunklines to downlines.

Taping

Two lengths of detonating cord may be connected by overlapping the ends by at least 10 cm (4 in.) and taping them together with electrician's tape.

Initiation Procedures

Detonating cord is designed to be initiated by a high-strength detonator. Follow the cord manufacturer's recommended initiation procedure.

Dual Initiation

Dual initiation is the use of two detonators at each initiation point. At least one manufacturer recommends this technique, and many blasters use it. The second detonator helps ensure a successful blast. Dual initiation should be used whenever the blast is critical and the charge is not easily recovered.

Attaching Detonators

In a dual-initiation set-up, detonators to initiate detonating cord should be attached as follows:

Bend about 20 cm (8 in.) of the initiating end of the detonating cord into a loop. (Be sure to bend the cord in the opposite direction to its natural curve from being on the spool.) Place the detonators in the loop with the base end (business end) pointing in the direction the shock wave will travel.
With the detonators held tightly in position, secure them in place with electrician's tape. Keep the base end of each detonator in contact with the detonating cord.
Leave about 6 mm (¼ in.) of the base end exposed. This allows checking for misfires without disturbing the connection.
If electric detonators are used, connect them in a single series circuit and thoroughly tape all connections.

Safety Procedures

Select the appropriate detonating cord for the job. The type of cord should match the blasting method and the types of explosives being used. If in doubt, consult the manufacturer or supplier. Do not use damaged detonating cord.

When using detonating cord, do the following:

Keep and store:
- Detonating cord with other explosives.
- Surface delay connectors with detonators.
Keep cord supply spools in the shipping case before and after use.
Handle detonating cord as carefully as other explosives.
Use a hand-held rod or a stand when removing cord from a reel.
Use a sharp, single-bladed device designed or approved for cutting detonating cord.
Cut the detonating cord downlines from the spool before loading the rest of the explosives into each hole. An accidental detonation within a hole could detonate the spool on the surface.
Cut the detonating cord trunklines from the spool before attaching downhole detonators.
After loading and cutting a downline from the spool, either stem the hole or secure the end of the downline to a wooden dowel or other anchor. This is an important step if there is any possibility of the downline slipping into the hole.
Avoid abrasion to the cord from hole collars or other sharp edges.
Connections between detonating cords should not occur within blast holes. These connections could be damaged when explosives or stemming is added.
Collect all scrap pieces of cord and destroy them as recommended by the manufacturer.
Do not attach detonators for initiating the blast to detonating cord until the blast area has been cleared and secured for the blast.
Attach the cord-initiating detonator at least 15 cm (6 in.) from the cut end of the detonating cord.
Use surface delay connectors designed for use with detonating cord.
Do not damage detonating cord prior to firing.

Blast Area Security