Chapter Two - Initiation and Priming

Initiation Systems

To Initiate An Explosive

A considerable amount of energy is required to initiate a high explosive such as dynamite or cap-sensitive slurry. In blasting, high explosives are initiated by a detonator, which is a capsule containing a series of relatively sensitive explosives that can be readily initiated by an outside energy source. Blasting agents which are the most common products used as the main column charge in the blasthole, are even less sensitive to initiation than high explosives. To assure dependable initiation of these products, the initiator is usually placed into a container of high explosives which in turn is placed into the column of blasting agents.

Three Parts of an Initiation System

An initiation system consists of three basic parts:

1. An initial energy source;
2. An energy distribution network that conveys energy into the individual blastholes; and
3. An in-the-hole component that uses energy from the distribution network to initiate a cap-sensitive explosive.

Types of Initial Energy Source

The initial energy source may be electrical, such as a generator or condenser-discharge blasting machine or a powerline used to energize an electric blasting cap, or heat source such as a spark generator or a match. The energy conveyed to and into the individual blastholes may be electricity, a burning fuse, a high-energy explosive detonation, or a low-energy dust or gas detonation.

Distribution Networks

Distribution networks or "business end" of the initiation system. This detonator, when inserted into a cap-sensitive explosive and activated, will initiate the detonation of the explosive column. Commercial detonators vary in strength from No. 4 to No. 12. Although No. 12 detonators are the most common, there is a trend toward higher strength detonators, particularly when blasting with cap-sensitive products which are less sensitive than dynamites.

Primer

The primer is the unit of a cap-sensitive explosive containing the detonator. Where the main blasthole charge is a high explosive, the detonator may be inserted into the column at any point. However, most of the products (blasting agents) used are insensitive to a No. 12 detonator. To detonate these products, the detonator must be inserted into a unit of cap-sensitive explosive, which in turn is inserted into the blasting agent column at the desired point of initiation.

The discussions of the various initiation and priming systems will concentrate primarily on common practice. With each system there are optional techniques and "tricks of the trade" that increase system versatility. It is a good idea to confer with the manufacturer before finalizing your initiation and priming program, so you fully understand how to best use a specific system.

Delay Series

Instantaneous Detonator

Figure 11 shows an instantaneous detonator. In this type of detonator, the base charge detonates within a millisecond or two after the external energy enters the detonator. However, in most types of blasting, time intervals are required between the detonation of various blastholes or even between decks within a blasthole. To accomplish this, (figure 12) a delay element containing a burning powder is placed immediately before the priming charge in the detonator. Figure 14 and 15 shows a delay detonator.

Delay Detonators

There are two basic delay series used in metal-nonmetal mining; slow or tunnel delays, and fast or millisecond delays. For all commercial delay detonators, the delay time is determined by the length and burning rate of the delay powder column. As a result, slow delay caps may be quite long in dimension whereas lower period millisecond delays are shorter. Although the timing of delay detonators is sufficiently accurate for most blasting needs, these delays are not precise, as indicated by recent research. Recently, however, manufacturers' tolerances for some delay caps have been tightened. It is important to use the manufacturer's recommended current level to initiate electric blasting caps. Current levels above or below the recommended firing level can further increase the scatter in delay cap firing times and can also cause hang fires due to arcing and rupture of the cap shell. Extremely high current can speed up delay firing times. Near the minimum firing current, delays can become extremely erratic.

Slow Delays

Slow delays are useful underground under tight shooting conditions where it is essential that the burden on one hole moves before a subsequent hole fires. This situation may occur in tunnels, shafts, underground metal-nonmetal mines, and in trenching. Slow delays are available with all initiation systems except surface detonating cord and delay cast primers. Delay intervals are typically 0.5 to 1 second.

Millisecond Delays

Millisecond delays are the most commonly used delays and are useful wherever the tight conditions previously mentioned are not present. Millisecond delays provide improved fragmentation, controlled throw, and reduced ground vibration and airblast, as compared with simultaneous firing. They are available with all initiation systems. In millisecond detonators, delay intervals are 25 to 50 milliseconds in the lower periods and are longer in the higher periods. In detonating cord delay connectors, the delay may be as short as 5 milliseconds.

Iron Leg Wires

Iron leg wires are available optionally with ordinary electric detonators and are used primarily to facilitate magnetic removal of the wires from the muckpile, such as in trona and salt mines.

Electric Initiation

Initiation Mechanism of an Electric Blasting Cap

Electric initiation has been used for many years in both surface and underground blasting. An electric blasting cap consists of two insulated leg wires that pass through a waterproof seal and into a metal capsule containing a series of explosive powders (Fig 14). Leg wires of various lengths are available to accommodate various borehole depths. Inside the capsule the two leg wires are connected by a fine filament bridge wire embedded in a highly heat-sensitive explosive. Upon application of electric current the bridge wire heats sufficiently to initiate the ignition mixture, which in turn initiates a series of less sensitive, more powerful explosives.

Electric Blasting Cap Leg Wires

Most electric blasting caps have copper leg wires. Iron leg wires are available for use where magnetic separation is used to remove the leg wires at the preparation plant. Atlas Powder Co. has prepared an excellent handbook that describes electric blasting procedures in detail (2).

Saf-T-Det and Magnadet

The Saf-T-Det and Magnadet electric blasting caps are two recent developments. The Saf-T-Det resembles a standard electric blasting cap but has no base charge. A length of 100 grain or less detonating cord is inserted into a well to act as a base charge just before the primer is made up. The device is similar to an electric blasting cap in regard to required firing currents and extraneous electricity hazards. The Saf-T-Det is manufactured in India and is not available in the United States at this time.

Magnadet

The Magnadet is also similar to a standard electric blasting cap, except that the end of each cap lead contains a plastic-covered ferrite toroidal ring. A special blasting machine is used to fire these detonators. The manufacturer, ICI of Scotland, claims ease of hookup and protection against extraneous electricity as advantages to this system.

Electric Blasting Circuits

In order to fire electric blasting caps, the caps must be connected into circuits and energized by a power source. There are three types of electric blasting circuits. In order of preference they are series, parallel series, and parallel. In series circuits all the caps are connected consecutively so that the current from the powder source has only one path to follow. The series circuit is recommended because of its simplicity. Also, all the caps receive the same amount of current.

To splice two small wires, the wires are looped and twisted together. To connect a small wire to a large wire, the small wire is wrapped around the large wire.

Series Circuit

The electrical resistance of a series of caps is equal to the sum of the resistances of the individual caps. For most blasting machines, it is recommended that the number of caps in a single series be limited to 40 to 50, depending on the leg wire length. Longer leg wires require smaller series. The limit for most small twist-type blasting machines is 10 caps with 30-foot leg wires.

Connecting Wire

Many blasters minimize excess wire between holes to keep the blast site from being cluttered. The ends of the cap series are extended to a point of safety by connecting wire, which is usually 20 gauge, but should be heavier where circuit resistance is a problem or when using parallel circuits. This connecting wire is considered expendable and should be used only once. The connecting wire is in turn connected to the firing line, which in turn is connected to the power source.

The Firing Line

The firing line contains two single conducting wires of 12 gauge or heavier and is reused from shot to shot. It may be on a reel mechanism for portability, or may be installed along the wall of a tunnel in an underground operation. Installed firing lines should not be grounded, should be made of copper rather than aluminum, and should have a 15-foot lightning gap near the power source to guard against premature blasts. The firing line should be inspected frequently and replaced when necessary.

Parallel Series Circuit

When the number of caps in a round exceeds 40 to 50, the parallel series circuit is recommended. In a parallel series circuit, the caps are divided into a number of individual series. Each series should contain the same number of caps or the same resistance to ensure even current distribution. The leg wires of the caps in each series are connected consecutively. Next, two bus wires are placed in such a position that each end of each series can be connected as shown in figures 16 and 17. The bus wire is usually about 14 gauge or heavier and may be either bare or insulated. Where bare wires are used, care must be exercised to prevent excessive current leakage to the ground. It is recommended that the insulation be cut away at point of connection with the blasting cap series. To ensure equal current distribution to each series, one bus wire should be reversed. With parallel series circuits, 14 gauge or heavier gauge connecting wire is used to reduce the total circuit resistance.

Parallel Circuit

The third type of blasting circuit is the straight parallel circuit. The straight parallel circuit is less desirable to use than the series or series parallel circuits for two reasons. First, its nature is such that it cannot be checked. Broken leg wires or faulty connections cannot be detected once the circuit has been hooked up. Second, because the available current is divided by the number of caps in the circuit, powerline firing must often be used to provide adequate current for large parallel circuits. The problems associated with powerline firing will be discussed later.

Parallel circuits are not appropriate for surface blasting but they are used to some extent for tunnel blasting. Parallel circuits are similar to parallel series except that instead of each end of a series circuit being connected to alternate bus wires, each leg wire of each cap is connected directly to the bus wires, as shown in Figure 18. In underground blasts using parallel circuits, bare bus wire is usually strung on wooden pegs driven into the face to avoid grounding. As with parallel series circuits, the bus wires are reversed as shown in Figure 18.

In a parallel circuit the lead wire (firing line) represents the largest resistance in the circuit. Keeping the lead wire as short as possible, consistent with safety, is the key to firing large numbers of caps with parallel circuits. Doubling the length of the lead wire reduces the number of caps that can be fired by almost half. Heavy (12 to 14 gauge) bus wires are used to reduce the resistance. A 14 gauge connecting wire, rather than a lighter gauge, is recommended to reduce the circuit resistance. Only the very basics of circuit calculations are covered here. For more detail on circuit calculations or other of the many intricacies of electrical blasting, the reader should refer to a detailed electric blasting handbook such as reference 2.

Circuit Resistance Calculations

Resistance of a Series Circuit

The resistance of a series circuit is the easiest to calculate. First, the resistance of a single cap, as specified by the manufacturer, is multiplied by the number of caps to determine the resistance of the cap circuit. To this is added the resistance of the connecting wire and that of the firing line to determine the resistance of the total circuit. Since the firing line contains two wires, there will be 2 feet of wire for every foot of firing line. Where bus wire is used (parallel or parallel series circuits) the resistance of one-half of the length of the bus wire is added to find the total circuit resistance.

When firing from a powerline, the voltage of the line divided by the resistance of the circuit will give the current flow. In a single series circuit, all of this current flows through each cap. The minimum recommended firing current per cap is 1.5 amperes of direct current (amp dc) or 2.0 amperes of alternating current (amp ac). The current output of condenser (capacitor) discharge blasting machines may vary with the circuit resistance, but not linearly. Consult manufacturer's specifications to determine the amperage of a specific machine across a given resistance. For a generator blasting machine, the manufacturer rates the machine in terms of the number of caps it can fire.

Resistance of a Parallel Series Circuit

The resistance calculation for a parallel series circuit is as follows. First the resistance of each cap series is calculated as previously described. Remember, in a good parallel series circuit the resistance of a single series is then divided by the number of series to find the resistance of the cap circuit. To this are added the resistance of half the length of bus wire used, the resistance of the connecting wire, and the resistance of the firing line to obtain total circuit resistance. The locations of the bus wire, connecting wire, and firing line. The current flow is determined either by dividing the powerline voltage by the circuit resistance or, in the case of a condenser discharge machine, by checking the manufacturer's specifications. The current flow is divided by the number of series to determine the current flow through each series.

Circuit Calculation of a Straight Parallel Circuit

For straight parallel circuits, the resistance of the cap circuit is equal to the resistance of a single cap divided by the number of caps. This is usually a very small value. For 20 short leg wire caps, the resistance is less than 0.1 ohm. The resistance of the connecting wire, the firing line, and one-half the bus wire are added to find the total resistance. The current flow is determined in the same manner as with series and parallel series circuits. The current flow is divided by the number of caps to determine the current flow through each cap.

Power Sources for Electric Blasting

Manufacturer Recommended Equipment

To energize electric blasting circuits use only manufacturer recommended equipment. Electric blasting by generator-type blasting machines, condenser-discharge blasting machines, and powerlines.

Storage and dry cell batteries are definitely not recommended for blasting because they cannot be depended on for a consistent output.

Generator blasting machines may be of the rack-bar (push down) or the key-twist type. The capacity of rack-bar machines ranges from 20 to 50 caps in a single series, while key-twist machines will normally initiate 10 or 20 caps in a single series. The actual current put out of these machines depends on the condition of the machine and the effort exerted by the shot firer. When using a rack-bar machine, the terminals should be on the opposite side of the machine from the operator. Both the rack-bar and the twist machines should be operated vigorously to the end of the stroke because the current flows only at the end of the stroke.

The condition of a generator blasting machine deteriorates with time, therefore, it is important that the machine be periodically checked with a rheostat designed for that purpose. The directions for testing with a rheostat itself. Although the generator machine has been a dependable blasting tool, its limited capacity and variable output have caused it to be replaced, for most applications, by the condenser (capacitor) discharge machine.

Capacitor Discharge (CD) Machine

As the name implies, the capacitor discharge (CD) machine employs dry cell batteries to charge one or more capacitors. The energy stored in the capacitor is then discharged into the blasting circuit. CD machines are available in a variety of designs and capacities, with some capable of firing over 1,000 caps in a parallel series circuit.

Operation of Capacitor Discharge Machines (General)

All CD machines operate in basically the same manner. One button or switch is activated to charge the capacitors and a second button or switch is activated to fire the blast. An indicator light or dial indicates when the capacitor is charged to its rated capacity. Ideally, the overall condition of a CD blasting machine should be checked with an oscilloscope. However, the current output can be checked by combining a rheostat and a resistor (2) or by using a capacitor discharge checking machine (7). The powder supplier should be consulted as to the availability of equipment for checking capacitor discharge machines.

Sequential Blasting Machine

A sequential blasting machine is a unit containing 10 capacitor discharge machines that will fire up to 10 separate circuits with a preselected time interval between the individual circuits. When used in conjunction with millisecond delay electric blasting caps, the sequential machine provides a very large number of separate delay intervals (3). This can be useful in improving fragmentation and in controlling ground vibrations and airblasts. Because blast pattern design and hookup can be quite complex, the sequential blasting machine should be used only by well-trained persons or under the guidance of a consultant or a powder company representative. A poorly planned sequential timing pattern will result in poor fragmentation and excessive overbreak, flyrock, ground vibrations, and noise.

Powerline Blasting

The third alternative for energizing electric blasting circuits is the powerline. Powerline blasting is often done with parallel circuits where the capacity of available blasting machines is inadequate. When firing off a powerline, the line should be dedicated to blasting alone, should contain at least a 15 feet lightning gap, and should be visually checked on a regular basis for damage and for resistance. Powerline shooting should not be done unless precautions are taken to prevent arcing. Arcing can result in erratic timing, a hangfire, or a misfire.

Circuit Testing

It is important to check the resistance of the blasting circuit to make sure that there are no broken wires or short circuits and that the resistance of the circuit is compatible with the capacity of the power source. There are two types of blasting circuit testers: a blasting multimeter, and a blasting galvanometer. The blasting galvanometer is used only to check the circuit resistance, whereas a blasting multimeter can be used to check resistance, ac and dc voltage, stray currents, and current leakage (2). Only a meter specifically designed for blasting should be used to check blasting circuits. The output of such meters is limited to 0.05 amp, which will not detonate an electric blasting cap, by using a silver chloride battery and/or internal current-limited circuitry.

Other equipment such as a "throw-away" go-no-go device for testing circuits and a continuous ground current monitor is available. The explosive supplier should be consulted to determine what specific electrical blasting accessory equipment is available and what equipment is needed for a particular job.

It is generally recommended that each component of the circuit be checked as hook up progresses. After each component is tested, it should be shunted. Each cap should be checked after the hole has been loaded and before stemming. In this way, a new primer can be inserted if a broken leg wire is detected. A total deflection of the circuit tester needle (no resistance) indicates a short circuit. Zero deflection of the needle (infinite resistance) indicates a broken wire. Either condition will prevent a blasting cap, and possibly the whole circuit, from firing.

Checking Resistance of the Circuit

Before testing the blasting circuit, its resistance should be calculated. After the caps have been connected into a circuit the resistance of the circuit is checked and compared with the calculated value. A zero deflection at this time indicates a broken wire or a missed connection and an excessive deflection indicates a short circuit between two wires.

Checking Connecting Wires

After the circuit resistance has been checked and compared, the connecting wire is then added and the circuit is checked again. If a parallel series circuit is used, the change in resistance should be checked as each series is added to the bus wire. In a straight parallel circuit a break in the bus wire can sometimes be detected. However, a broken or a shorted cap wire cannot be detected in a straight parallel circuit because it will not significantly affect the resistance.

A final check of the circuit is made at the shot firer's location after the firing line has been connected. If a problem is found in a completed circuit, the circuit should be broken up into separate parts and checked to isolate the problem. The firing line should be checked for a break or a short after each blast, or at the end of each shift, as a minimum.

To Test for a Break in the Firing Line

To check for a break in the firing line, the two wires at one end of the line are shunted and the other end is checked with a blasting meter. A large deflection indicates that the firing line is not broken; a zero deflection indicates a broken wire. To test for a short, the wires at one end of the lead line are separated and the other end is checked with the meter. A zero deflection should result. If there is a deflection, the lead line has a short circuit. Hazardous, and costly misfires can be avoided through proper use of the blasting galvanometer or blasting multimeter.

Current Leakage

Certain conditions such as damaged insulation, damp ground, a conductive ore body, water in a borehole, bare wires touching the ground, or bulk slurry in the borehole may cause current to leak from a charged circuit. Although this is not a common occurrence, it may be checked if unexplained misfires are occurring. To properly check for current leakage you should check with a consultant or an electric blasting handbook (2). Measures for combating current leakage include using fewer caps per circuit, using heavier gauge lead lines and connecting wires, keeping bare wire connections from touching the ground, and not slicing leg wires in the borehole or using a nonelectric initiation system.

Electrical Hazards

The principal hazard associated with electric blasting systems is lightning. Extraneous electricity in the form of stray currents, static electricity, radiofrequency energy, and high-voltage powerlines can also be a hazard. Electric blasting caps should not be used in the presence of stray currents of 0.05 amp or more. Stray currents usually come from heavy equipment or power systems in the area, and are often carried by metal conductors or high-voltage powerlines. Atlas Powder Co. outlines techniques for checking for stray currents. Instruments have recently been developed which continuously monitor ground currents and sound an alarm when an excess current is detected.

Static Electricity

Static electricity may be generated by pneumatic loading, particularly in a dry atmosphere, and by rubbing of a person's clothes. Most electric blasting caps are static resistant. When pneumatically loading blasting agents with pressure pots or venturi loaders, a semiconductive loading hose must be used, and the loading vessel should be grounded.

Electrical Storms

Electrical storms are a hazard regardless of the type of initiation system being used. Even underground mines are susceptible to lightning hazards. Upon the approach of an electrical storm, loading operations must cease and all personnel must retreat to a safe location. The powder manufacturer should be consulted on the availability of commercial storm warning devices. Some operators use static on an AM radio as a crude detector of approaching storms. Weather reports are also helpful.

Radio Frequency Energy Hazard

Broadcasting stations, mobile radio transmitters, and radar installations present the hazard of radio frequency energy. The IME has prepared charts giving transmission specifications and potentially hazardous distances.

Powerlines

High-voltage powerlines present the hazards of capacitive and inductive coupling, stray current and conduction of lightning. Atlas Powder Co. (2) details precautions to be taken when blasting near high-voltage powerlines. A specific hazard with powerlines is the danger of throwing part of the blasting wire onto the powerline. This shorts the powerline to the ground and has been responsible for several deaths. Care should be exercised in laying out the circuit so that the wires cannot be thrown on a powerline. Other alternatives are to weigh down the wires so they cannot be thrown or attach a charge that cuts the blasting wire.

Advantages of Electric Blasting

Electric blasting is a safe, dependable system when used properly under the proper conditions. Advantages of the system are its reasonably accurate delays, ease of circuit testing, control of blast initiation time, and lack of airblast or disruptive effect on the explosive charge. In addition to extraneous electricity, one should guard against kinks in the cap leg wires, which can cause broken wires, especially in deep holes. Different brands of caps may vary in electric properties, so only one brand per blast should be used. It is recommended that the blaster carry the key or handle to the power source while he or she is checking out the shot so the shot cannot be inadvertently fired.

Exploding Bridge Wire

A device called an exploding bridge wire is available for use where a single cap is used to initiate a nonelectric circuit. This device has the safety advantages of a lack of primary explosive in the cap and a high voltage required for firing. A special firing box is required for the system. The high power required and high cost make it unsuitable for use in multicap circuits.

Detonating Cord Initiation

Detonating cord initiation has been used for many years as an alternative to electric blasting where the operator prefers not to have an electric initiator in the blasthole. Detonating cord consists of a cord of high explosive, usually PETN, contained in a waterproof plastic sheath enclosed in a reinforcing cover of various combinations of textile, plastic, and waterproofing. Detonating cord is available with PETN core loadings ranging from 1 to 400 grains per foot.

Detonation and Detonation Velocity

All cords can be detonated with a blasting cap and have a detonation velocity of approximately 21,000 feet per second. Detonating cord is adaptable to most surface blasting situations. When used in a wet environment the ends of the cord should be protected from water. PETN will slowly absorb water and as a result will become insensitive to initiation by a blasting cap. Even when wet, however, detonating cord will propagate if initiated on a dry end. Understanding the function of a detonating cord initiation system requires a knowledge of the products available. The Ensign Bickford Co. has published a manual (8) that describes detonating cord products in detail. Technical data sheets are available from Austin Powder Co. and Apache Powder Co.

Trunklines

The most common strengths of detonating cord are from 25 to 60 grains per foot. These strengths are used for trunklines, which connect the individual blastholes into the pattern, and for downlines, which transmit the energy from the trunkline to the primer cartridge. The lower strength cord may be somewhat less dependably initiated by 25-grain cord or lighter cord. However, under normal conditions, the lighter core loads offer economy and their greater flexibility makes field procedures such as primer preparation and knot tying easier.

Cord Strengths Less Than 25 Grains Per Foot

Detonating cord strengths lower than 25 grains per foot are sometimes used. Fifteen- to twenty-grain products may be used for small-diameter holes, for secondary blasting, and in the shock tube system. A 4 grain per foot product is used as part of an assembly called a Primaline Primadet. A Primaline Primadet consists of a length of 4-grain cord crimped to a standard instantaneous or delay blasting cap. The cap is inserted into the primer and the 4-grain cord serves as a downline. Various cord lengths are available to suit specific borehole depths. These Primadets are primarily used in underground mines (such as salt) where Shock tubes would be a product contaminant. Du Pont's new Detaline System utilized a 2.4 grain cord.

Downline to Trunkline Connections

Recommended knots for detonating cord:

• Clove hitch - for downline to trunkline
• Double-wrap clove hitch - for stiff cord downline
• Square knot - for cord-to-cord connection

Downlines

Detonating cord strengths of 100 to 200 grains per foot are occasionally used where continuous column initiation of a blasting agent is desired. Cords with 200 to 400 grains of PETN per foot are occasionally used as a substitute for explosive cartridges in very sensitive or small, controlled blasting jobs. Controlled blasting is described in the "Blast Design" chapter.

Millisecond Delays

Millisecond delay surface connectors are used for delaying detonating cord blasts. To place a delay between two holes, the trunkline between the holes is cut and the ends are joined with a delay connector. One type of delay connector is a plastic assembly containing a delay element. At each end of the element is an opening into which a loop of the severed trunkline can be inserted. A tapered pin is used to lock the trunkline cord into place. A shock delay connector has also been developed for detonating cord blasting. This connector consists of two plastic blocks, each containing a delay initiator, connected by a short length of Shock tubing. Each end of the severed trunkline is wrapped around the notch in one of the plastic blocks. Both types of delay connectors are bidirectional.

Line Considerations: Slack and Tight

Slack Line:

• Potential cutoff where cord crosses itself

Tight Line:

• Acute angle at knot can cause cutoff
• Low-energy cords will not propagate through knots

Laying Out Downlines

After the primer has been lowered to its proper location in the blasthole, the detonating cord is cut from the spool. About 2 or 3 feet of cord should extend from the hole to allow for charge settlement and tying into the trunkline. The trunkline is laid out along the path of desired initiation progression. Trunkline-to-trunkline connections are usually made with a square knot. A tight knot, usually a clove hitch, a half hitch, or a double-wrap half hitch is used to connect the downline to the trunkline (Fig 23 and 24). Any excess cord from the downline should be cut off and disposed.

If Primadets or other in-hole delay assemblies are used, a plastic connector often serves as the connection to the trunkline. The cord lines should be slack, but not excessively so. If too much slack is present, the cord may cross itself and possibly cause a cutoff. If the lines are too tight and form an acute angle, the downline may be cut off without detonating and also, low-energy cords will not propagate through knots.

Axial Initiation when using Cap-Sensitive Explosives

Downlines of detonating cord can adversely affect the column charge of explosives in the blasthole. With cap-sensitive explosives, continuous, axial initiation will occur with any cord containing 18 or more grains of PETN per foot of cord. Lower strength cords may also cause axial initiation. Four-grain cord will not initiate most cap-sensitive explosives. With blasting agents, the effects of detonating cord is less predictable. The blasting agent may be desensitized or it may be marginally initiated.

Hagen has studied this problem. The effect depends on the cord strength, blasting agent sensitivity, blasthole diameter, and position of the cord within the blasthole. As a general rule:

The manufacturer should be consulted for recommendations on the use of detonating cord with explosive products. A low-energy initiation system called Detacord, developed by du Pont, is described later in this chapter.

Surface Delay Connectors

Surface delay connectors offer an unlimited number of delays. For instance, a row of 100 holes could be delayed individually by placing a delay between each hole and initiating the row from one end. Typical delay intervals for surface connectors are 5, 9, 17, 25, 35, 45, and 65 milliseconds. Since these connectors are normally used for surface blasting, half-second delay periods are not available.

Surface Delays and Cutoffs

Cutoffs may be a problem with surface delay connectors. When the powder column in one hole detonates, the connections between holes to be fired later may be broken by cratering or other movement of the rock mass. This may cause a subsequent hole misfire. To correct this situation, MSHA requires that each blasthole can be reached by two paths from the point of initiation of the blast round. The patterns can become somewhat complex and should be laid out and carefully checked on paper before attempting to lay them out in the field. Where possible the pattern should be designed so that the delay sequence in which the holes fire is the same no matter which path is taken from the point of blast initiation. The "Blast Design" chapter gives suggestions for selecting the actual delay intervals between blastholes.

Detonating Rate of Detonating Cord

A time of one millisecond is required for 21 feet of detonating cord to detonate. This time is not sufficient to significantly alter the delay intervals between holes.

Detonation Proceeds From the Top Down When Detonating Cord Downlines are Used

When detonating cord downlines are used, detonation of the cord in the blasthole proceeds from the top down. This presents two disadvantages:

1. The detonation of the cord may have an undesirable effect on the column charge as it proceeds downward and the stemming may be loosened.
2. If the hole is cut off by burden movement caused by detonation of an earlier hole (Fig 26) the powder in the lower portion of the hole will not detonate.

The use of an in-hole delay will correct both of these problems.

Primaline Primadet

Primaline Primadet is a delay cap attached to a length of 4-grain per foot detonating cord. It is available in both millisecond and long delay periods. The Primaline Primadet is connected to the trunkline with a plastic connector or a double-wrap half hitch. If the delay pattern of the blast is such that the number of available Primadet delay periods is adequate, an undelayed trunkline may be used. The delay period of the hole.

As an example, to attain the delay pattern in Figure 25, cap delay periods one through nine would be placed in the appropriate holes and trunklines would contain no delays. In this situation, the delay in every cap would be actuated before the first hole detonates. This would reduce the chance of a cutoff. The 4-grain Primaline Primadet is steadily being replaced by other nonelectric systems, described later in this chapter.

Combination Surface and In-Hole Delays

Another alternative to obtain the delay pattern and avoid the cutoff problem, would be to use the array of surface delays shown in figure 25 and an in-hole delay of an identical period in each blasthole.

Delay Cast Primers

For instance, if a 75 millisecond delay is used in each hole, and the trunkline delays are each 9 milliseconds, the delays in all of the holes except the two rear corner holes will be actuated before the first hole in the pattern fired, thus alleviating the cutoff problem. More complex patterns involving both surface and in-hole delays can be designed where desirable.

An alternative method of obtaining in-hole delays with detonating cord is to use delay cast primers. These are cast primers with built-in nonelectric millisecond delays. They can be strung on detonating cord downlines of 25-grain or more and are particularly useful in obtaining multiple delayed deck charges with a single downline. It bears repeating that delay patterns involving both surface and in-hole delays can be somewhat complex and should be carefully laid out on paper before attempting to install them in the field.

Primary Advantages of Detonating Cord Initiation Systems

Two of the primary advantages of detonating cord initiation systems are their ruggedness and their insensitivity. They function well under severe conditions such as in hard abrasive rock, in wet holes, and in deep, large-diameter holes. They are not susceptible to electrical hazards, although lightning is always a hazard while loading any blast.

Detonating cord is quite safe from accidental initiation until the initiating cap or delay connectors are attached. Available delay systems are extremely flexible and reasonably accurate.

Disadvantages

There are several disadvantages that may be significant in certain situations:

• Systems employing only surface connectors for delays present the potential for cutoffs
• Surface connectors also present the hazard of accidental initiation by impact
• Detonating cord trunklines create a considerable amount of irritating, high-frequency airblast (noise). In populated areas the cord should be covered with 15 to 20 inches of fine material or alternative, noiseless systems should be used
• Detonating cord downlines present the problem of charge or stemming disruption. As discussed previously, this depends on the borehole diameter, the type of explosive and core load of explosive in the cord
• The means of checking the system is visual examination

Safety Precaution

Vehicles should never pass over a loaded hole because the detonating cord lines may be damaged, resulting in a misfire or premature ignition. A premature ignition could result from driving over a surface delay connector.

Cap-and-Fuse Initiation

Cap and fuse is the oldest explosive initiation system; however, its use has dwindled steadily. Its primary remaining use is in small underground mines, although a few large mines still use it. Surface applications are limited to secondary blasting and the initiation of detonating cord rounds with a single cap.

Detonator Used in Cap and Fuse System

The detonator used in a cap-and-fuse system is a small capsule that is open at one end (figure 27). The capsule contains a base charge and a heat-sensitive primer charge of explosive. The powder charge in the cap is initiated by a core of flammable powder in the safety fuse. Safety fuse has an appearance somewhat similar to detonating cord except that the surface of safety fuse is smoother and waxy and the cord load is black. The core load of detonating cord is white.

Assembly of Cap and Fuse

To assemble a cap and fuse, the fuse is squarely cut and inserted into the cap until it abuts against the explosive charge in the cap. The cap is then crimped near the open end with an approved hand or bench crimper. The crimp should be no more than three-eighths of an inch from the open end of the cap.

Burning Rate of Safety Fuse

Currently, safety fuse burns at the nominal rate of 130 seconds per meter. Both dampness and high altitude will cause the fuse to burn more slowly. Fuse should be test burned periodically so that the blaster can keep a record of its actual burning rate. "Fast fuse" has been blamed for blasting accidents but this rarely if ever occurs. However, pressure on the fuse may increase its burning rate.

Lighting Mechanism for Cap and Fuse

One of the most important considerations in the use of cap and fuse systems is the use of a positive, approved lighting mechanism. Matches, cigarette lighters, carbide lamps, or other open flames are not approved for lighting fuse. MSHA regulations specify hotwire lighters, lead spitters, and Ignitacord as approved ignition systems. The safest, most controllable lighting method is Ignitacord. In South Africa, where safety fuse is most often sold as an assembly with an Ignitacord connector attached, the safety record with cap and fuse is much better than it is in the United States.

Ignitacord Connector

The Ignitacord connector fits over the end of the fuse and is crimped in a manner similar to the cap. The cap is attached to the fuse with a bench or hand crimper (never with the teeth or pliers). When crimping the cap, care should be taken not to crimp the zone containing the powder. Ignitacord connector is crimped to the other end of the fuse with a hand crimper. The Ignitacord is inserted into the notch near the end of the connector and the notch is closed using the thumb.

Water Deterioration

To guard against water deterioration, it is a good idea to cut off a short length of fuse immediately before making cap and fuse assemblies. In deciding the length of fuse to cut for each primer, the lighting procedure must be considered. Ignitacord is strongly recommended because of its safety record.

When Using Ignitacord or Quarry Cord

When Ignitacord (also called Quarry cord) is used, each fuse must have a burning time of at least 2 minutes. To make sure of this time, the fuse must be calibrated periodically by test burning. The Ignitacord is attached to the Ignitacord connectors in the desired order of firing. If all the fuses are cut accurately to the same length, the desired order of firing will be achieved.

Fuse Lighting

With Ignitacord, only one lighting is required before the shot firer returns to a safe location. Hotwire lighters and lead spitters require that each fuse be lit individually. The primary hazard of using safety fuse is the tendency of blasters to linger too long at the face, making sure that all the fuses are lit. To guard against this, MSHA regulations specify minimum burning times for fuses, depending on how many fuses one person lights. Keep in mind that two persons are required to be at the face while lighting fuse rounds.

To Avoid Misfires

To avoid misfires due to cutoff fuses, MSHA requires that the fuse in the last hole to be fired is burning within the hole before the first hole fires. Kinks and sharp bends in the fuse should be avoided because they may cut off the powder train and cause misfire. Many people who use cap and fuse do so because they feel that it is simpler to use than other initiation systems. However, proper use of cap and fuse requires as much or more skill and care as the other systems.

Timing of Holes

Cap and fuse is the only initiation system that offers neither flexibility nor accuracy in delays. Because of variations in lengths of fuse, burning rates, and time of lighting the individual holes will fire at erratic intervals at best, and out of sequence at worst. It is impossible to take advantage of the fragmentation benefits of millisecond delays when using the cap-and-fuse system.

Disadvantages of Cap and Fuse

There is no situation in which cap and fuse can be recommended as the best system to use. The system has two overpowering flaws--inaccurate timing and a poor safety record. The former results in generally poor fragmentation, a higher incidence of cutoffs, and less efficient pull of the round. All of these factors nullify the small cost advantage derived from the slightly lower cost of the system components.

The poor safety record of cap and fuse is an even more serious drawback. It is the only system that requires the blaster to activate the blast from a hazardous location and then retreat to safety. The use of Ignitacord rather than individual fuse lighting alleviates this problem. A Bureau study determined that the accident rate with cap and fuse is 17 times that of electric blasting, based on the number of units used. Too often, the person lighting the fuse is still at the face when the round detonates. The time lag between lighting the fuse and the detonation of the round makes security more difficult than with other systems.

Advantages of Cap and Fuse

Cap and Fuse does have the advantages of lack of airblast, no charge disruption, somewhat lower component costs, and protection from electrical hazards. If an operator decides to use the cap and fuse system, incorporation of Ignitacord for lighting multiple holes is strongly recommended because of its safety record.

Shock Tube Initiation

Shock Tube Initiation System

The hookup of the shock tube system is similar to the detonating cord system. The cap used in the system is higher strength than most electric blasting caps. Instead of leg wires, a single hollow tube protrudes from the cap. The shock tube has a thin coating of reactive material on its inside surface, which detonates at a speed of 6,000 feet per second (2000 meters per second). This is a very mild (19 grams per km or 1 ounce per mile) dust explosion that has insufficient energy to damage the tube. Several variations of the shock tube system can be used, depending on the blasting situation. In addition to the shock tube, tube-cap assembly, system accessories include noiseless trunklines with built-in delays, noiseless lead-in lines, and millisecond delay connectors for detonating cord trunklines.

Noiseless Lead-in Line Detonators (Nonelectric Instantaneous Detonator Assembly)

Features:

• Low strength PETN base charge
• Yellow shock tube shock tubing
• Plastic hinged white bunch block connector
• Comes in a multitude of different lengths

Initiation of Noiseless Lead-in Line (Bulk Shock Tube)

Lead-in line can be initiated with a nonelectric starter device or with a nonelectric or electric detonator.

Cutting and Splicing

EXEL Lead-in Line can be cut with a sharp knife or with an anvil-type pruning shear. Cuts should be made clean and at right angles taking care not to crush or collapse the cut-ends. The cut end on the spool is capped to prevent moisture penetrating the tube. Ends to be spliced are pushed into the splicing sleeve so that they butt together in the centre. Do not place tubing splices under tension.

Shock Tube and Detonating Cord Trunkline

One shock tube system for surface blasting uses a shock tube Primadet in each blasthole with 25 to 60 grain per foot detonating cord as a trunkline. The shock tube cap used in this system is factory crimped to 24 inch length of shock tube with a loop in the end. The caps are available in a variety of millisecond delay periods. A 7.5-grain detonating cord downline is attached to the loop with a double-wrapped square knot. The 7.5 grain detonating cord extends out of the borehole. This downline will not disrupt a column charge of blasting agent but it may initiate dynamite and other cap-sensitive products.

Connecting Shock Tube Trunkline Delay Units

The sleeve is attached to the initial hole to be fired and the shock tube is extended to the next hole in sequence. The downline from this next hole is connected to the plastic block containing the delay cap, using about 6 inches of cord at the end of the downline. Another delay unit is selected and the sleeve is attached to the downline below the plastic block. The shock tube is extended to the next hole, where the delay assembly is connected to the downline. The process is repeated until all the holes are connected.

Initiating the System

The downlines and the plastic blocks containing the delay cap should be covered to reduce noise and flying shrapnel. When the blaster is ready to fire the shot, an initiating device is attached to the downline of the first hole. This device may be an electric blasting cap, a cap and a fuse, or a shock tube noiseless lead-in. A noiseless lead-in is a length of shock tube, up to 610 meters long, crimped to an instantaneous blasting cap. The shock tube is initiated by using a shock tube starter, or other initiating device recommended by the manufacturer.

Underground Initiation

For underground blasting, millisecond and long period delay caps are available with 10 to 20 foot lengths of shock tube attached. Common practice is to use a trunkline of 18 or 25 grain detonating cord. The shock tube from each blasthole is attached to the trunkline with a J-connector. A simpler method is to use the bunching system, where up to 30 tubes are tied together parallel, in a bunch, and detonating cord is clove-hitched around the bunch. The manufacturer should be consulted to demonstrate the bunching technique and to determine the number of wraps of detonating cord required for a given size bunch.

Pneumatic Loading

When pneumatic loading is used, a plastic cap holder can be utilized to center the cap in the hole and to reduce movement of the cap. It is important that the shock tube is in a straight line, fairly taut, and that crossovers or contact with the trunkline are avoided. This is true in all shock tube blasting but particularly in heading rounds, where the blast face is more crowded. Just before blasting an electric cap or cap and fuse are connected to the trunkline.

Advantages of the Shock Tube System

The shock tube system has the advantages of:

• No airblast (when a noiseless trunkline is used)
• No charge disruption (when shock tube or a 7.5-grain cord in conjunction with a shock tube Primadet is used as a downline)
• No electrical hazards
• A versatile delay capability

Keep in mind that electrical storms are a hazard with any initiation system. System checkout is done through visual inspection. Shocktube assemblies should never be cut or trimmed, as that may cause the system to malfunction. The shock tube will initiate nothing but the cap crimped onto it. Because of the variations available and new concepts involved, specific crew training by a manufacturer's representative is highly recommended before using the shock tube system.

Priming

Primer

Essentially, the term primer is used to describe a unit of cap sensitive explosive that contains a detonator, while the term booster describes a unit of explosive that may or may not be cap-sensitive, and is used to intensify an explosive reaction but does not contain a detonator. Although a primer is generally thought of as containing a blasting cap, the primer cartridge may also be detonated by a downline of detonating cord.

Effect of Cord on Blasting Agents

The possible undesirable effect of the cord on blasting agents, described in the "Detonating Cord Initiation" section, must be considered. If the column charge is cap-sensitive, detonating cord will cause initiation to proceed from the top down. The manufacturer should be consulted to determine the minimum strength detonating cord that will reliably initiate a specific type of primer. Most cast primers require a detonating cord strength greater than 25 grains per foot for reliable initiation.

Primer Selection

The primer should have a higher detonation velocity than that of the column charge being primed. Some experts feel that priming efficiency continues to increase as the primer's detonation velocity increases.

Small Diameter Holes (3-inch and less)

In blastholes of 3-inch diameter and less, cartridged dynamites and cap-sensitive cartridged slurries are commonly used as primers. For maximum efficiency, the diameter of the cartridge explosive should be as near to the blasthole diameter as can be conveniently loaded.

Gelatin dynamites are preferred over granular types because of their higher density, velocity, and water resistance. Some granular dynamites may be desensitized when submitted to prolonged exposure to water or to the fuel oil in ANFO. Cast primers (fig 6) may be used if the borehole is large enough to accommodate them. Small units of explosive that fit directly over the shell of a blasting cap can be used for priming bulk blasting agents in small-diameter holes.

In some situations, where boreholes are dependably dry, a high-strength cap alone has been used to prime bulk-loaded ANFO in a small-diameter hole. However, it is strongly recommended that a small booster fitting directly over the shell of the cap be used rather than a high-strength blasting cap alone. The cap manufacturer should be consulted for a recommendation if you are in doubt.

Large Diameter Blastholes

In large diameter blastholes, cast primers are predominantly used, although some operators prefer to use cartridged high explosives. Ideally, the primer should fill the diameter of the blasthole as nearly as possible. However, primers are relatively expensive in comparison to the blasting agents used in larger boreholes, so economics are a factor in primer choice.

Transient Detonation Velocity

All blasting agents are subject to transient detonation velocities. That is, they may begin detonating at a relatively low velocity at the point of initiation with the velocity rapidly building up until the blasting agent reaches its stable velocity, called the steady state velocity. This buildup occurs within about three charge diameters. A low initial velocity probably causes some loss of energy at the primer location. Low initial velocities can result when the primer is too small or of inadequate strength, or when the blasting agent is poorly mixed or partially desensitized by water.

Large-Diameter Slurry Columns

In large-diameter slurry columns, a 1-pound cast primer or a cartridge of gelatin dynamite is often an adequate primer. In ANFO columns where conditions are dependably dry, a 1-pound primer is sometimes adequate. However, where dampness exists, or where low transient velocities are a particular concern, it is recommended that a 25 or 50 pound charge of high-energy slurry or aluminized ANFO be poured around the primer. This is called combination priming. Bureau of Mines research indicates that each type of product does a good job of raising the velocity in the transient zone. An added benefit of combination priming is the margin of safety in damp boreholes that may partially desensitize ANFO.

Delay Cast Primers

Cast primers have been developed which incorporate an internal millisecond delay. The cast primers and the delay devices are supplied separately, with directions for assembly. These delay primers are slipped onto a detonating cord downline and are especially useful in providing multiple delays in the blasthole on a single downline.

Primer Makeup

Proper care and technique in making primers is very important because this is the time in the blasting the sensitive initiator and the powerful explosive cartridge are first combined. Because of the additional hazard involved, primers should be made up as close to the blast site as practical and immediately before loading.

Outside Primer Makeup Facility

In large tunnel projects, it is generally agreed that an outside primer makeup facility is best, assuming that transportation from the facility to the working face is safeguarded. Extra primers should be dismantled before removal from blasting site. An adequate hole must be punched into the cartridge to ensure the detonator can be fully imbedded. Care must be taken to ensure that the detonator does not come out of the primer cartridge during the loading.

The primer cartridge should never be tamped or dropped down the borehole.

One or more cartridges or a few feet of ANFO should be placed above the primer cartridge before dropping or tamping begins.

For Small Diameter Holes

In small diameter holes, it is especially important that the end of the cap points in the direction of the main charge. It is also strongly recommended in small-diameter holes that the primer cartridge be the first cartridge placed into the blasthole. When priming small-diameter cartridges, check tips figures E & G.

Procedure for Safety Fuse

Some safety fuses will not stand the sharp bend required for end priming. In this case, a diagonal hole is punched all the way through the cartridge and a second diagonal hole is punched partially through. The cap and fuse is strung through the first hole, placed into the second hole, and pulled secure. Here again, taping of the fuse to the cartridge will ensure that the cap is not pulled out during loading.

Attaching Detonating Cord to a Small-Diameter Primer Cartridge

When attaching detonating cord directly to the small-diameter primer cartridge, the detonating cord is usually inserted into a deep axial hole in the end of the cartridge. The cord is then either taped to the cartridge, passed through a diagonal hole in the cartridge, or secured with a half hitch to ensure that the cord will not pull out.

Primer Location

General Consideration

Proper location of the primer is important from the standpoint of both safety and efficiency (1,6). When using cartridged products in small-diameter blastholes, the primer should be the first cartridge placed into the hole, with cap pointing toward the collar. This ensures maximum confinement and the most efficient use of the explosive's energy. Placing the primer in the bottom minimizes bootlegs and also protects against leaving undetonated explosives in the bottom of the hole if the cartridges become separated. The primer cartridge must not be cut, deformed, or tamped. If bulk products are being loaded, the primer may be raised slightly from the bottom of the hole.

Bench Blasting, Bulk Loaded Products, and Subdrilling

In bench blasting with a bulk loaded product, where subdrilling is used, the primer should be placed at toe level, rather than at the bottom of the hole, to reduce ground vibrations. If there is some compelling reason to place the primer at the collar of the hole the detonator should be pointed toward the bottom of the hole.

Large-Diameter Blastholes

In large-diameter blastholes, the location of the primer is a matter of choice, although bottom initiation is recommended to maximize confinement of the charge. To help reduce vibrations, the primer should be at toe level rather than in the bottom of the hole, where subdrilling is used.

Bottom-initiated holes tend to produce less flyrock and airblast than top-initiated holes, assuming that all other blast dimensions are equal. If pulling the toe is not a significant problem, some operators prefer to place the primer near the center of the charge. This gives the quickest total reaction of the explosive column and may yield improved fragmentation.

Top priming is seldom recommended except where the only fragmentation difficulty is a hard band of rock in the upper portion of the bench. A rule of thumb, when using a single primer in a large-diameter blasthole, is to place the primer in the zone of most difficult breakage. This will normally be the toe area.

Deck Charges

In many blasting situations, single-point priming may be adequate. However, there are some situations in which multiple primers in a single borehole may be needed. The first is where deck charges are used:

1. To reduce the powder factor in a blast while still maintaining satisfactory powder distribution
2. To break up boulder-prone cap rock in the stemming area of the blast
3. To reduce the charge weight per delay to reduce vibrations

In situation 3, each deck in the hole is on a different delay period. In 1 and 2 the decks within a single hole may be on the same or on different delays. In any case, each deck charge requires a separate primer. Some states, such as Pennsylvania, require at least two primers per blasthole.

Multiple Priming as a Safety Factor

The second reason for multiple priming is as a safety factor to ensure total column detonation. With modern explosives and blasting agents, once detonation has been established it will proceed efficiently through the entire powder column. However, an offset in the powder column (Fig 26) may occur before detonation and cause part of the column not to propagate. This is most likely to occur with very long, thin charges or where slip planes are present in the burden area. In these cases, two or more primers should be spaced throughout the powder column. Frequently, these primers will be on the same delay. Where single point priming is preferred, but one or more additional primers are needed to ensure total column propagation, the additional primers are not on a later delay period.

Detonation Sequence of Multiple Delayed Decks

With multiple delayed decks in a blasthole, detonation should proceed from the bottom up where a good free face exists. Where the shot is tight detonation from the top down will give some relief to the lower decks.

Axial Priming

Axial priming, which employs a central core of primer throughout an ANFO column, has been used successfully but appears to have no particular advantage over single point or multiple point priming. Axial priming is more expensive than conventional priming.