Appendix F: Electric Blasting Circuit Design, Calculations, and Hazard Evaluation

Electric detonators require the application of a proper electric firing current as prescribed by the manufacturer. Electric initiation systems offer the benefit of verification of circuit continuity and circuit parameters prior to the blast. Therefore, circuit calculations are very important for the blaster-in-charge to compute to field circuit measurements with an approved "blasters' " multimeter, blasting galvanometer, or blasting ohmmeter. If and when discrepancies are discovered, corrective actions can be taken to ensure the initiation system performs safely and according to the blasting plan.

Caution The three "blasters' " meter reading possibilities are (1) a lower reading would indicate some electric detonators are not connected in the series that planned, (2) a higher reading would indicate that there are more electric detonators in the series than planned, loose or dirty connections, or that wire insulation within the circuit has been damaged; and (3) no reading would indicate there is a break in the circuit (See overview: Troubleshooting Circuit Breaks).

ELECTRIC CIRCUIT PROPERTIES

The flow of electric current obeys two basic electrical laws. The first is Ohm's law, which relates (1) voltage, (2) current, and (3) resistance within the blasting circuit. The second is Kirchhoff's law, which states that the applied voltage (electrical force) is completely consumed within the circuit, as current passes through each stage of the circuit. Applications in the form of solved example problems of these laws are found in the appropriate sections in this appendix.

Electric blasting circuits are described based on whether there are one or multiple paths available for the current to flow. Single path systems are the simplest circuits. Multiple path circuits require more initial current, since the current divides among the paths according to the path resistance.

An understanding of both Ohm's and Kirchhoff's laws helps the blaster-in-charge's understanding of the problem-solving methods used to make for accurate safety calculations for electric circuits. Electric power and energy calculations are made using the power line initiation method described in the Power Line Blasting Initiation section in this appendix.

Caution The ends of all detonators and circuits must be shunted during the circuit hookup process and up to the time of final tie-in to the power source to prevent the introduction of extraneous current sources.

Ohm's Law

Ohm's law describes the mathematical relationship among voltage, current, and resistance. It is represented by equation F.1. The blaster-in-charge usually knows two of the three variables of this equation and needs to calculate the third. For this purpose Ohm's law may also be expressed as either equation F.2 or F.3.

$$V = I \times R$$

Equation F.1

Where:

• V = Applied voltage (volts)
• I = Current (amperes)
• R = Resistance (ohms)

$$I = \frac{V}{R}$$

Equation F.2

$$R = \frac{V}{I}$$

Equation F.3

Kirchhoff's Law

Kirchhoff's law states that the applied voltage is completely consumed as voltage "drops" across each resistance element of the circuit (e.g. detonator, connecting wire, bus wire, or lead line). Equation F.4 shows that the applied voltage (VA) is completely consumed by all of the voltage drops (I x R) in the circuit.

$$V_A = IR_1 + IR_2 - IR_3 - ... - IR_m = 0$$

Equation F.4

Where:

• $V_A$ = Applied voltage (volts)
• $I_1, I_2, I_3, ... I_m$ = Current through each circuit detonator or wire (amperes)
• $R_1, R_2, R_3, ... R_m$ = Resistance of each detonator or wire (ohms)

The algebraic sum of all currents flowing to any point in a circuit is equal to "zero" (as indicated in equation F.5). This fact is important when analyzing parallel or series-in-parallel circuits, where the applied current divides among circuit paths (branches or series). For this, the equation can be rewritten as equation F.6.

$$I_A + I_1 + I_2 + I_3 + ... - I_m = 0$$

Equation F.5

Where:

• $I_A$ = Applied current (amperes)
• $I_1, I_2, I_3, ... I_m$ = Current through each parallel circuit branch (amperes)

$$I_A = I_1 + I_2 + I_3 + ... - I_m$$

Equation F.6

ELECTRIC BLASTING CIRCUIT CALCULATIONS

Electric blasting circuits are designed and hooked up to (1) ensure proper current distribution and (2) prevent the introduction of extraneous currents. This appendix contains information and solved example problems to help the blaster-in-charge make necessary calculations when using electric blasting circuits. The example problems in this appendix follow a problem-solving methodology that break down these solutions into steps of basic calculations.

Caution It is imperative that circuit voltage, current, and resistance values are calculated in advance to ensure each electric detonator in the blasting circuit receives its minimum "firing" current as prescribed by the manufacturer.

This section describes circuit wiring hookup techniques and the method to calculate the Ohm's law variables.

Caution Tables in this chapter give values in U.S. units. Therefore blasters using the metric system will need to convert their factors into metric units.

To calculate the circuit resistance, the nominal resistances of each electric detonator and all wire types to be used must be known. Table F.1 illustrates a typical listing of nominal electric detonator resistance by legwire type (e.g. copper or iron) and length. Values in this table are used in the example problems in this appendix.

Caution Each electric detonator manufacturer publishes its own nominal resistances for electric detonators by wire type and legwire length.

The nominal resistance of an electric detonator is the resistance measured at the end of its legwires when using a "blasters' " multimeter, blasting ohmmeter, or blasting galvanometer. When making circuit calculations, always use the nominal resistance and maximum "firing" current recommendations of the manufacturer.

Table F.1 – Nominal Resistances Of Typical Electric Detonators

Table F.1 – Nominal resistances of typical electric detonators. (Source: ISEE Blasters' Handbook™, 17th Ed. table 16.1)

Wire Resistance Calculations

Additional wire is used to connect the ends of detonator circuits to the blasting machine or power source. Wire may be in the form of connecting wire, bus wire, or a lead line. Its resistance depends on the wire metal and diameter (gauge) and resistance. Table F.2 lists resistance values for various gauge copper wires used in most electric detonator blasting circuits.

Table F.2 – Copper Wire Resistance (at 68°F)

Table F.2 – Nominal resistance for copper wire (at 68°F). (Source: ISEE Blasters' Handbook™, 17th Ed. table 16.2)

The recommended method of splicing wires is described in chapter 13. Proper splicing techniques ensure the electrical properties of the circuit are preserved. Equation F.7 is used to calculate the total resistance of any length of wire.

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

Equation F.7

Where:

• $R_{total}$ = Total wire resistance (ohms)
• L = Length of wire used (feet)
• $R_{1,000}$ = Wire resistance/1,000 feet (ohms)

Caution When using duplex or two-conductor wire, the length measured for use is doubled to obtain the total wire length for resistance calculations.

EXAMPLE F.1

Calculate the total resistance of 300 meters of 16-gauge single conductor copper connecting wire in a blasting circuit.

Step 1 Convert the wire length in meters ($l_m$) to feet ($l_ft$) by multiplying by 3.2808 as follows:

$l_{ft} = l_m \times 3.2808$

$l_{ft} = 300 \times 3.2808$

$l_{ft} = 984$

Step 2 Use 16-gauge wire resistance (4.02 ohms) in table F.2 and calculate the total wire resistance using equation F.7.

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(984 \times 4.02)}{1,000}$$

$R_{total} = 3.96$

The total resistance is 3.96 ohms.

Series Circuit Calculations

The three basic electric blasting circuit configurations are the (1) series, (2) parallel, and (3) series-in-parallel. As the series-in-parallel name implies, it has the combined characteristics of the series and the parallel circuits. These circuit calculations are discussed next.

Series Circuit Configuration

A series circuit is the single path circuit illustrated in figure F.1 (straight series). The nature of the series circuit is that the applied current flows through each element of the circuit. Figures F.1 through F.5 show three examples of acceptable series circuit wiring hookups and table F.3 summarizes the specific recommendations and limitations for use of each. The total resistance of a series circuit is equal to the sum of element resistances (detonators, connecting wire, bus wire, and lead line) in the circuit as shown in equation F.7.

Table F.3 – Series Circuit Wiring Hookup Technique Limitations

$$R_{total} = R_1 + R_2 + R_3 + ... + R_m$$

Equation F.8

Where:

• $R_{total}$ = Total resistance (ohms)
• $R_1, ..., R_m$ = Resistance of individual elements in the circuit (ohms) (e.g. Detonators, lead line, connecting wire, bus wire)

When all electric detonators have the same resistance this equation can be rewritten for simplicity as equation F.9.

$$R_{total} = R \times m$$

Equation F.9

Where:

• $R_{total}$ = Total resistance series circuit (ohms)
• R = Resistance of an individual electric detonator (ohms)
• m = Total number of electric detonators of equal resistance

EXAMPLE F.2

Calculate the series circuit resistance that contains 25 each 40-foot copper wire electric delay detonators using a 600 foot 14-gauge copper lead line.

Step 1 Calculate the resistance of 600 feet of 14-gauge copper lead line using equation F.7, where 600 feet of lead line = 1,200 feet of wire.

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(1,200 \times 2.525)}{1,000}$$

$R_{total} = 3.03$

The lead line resistance is 3.03 ohms.

Step 2 Calculate the total resistance of the detonator circuit using table F.1 and equation F.9.

$R_{total} = R \times m$

$R_{total} = 2.06 \times 25$

$R_{total} = 51.5$

The total detonator resistance is 51.5 ohms. This is the resistance, lead line not included, measured on the "blasters' " multimeter, blasting galvanometer, or blasting ohmmeter, between the open ends of the circuit.

Step 3 Calculate the total series circuit resistance by adding the resistance of the circuit and the lead line as follows:

$R_{total} = R_{lead line} + R_{detonator circuit}$

$R_{total} = 3.03 + 51.5$

$R_{total} = 54.53$

The total series circuit resistance is 54.53 ohms. This is the resistance measured on the "blasters' " multimeter, blasting galvanometer, or blasting ohmmeter should then read approximately 54 ohms to 55 ohms.

Parallel Circuit Calculations

The parallel circuit is a circuit with two or more paths for the current to flow as illustrated in figure F.4. Parallel circuits generally use bus wires to connect individual electric detonators. The nature of the parallel circuit is to divide the total current among the circuit branches.

Resistance of a parallel circuit is calculated using equation F.10. Four parallel circuit hookups are illustrated in figure F.4 through F.7. Specific hookup recommendations and their limitations are summarized in table F.4. Recommended safety practices when using parallel circuits are summarized on this page.

Table F.4 – Parallel Circuit Wiring Hookup Technique Limitations

Table F.4 – Parallel circuit wiring hookup technique limitations.

The total resistance of a parallel circuit can be calculated using equation F.10. This equation is used when circuit branches have unequal resistances ("unbalanced" branches).

$$R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + ... + \frac{1}{R_m}}$$

Equation F.10

Where:

• $R_{total}$ = Total circuit resistance (ohms)
• $R_1, ... R_m$ = Resistance of individual electric detonators (ohms)

Equation F.10 can be simplified to equation F.11 if only two detonators are wired in parallel.

$$R_{total} = \frac{R_1 \times R_2}{R_1 + R_2}$$

Equation F.11

Where:

• $R_{total}$ = Total detonator circuit resistance (ohms)
• $R_1$ and $R_2$ = Resistance of individual electric detonators (ohms)

Equation F.10 can again be simplified again to equation F.12 when all detonator resistances are equal (branch resistances are balanced).

$$R_{total} = \frac{R_1}{m}$$

Equation F.12

Where:

• $R_{total}$ = Total detonator circuit resistance (ohms)
• $R_1$ = Resistance of a single detonator (ohms)
• m = Number of detonators with $R_1$ resistance

EXAMPLE F.3

Determine the total resistance of a parallel circuit containing 40 each 10-foot copper wire delay detonators and 50 feet of 12-gauge copper bus wire.

Step 1 Calculate the resistance of the bus wire (1.588 ohms/1,000 feet from table F.2) using equation F.7.

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(50 \times 1.588)}{1,000}$$

$R_{total} = 0.0794$

The bus wire resistance is 0.0794 ohms.

Step 2 Calculate the parallel circuit resistance (detonator resistance = 1.40 ohms from table F.1) using equation F.12.

$$R_{total} = \frac{R_1}{m}$$

$$R_{total} = \frac{1.40}{40}$$

$R_{total} = 0.035$

The parallel circuit resistance is 0.035 ohms.

Step 3 Calculate the total parallel circuit resistance by adding the resistance of the bus wire to the detonator parallel circuit resistance as follows:

$R_{total} = R_{bus wire} + R_{detonator circuit}$

$R_{total} = 0.0794 + 0.035$

$R_{total} = 0.114$

The total parallel circuit resistance is 0.114 ohms.

Caution The voltage, wire size, and number of detonators that can be initiated is greatly affected by the choice of parallel circuit wiring method.

Parallel circuits are rated by their ability to uniformly distribute the current.

Parallel circuit calculations can be complex. In spite of this, accurate calculations are necessary to ensure every detonator in the circuit receives its proper "firing" current for reliable initiation. This requires solving simultaneous equations, which can be accurately performed by a computer. In this appendix, such simultaneous calculations are made multiple times to verify circuit information. This information can be provided in a form (See figure F.8) presented here again from chapter 13.

Analysis of this information helps the blaster-in-charge determine the three essential circuit factors: (1) the most appropriate circuit configuration, (2) the lead and bus wire gauges, and (3) the type of blasting machine or power source most appropriate for the blasting operation. Many detonator manufacturers or blasting consultants can make the necessary calculations for the blaster from the information provided in the form.

A parallel circuit cannot be accurately tested with the "swing needle" "blasters' " multimeter, blasting galvanometer or blasting ohmmeter, when the total resistance of the circuit is too small. In the low range it will read close to "zero" ohms and will not indicate an accurate value. For these small measurements, a digital "blasters' " multimeter, blasting galvanometer, or blasting ohmmeter is required.

Recommended Safety Practices When Using Parallel Circuits

• Use a closed-loop reverse parallel circuit (See figure F.7).
• Use copper insulated lead lines large enough to carry the current.

Series-In-parallel Circuit Calculations

The series-in-parallel circuit is the most common type of circuit used in electric blasting. A series-in-parallel circuit is one where electric detonators are divided into two or more balanced groups, each connected together in series and the series then connected together in parallel (IME SLP 12, 2010). Therefore, the methods of calculating individual series and parallel circuits already discussed apply to this combination circuit.

A simple series-in-parallel circuit is created when single series is divided into two smaller series and then connected in parallel as shown in figure F.9. At the point where the two series are connected a "blasters' " multimeter, blasting galvanometer, or blasting ohmmeter reading should be taken to verify circuit continuity and resistance. The two power source ends of the lead must be shunted before connecting the lead line to the circuit.

When large numbers of detonators are involved, they can be divided into more than two series and connected as shown in figure F.10.

Use copper insulated lead lines large enough to carry the current.

Caution Always check with the manufacturer for minimum "firing" current requirements and recommendations related to the detonators used.

When using connecting wire in a series-in-parallel circuit, it is best to extend the ends of each individual series with connecting wire to the lead line, rather than extend the lead line to the series-to-parallel connection point. This adds some resistance to each series, but the added resistance will be small compared to the resistance of the lead line.

Bus wires are sometimes used to connect individual series in parallel. This method offers some advantages in wiring simplicity, but bus wires need to be kept offhand (See figure 16.6). The sure use of bus wire can result in an uneven current distribution, which can cause misfire in one or more series, usually those series located farthest from the power source. This condition should be confirmed by the calculations. The bus wire resistance to total detonator resistance relationship.

Bus Wire To Total Detonator Resistance Relationship

The maximum allowable resistance of the bus wire should not exceed the sum of all resistances of detonators in the blast divided by 1500. This is represented in the expression below. If the bus wire resistance is too high, the result may be shortened or a lower resistance bus wire must be used. This is represented in the following mathematical expression:

$$R_W < \frac{R_T}{1500}$$

Where:

• $R_W$ = Maximum allowable bus wire resistance (ohms)
• $R_T$ = Total of all detonator resistances in the blast (ohms)

This bus wire to detonator resistance relationship is represented in the following mathematical expression:

EXAMPLE F.4

Determine if a 300 foot 20-gauge copper bus wire should be used in a blasting circuit using 500 each 40-foot MS delay detonators.

Step 1 Calculate the total resistance of all the detonators in the blast as follows where from table F.1 the resistance of one detonator is 2.06 ohms:

$R_{total} = R \times m$

$R_{total} = 2.06 \times 500$

$R_{total} = 1,030$

The total resistance of all detonators in the blast is 1,030 ohms.

Step 2 Calculate the bus wire resistance using equation F.7.

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(300 \times 10.15)}{1,000}$$

$R_{total} = 3.045$

The bus wire resistance is 3.045 ohms.

Step 3 Calculate the maximum allowable bus wire resistance as follows:

$$R_W = \frac{R_T}{1500}$$

$$R_W = \frac{1,030}{1500}$$

$R_W = 0.687$

The maximum allowable bus wire resistance is 1.030 ohms.

Since the bus wire resistance is almost three times as large as the maximum allowable bus wire, the 20-gauge bus wire should not be used. Failures would be expected. If the diameter of the 300 foot bus wire were increased to 14-gauge, its resistance would be 0.76 ohms and the bus wire to detonator resistance relationship would be satisfied.

In a series-in-parallel circuit, it is "best practice" that each series be electrically balanced with each series reading the same number of ohms resistance (max 10% difference). Usually, an equal number of detonators in each series will produce a balanced series. Where connecting wire is used, its resistance must be added to resistance of each series in which it is used.

EXAMPLE F.5

Calculate the resistance of a series-in-parallel circuit for a blast using 300 each 50-foot copper wire MS delay detonators connected in 6 balanced series and a 700 foot 14-gauge copper wire lead line.

Step 1 Determine the number (N) of detonators in each balanced series as follows:

$$N_{detonators/series} = \frac{N_{total detonators}}{N_{series}}$$

$$N_{detonators/series} = \frac{300}{6}$$

$N_{detonators/series} = 50$

The number of detonators in each balanced series is 50.

Step 2 Calculate the total resistance of each balanced series using equation F.9 where the resistance of each detonator is 2.32 ohms from table F.1.

$R_{total} = R \times m$

$R_{total} = 2.32 \times 50$

$R_{total} = 116$

The resistance of each balanced series is 116 ohms.

Step 3 Calculate the resistance of the series-in-parallel circuit as circuits are added one by one using equation F.12. Table F.6 summarizes these calculations.

Table F.6 – Summary Of Series-In-Parallel Circuit Resistances For Example F.5

Divide the resistance of one balanced series by the number of balanced series as they are connected using Equation F.12: $R_{total} = \frac{R_1}{m}$

Where:

• $R_1$ = Series resistance (ohms)
• m = number of series connected

Table F.6 – Summary of series-in-parallel circuit resistances for example F.5.

It is evident from table F.5 that as the number of balanced series increases, the total circuit resistance decreases. It becomes very difficult to read the resistance on a "blasters' " multimeter, blasting galvanometer, or blasting ohmmeter with accuracy with a swing needle type meter as the resistance becomes very low. However, it is possible to see the meter move as each series is connected, and this should be clearly observed during hookup. With digital "blasters' " multimeters, blasting galvanometers, or blasting ohmmeters the change in resistance should be accurately indicated.

Step 4 Calculate the resistance of the lead line using equation F.7, where the resistance of 14-gauge copper wire from table F.2 is 2.525 ohms/1,000 feet. (A 700 foot two wire lead line has 1,400 feet of wire.)

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(1,400 \times 2.525)}{1,000}$$

$R_{total} = 3.535$

The resistance of the lead line is 3.535 ohms.

Step 5 Calculate the total resistance of the blasting circuit by adding the resistances of the circuit and lead line as follows:

$R_{total} = R_{lead} + R_{circuit} + R_{connecting}$

$R_{total} = 19.3 + 3.535$

$R_{total} = 22.835$

The total circuit resistance is 22.835 ohms. This should be the resistance that is read by the "blasters' " multimeter, blasting galvanometer, or blasting ohmmeter at the ends of the lead line.

CAPACITOR DISCHARGE BLASTING MACHINE INITIATION

The firing limits for the capacitor discharge (CD) blasting machine have been determined from experience and computer analysis to assist the blaster-in-charge in designing the electrical circuitry for blasting with electric detonators. These limits are shown graphically for the example machine "A" in figure F.11. The recommended procedures when using a CD blasting machine is summarized in table F.7. The graph in figure F.11 is based on a detonator resistance of 2.0 ohms. The figure may be used for instantaneous or delay detonators of any length. To evaluate CD blasting machine compatibility when using detonators with resistances other than 2.0 ohms, make the figure calculations listed in table F.8. These evaluations are discussed in the examples in this section.

Recommended Procedure When Using a Capacitor Discharge Blasting Machine

Table F.7 – Recommended procedure when using a capacitor discharge blasting machine "A".

CD Blasting Machine Compatibility Calculations

Table F.8 – CD blasting machine compatibility calculations.

For normal blasting conditions use a number of series which falls midway between the straight line and the curved line specifying the total blasting line resistance. The total number of 2.0-ohm detonators in the blast is shown in the horizontal axis of the graph in figure F.11 and the number of series to be used is shown on the vertical axis. The area below the Blasting machine compatibility when using detonator recommended firing range and should not be exceeded. The heavy curved lines represent recommended firing limits for the detonators used line in this section.

$$E_{equiv} = \frac{T_{total} \times R_{detonator}}{2}$$

Equation F.13

Where:

• $E_{equiv}$ = Equivalent number of 2.0 ohm detonators
• T = Total number of detonators in the blast
• R = Resistance of each detonator

EXAMPLE F.6

Determine if the CD blasting machine "A" in figure F.11 is adequate to initiate a blast of 50 each 16-foot copper wire MS delays, a 300-foot 14-gauge copper wire lead line, and 200 feet of 20-gauge connecting wire wired in a single series.

Step 1 Calculate the equivalent number of 2-ohm detonators using equation F.13, where the resistance of a 16-foot electric detonator from table F.1 is 1.65 ohms.

$$E_{equiv} = \frac{T_{total} \times R_{detonator}}{2}$$

$$E_{equiv} = \frac{50 \times 1.65}{2}$$

$E_{equiv} = 41.25$

The equivalent number of 2-ohm detonators is 41.

Step 2 Calculate the lead line resistance using equation F.7, where the resistance of 14-gauge copper from table F.2 is 2.525 ohms/1,000 feet. (The 300 foot 2 wire lead line has 600 feet of wire).

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(600 \times 2.525)}{1,000}$$

$R_{total} = 1.515$

The lead line resistance is 1.515 ohms.

Step 3 Calculate the connecting wire resistance using equation F.7, where the resistance of 20-gauge copper wire from table F.2 is 10.15 ohms/1,000 feet.

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(200 \times 10.15)}{1,000}$$

$R_{total} = 2.03$

The total resistance of the connecting wire is 2.03 ohms.

Step 4 Calculate total wire resistance as follows:

$R_{total} = R_{lead line} + R_{conn. wire}$

$R_{total} = 1.515 + 2.03$

$R_{total} = 3.545$

The total wire resistance is 3.545 ohms.

Step 5 Compare the number of equivalent 2-ohm detonators from step 1 to the allowable lead line resistance in figure F.11. Since it falls within the allowable lead line (total wire resistance) of 3.545 ohms from step 4 this blast can be initiated in a "single" series with blasting machine "A".

EXAMPLE F.7

Determine the number of series to hookup in parallel for a blast using 500 each 50-foot copper wire MS delay detonators and a 750 foot 14-gauge copper lead line using the example blasting machine in figure F.12.

Step 1 Calculate the equivalent number of 2.0-ohm detonators using equation F.13, where the detonator resistance from table F.1 is 2.0 ohms.

$$E_{equiv} = \frac{T_{total} \times R_{detonator}}{2}$$

$$E_{equiv} = \frac{500 \times 2.0}{2}$$

$E_{equiv} = 500$

The equivalent number of 2-ohm detonators is 500.

Step 2 Calculate the lead line resistance using equation F.7, where the resistance of 14-gauge copper wire from table F.2 is 2.525 ohms/1,000 feet. (The 750 foot 2 wire lead line has 1,500 feet of wire.)

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(1,500 \times 2.525)}{1,000}$$

$R_{total} = 3.79$

The lead line resistance is 3.79 ohms.

Step 3 Determine the capability of the blasting machine of figure F.11. Locate 500 on the horizontal axis of the graph. Then follow the 500 detonator line vertically into the area above the straight line until it intersects the 4.0 ohm total line resistance curve. From the bottom and top intercept points follow across the graph to determine the number of balanced series that will be within the energy limits of the blasting machine. As indicated, the acceptable limits are between 6 and 18 series. The optimum energy would be delivered by choosing a circuit arrangement approximately midway between the extreme limits of 6 to 18 series. In this example, 8 would be a good choice.

Step 4 Divide the total number of detonators (N) in the blast by the number of series to determine the number of detonators per series as follows:

$$N_{detonators/series} = \frac{N_{total}}{N_{series}}$$

$$N_{detonators/series} = \frac{500}{12}$$

The number of detonators/series is 41.6 or 42. Therefore, hookup 42 detonators in each series.

Step 5 Calculate the resistance of each series using equation F.9.

$R_{total} = R \times m$

$R_{total} = 1.65 \times 50$

$R_{total} = 82.5$

The series resistance is 97.4 ohms.

It is always desirable to electrically balance the series resistances as closely as possible. However, minor differences of one or two detonators per series will not affect the results of the blast.

Caution The difference in resistance among series should never exceed 10%. A variance of 10% cannot be tolerated if the circuit is near the limits of the blasting machine. For normal blasting it is customary to limit the number of copper legwire detonators to 50 detonators/series (100 detonators/series).

For blasting machine "A", this is readily accomplished when firing up to 800 detonators with a total line resistance of 3 ohms or less. For a greater number of detonators, a larger number of detonators/series is required.

EXAMPLE F.8

Determine the number of series to hookup in parallel for a blast using 120 each 16-foot iron wire MS delay detonators and a 500 foot 14-gauge copper lead line using blasting machine "A" in figure F.11.

Step 1 Calculate the equivalent number of 2.0 ohm detonators using equation F.13, where the detonator resistance from table F.1 is 4.98 ohms.

$$E_{equiv} = \frac{T_{total} \times R_{detonator}}{2}$$

$$E_{equiv} = \frac{120 \times 4.98}{2}$$

$E_{equiv} = 299$

The equivalent number of 2-ohm detonators is 299.

Step 2 Calculate the lead line resistance using equation F.7, where the 14-gauge copper wire resistance from table F.2 is 2.525 ohms/1,000 feet (a 500 foot lead line has 1,000 feet of wire.)

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(1,000 \times 2.525)}{1,000}$$

$R_{total} = 2.525$

The lead line resistance is 2.525 ohms.

Step 3 Determine the number of series from figure F.11. Since the equivalent number of detonators is 299, the graph indicates many choices for the number of series-in-parallel. One option is to limit the equivalent number of copper wire detonators to 50 detonators/series or 6 series in this example.

Step 4 Calculate the number of actual detonators/series as follows:

$$N_{det/series} = \frac{N_{total}}{N_{series}}$$

$$N_{det/series} = \frac{120}{6}$$

There are 20 detonators/series.

Step 5 Calculate the resistance of each series, using equation F.9.

$R_{total} = R \times m$

$R_{total} = 4.98 \times 20$

$R_{total} = 99.6$

The resistance of each series is 99.6 ohms.

From example F.8, it is obvious the equivalent number of 2.0-ohm detonators must be calculated even when the total number of detonators in the blast is relatively small. The high resistance of iron wire detonators becomes a major factor in the choice of correct arrangement. Use this number to find recommended number of series. For normal blasting conditions, use a number of series, which falls midway between the straight line and the curved line specifying the total blasting line resistance.

POWER LINE INITIATION

Installing and maintaining a safe, dependable power line blasting system requires close attention to details and should be supervised by an experienced electrician in conjunction with the mine safety department. A recommended power line initiation station setup is illustrated in figure F.12, and station construction recommendations are listed in table F.9. Hookup and testing procedures for power line initiation are listed in table F.10.

Table F.9 – Power Initiation Station Construction Recommendations

Table F.9 – Power station construction recommendations.

Table F.10 – Hookup and Testing Procedure For Power Line Blasting

Recommended Power Line Initiation Procedures

All unnecessary personnel and equipment should be removed from the blasting site before any loading is started and follow the procedures outlined in table F.11. After the blast the procedures in table F.12 should be followed.

Table F.11 – Circuit Problem-Solving Procedure For Power Line Initiation

Table F.11 – Circuit problem solving procedure for power line initiation.

Table F.12 – Recommended After The Blast Procedure For Power Line Blasting

Table F.12 – Recommended procedure after the blast for power line blasting.

Power Line Blasting Circuit Calculations

Power line blasting utilizes the same basic circuitry previously discussed. However due to the nature of the power source, additional calculations are made to determine the adequacy of both the (1) electrical power and (2) electrical energy capability of the power source. These calculations are illustrated in the examples in this section. Electrical power measured in watts is calculated using equation F.14.

$$P = I \times V$$

Equation F.14

Where:

• P = Power (watts)
• I = Current (amperes)
• V = Voltage (volts)

When the value of V in Ohm's law (equation F.1) is substituted into equation F.14, electrical power is calculated using equation F.15.

$$P = I^2 \times R$$

Equation F.15

Where:

• P = Power (watts)
• I = Current (amperes)
• R = Resistance (ohms)

Electrical energy is the measure of electrical power used over time and is calculated using equation F.16 provided for reference.

$$E = I \times V$$

Equation F.16

Where:

• E = Electrical energy (watt-seconds) or (joules)
• I = Current (amperes)
• V = Voltage (volts)

This equation can be represented alternately to calculate electrical energy as equation F.17 provided here for reference.

$$E = I^2 \times R \times t$$

Equation F.17

Where:

• E = Electrical energy (watt-seconds) or (joules)
• I = Current (amperes)
• R = Resistance (ohms)
• t = Time of application (seconds)

Low Series Circuit Calculations

Special precautions must be taken when power lines are used to initiate a series circuit containing only delay detonators. The applied voltage should be limited through suitable transformers to keep the calculated current delivered to a series of delays greater than 1.5 and less than 10 amperes. The possibility of arcing damage increases as the current exceeds 10 amperes per detonator.

For example, having a minimum of 22 ohms series when using a 220-volt power source and 44 ohms series with a 440-volt source can usually control this. As an additional precaution it is recommended that each series contains a "stem" period of 25 millisecond delay detonator. When the "stem" or "zero" or 25 millisecond delay detonator is initiated, it will break the circuit before arcing damage can occur. Arcing problems can be eliminated by using a capacitor discharge blasting machine because the energy stored on the capacitor is discharged so rapidly that arcing damage cannot occur.

EXAMPLE F.9

Calculate the voltage and power required for a single series circuit of 50 each 16-foot copper wire MS delay detonators with 200 feet of 20-gauge copper connecting wire and a 700 foot 14-gauge copper wire lead line.

Step 1 Calculate the circuit resistance using equation F.9, where the detonator resistance from table F.1 is 1.65 ohms.

$R_{total} = R \times m$

$R_{total} = 1.65 \times 50$

$R_{total} = 82.5$

The detonator circuit resistance is 82.5 ohms.

Step 2 Calculate the resistance of the lead line using equation F.7 where the resistance of 14-gauge copper wire from table F.2 is 2.525 ohms/1,000 feet (A 700 foot lead line has 1,400 feet of wire)

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(1,400 \times 2.525)}{1,000}$$

$R_{total} = 3.535$

The lead line resistance is 3.535 ohms.

Step 3 Calculate the resistance of 200 feet of 20-gauge connecting wire using equation F.7, where the resistance of 20-gauge copper wire is 10.15 ohms.

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(200 \times 10.15)}{1,000}$$

$R_{total} = 2.03$

The connecting wire resistance is 2.03 ohms

Step 4 Calculate the total resistance of the blasting circuit using equation F.8 as follows:

$R_{total} = R_1 + R_2 + R_3$

$R_{total} = R_{series} + R_{lead line} + R_{conn. wire}$

$R_{total} = 82.5 + 3.535 + 2.03$

$R_{total} = 88.06$

The total circuit resistance is 88.06 ohms.

Step 5 Calculate the voltage required using equation F.1 with minimum firing current of 1.5 amperes.

$V = I \times R$

$V = 1.5 \times 88.06$

$V = 132.09$

The required voltage is 132.09 volts.

Step 6 Calculate the power required using equation F.15.

$P = I^2 \times R$

$P = 1.5^2 \times 88.06$

$P = 198.135$

The power required is 198 watts.

Therefore, a 220-volt, 200-watt transformer or generator would be sufficient to initiate the blast. If a "mixed" series containing instantaneous and delay detonators were used, 2.0 amperes would be required for the circuit and the voltage and power requirements must be recalculated as follows in example F.10.

EXAMPLE F.10

Recalculate the voltage and power requirements for example F.9 where the current of 2.0 amperes is required for a circuit with "mixed" detonators.

Step 1 Calculate the voltage using equation F.1.

$V = I \times R$

$V = 2.0 \times 88.06$

$V = 176.1$

The required voltage is 176.1 volts. The voltage required is increased from 132.09 to 176.1 volts.

Step 2 Recalculate the power required using equation F.15.

$P = I^2 \times R$

$P = 2.0^2 \times 88.06$

$P = 352.24$

The power required is 352.24 watts.

It is evident from this example, that the increased current requirement for the "mixed" series circuit imposes a greater voltage requirement on the transformer or generating system.

Power line Series-In-Parallel Circuit Calculations

It is normal practice to limit the number of detonators per series to 50 or less when initiating these circuits from a power line. Under normal mining conditions, each heading will usually be connected in one series and each heading will have a different number of boreholes/round, resulting in a series-in-parallel circuit that is extremely unbalanced. It is a good practice to connect each series from the individual headings directly to the lead line at one central point as shown in figure F.13. This involves the use of additional connecting wire, but yields more dependable results by eliminating bus wire from the circuit.

EXAMPLE F.11

Calculate the voltage and power requirements for a series-in-parallel circuit containing 220 each 20-foot copper wire MS delay detonators wired in five series of 44 detonators/series with a 1,500 foot 14-gauge copper wire lead line.

Step 1 Calculate the resistance of each series using equation F.9, where the detonator resistance from table F.1 is 1.56 ohms.

$R_{total} = R \times m$

$R_{total} = 1.56 \times 44$

$R_{total} = 68.64$

The series resistance is 68.64 ohms.

Step 2 Calculate the total circuit resistance using equation F.12.

$$R_{total} = \frac{R_1}{m}$$

$$R_{total} = \frac{68.64}{5}$$

$R_{total} = 13.73$

The circuit resistance is 13.73 ohms.

Step 3 Calculate the lead line resistance using equation F.7, where the resistance of 14-gauge copper wire from table F.2 is 2.525 ohms/1,000 feet. (A 1,500 foot lead line has 3,000 feet of wire.)

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(3,000 \times 2.525)}{1,000}$$

$R_{total} = 7.575$

The lead line resistance is 7.575 ohms.

Step 4 Calculate the total blasting circuit resistance as follows:

$R_{total} = R_{circuit} + R_{lead line}$

$R_{total} = 13.73 + 7.575$

$R_{total} = 21.3$

The total circuit resistance is 21.3 ohms.

Step 5 Calculate the voltage required using equation F.1, where 10 amperes is the maximum current requirement (2 amperes/series × 5 series = 10 amperes).

$V = I \times R$

$V = 10 \times 21.3$

$V = 213$

The voltage required is 213 volts.

Step 6 Calculate the power required using equation F.15.

$P = I^2 \times R$

$P = 10^2 \times 21.3$

$P = 2,130$

The power required is 2,130 watts.

In this case, a 220-volt power line will not supply the necessary current, and a 440-volt power line would be necessary. If a 440-volt power line is used to initiate the series-in-parallel circuit using both instantaneous and delay detonators, the possibility of arcing should be investigated. In this case, the total circuit (a 79.6 ohm series circuit with 7.6 ohms of lead line = 87.2 ohms) receives only 5.04 amperes and arcing will not occur, because the current is less than 10 amperes as calculated using equation F.2 here:

$$I = \frac{V}{R}$$

$$I = \frac{440}{87.2}$$

$I = 5.04$

When series are not balanced, the current delivered to each series must be determined to ensure each detonator receives adequate "firing" current. Example F.13 illustrates this analysis. The calculations are repetitive to the extent of the number of individual series used.

EXAMPLE F.13

Calculate the voltage and power required for a circuit using 200 each 16-foot copper wire MS delay electric detonators. The circuit uses sufficient 20-gauge connecting wire to connect to an 800 foot 14-gauge lead line, and is to be initiated from a 220 volt DC power line used in a block's heading (5 series each). The detonators and connecting wire totals for this example are listed in table F.13.

Table F.13 – Summary Of Circuit Element Requirements For Example F.13

Table F.13 – Summary of circuit element requirements for example F.13.

Step 1 Calculate the resistance of each individual series. The resistance calculations are summarized in table F.14.

Table F.14 – Summary of Series Resistance Calculations For Example F.13

Table F.14 – Summary of series circuit resistance calculations for example F.13.

Step 2 Calculate the total resistance of the five series (unbalanced) using equation F.10.

$$R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \frac{1}{R_4} + \frac{1}{R_5}}$$

$$R_{total} = \frac{1}{\frac{1}{53.56} + \frac{1}{86.56} + \frac{1}{51.02} + \frac{1}{84.53} + \frac{1}{72.09}}$$

$$R_{total} = \frac{1}{0.01867 + 0.01155 + 0.01960 + 0.01183 + 0.01387}$$

$$R_{total} = \frac{1}{0.07552}$$

$R_{total} = 13.24$

The total circuit resistance is 13.24 ohms.

Step 3 Calculate the resistance of the lead line using equation F.7 where the resistance of 14-gauge copper from table F.2 wire and find 2.525. (an 800 foot lead line has 1,600 feet of wire).

$$R_{total} = \frac{(L \times R_{1,000})}{1,000}$$

$$R_{total} = \frac{(1,600 \times 2.525)}{1,000}$$

$R_{total} = 4.04$

The lead line resistance is 4.04 ohms.

Step 4 Calculate the total resistance of the blasting circuit as follows:

$R_{total} = R_{circuit} + R_{lead line}$

$R_{total} = 13.24 + 4.04$

$R_{total} = 17.28$

The total circuit resistance is 17.28 ohms.

Step 5 Calculate the current for the entire circuit when using a 220-volt DC power line using equation F.2

$$I = \frac{V}{R}$$

$$I = \frac{220}{17.28}$$

$I = 12.73$

The current is 12.73 amperes.

Step 6 Calculate the voltage loss (drop) in the lead line using equation F.1.

$V = I \times R$

$V = 12.73 \times 4.04$

$V = 51.43$

The voltage drop in the lead line is 51.43 volts.

Step 7 Calculate the voltage to each series by subtracting he voltage drop in the lead line from 220 volts as follows:

$V = 220 - V_{lead line}$

$V = 220 - 51.43$

$V = 168.57$

The voltage delivered to each series is 168.57 volts.

Step 8 Calculate the current available to each series using equation F.2. The results of this repeated calculation are summarized in table F.15.

Table F.15 – Summary Of Series Resistance Calculations For Example F.13

Table F.15 Summary of series resistance calculations for example F.13.

Step 9 Calculate the power required using equation F.15.

$P = I^2 \times R$

$P = 12.73^2 \times 17.28$

$P = 2,800$

The power required is 2,800 watts.

The minimum current requirement is 1.5 amperes per series. Therefore, this circuit would receive sufficient current. If this circuit contained both delay and instantaneous detonators, it would require 2.0 amperes/series. There could be failures in series 1 and 2 as they are receiving less than the 2.0 amperes required for reliable initiation of all detonators in the circuit.

Power Line Parallel Circuits

The parallel current fired from a power are used primarily in tunnel driving and shaft sinking. The advantage of the parallel circuit is that it can be "hooked up" by several blasting crewmembers, working in a confined area, with a minimum of confusion. As explained earlier in this chapter, in the parallel circuit one legwire from each detonator is connected to one copper bus wire and the other legwire of the detonator is connected to the other copper bus wire. Usually, one copper bus wire will be designated for the blue legwires and the other copper bus wire, for the yellow legwires. Remember that colors of legwires will vary with manufacturer. This allows rapid visual inspection of the circuit after all connections are completed.

Figure F.14 illustrates how many electric detonators are "hooked up" to the bus wires. Two colored legwires and this hookup lend itself to projects where several blasting crewmembers are involved in the circuit hookup. In this example, one copper bus wire can be designated for the yellow legwires and the other for the blue legwires.

Caution Legwire colors of various manufacturers may differ from the yellow and blue of this example.

Copper bus wires should be installed on stakes as shown in figure F.14 (See Power Line Hazards section of this appendix). It is extremely important to ensure that the stakes are dry, as water-soaked stakes may allow a conductive path between the bus wires. If necessary, electricians' tape should be wrapped around the stake at the contact points to avoid any current leakage between bus wires. The bus wires should be shunted (twisted together) during the loading operation, and special attention should be given to the removal of the shunt prior to firing.

Caution Only the closed-loop reverse parallel hookup should be considered for power line firing. This type of circuit hookup greatly reduces the (1) size (gauge) of wire required for the lead line, (2) size of transformer, and (3) total voltage requirements.

In addition to supplying sufficient energy to the detonators, it is necessary to interrupt the power supply to the circuit to prevent arcing damage in any of the detonators. Arcing damage can be prevented if the power supply to the circuit is broken in 25 milliseconds or less. This can be accomplished by wrapping one or both of the lead lines around a cartridge of explosive as shown in figure F.15. The cartridge primer must use a zero delay electric detonator. The cartridge will detonate and break one lead line and thus effectively interrupt the power to the circuit before any detonator can be damaged by arcing. It is important that the cartridge be firmly attached to the lead line and not the bus wire. Merely breaking one of the copper bus wires may not cut off the current and prevent arcing.

Caution It is important to electrically connect the zero period "breaker" detonator to the copper bus wires as shown in figure F.15 and not to the lead line.

CURRENT LEAKAGE

Current leakage is the loss of part of the firing current through the ground. The three reasons this occurs are when (1) the insulation on the detonator wires has been damaged or abraded during loading, (2) bare connections between boreholes contact the ground, or (3) poorly insulated splices are placed in a borehole. Detonator failures are likely to occur unless the condition is recognized and preventive measures are taken. Leakage can occur in relatively nonconductive formations if the ground is wet. Most ANFO and water gels are conductive and will permit current flow if they come into contact with exposed wire. Excessive leakage of exposed wire contaminated by ammonium nitrate or gels can result in the borehole.

Before current leakage can be controlled, it must first be identified as a problem using the "blasters' " multimeter as the procedure. Experience has shown that current leakage can be a potential problem wherever the true resistance between a detonator series and ground is less than 25 times the resistance of one series. It should be recognized, however, that because of polarization effects at the electrodes, a DC meter such as the "blasters' " multimeter may show a ground resistance several times higher than the true resistance. The polarization effects will also cause the readings on the three scales to be different and only the 3,000 and 6,000 scale on the "blasters' " multimeter should be used. If resistance readings less than 5,000 ohms are obtained, find the source of leakage and correct it. The preventive measures summarized in table F.16 go a long way to preventing a current problem.

Preventive Measures To Control Current Leakage

Table F.16 – Preventive measures to control current leakage.

TROUBLESHOOTING CIRCUIT BREAKS

Use either a "Blasters' " multimeter, blasting ohmmeter, or blasting galvanometer to locate a break in a series circuit. When testing for a break in the circuit, use the technique shown in figure F.16, attach a connecting wire to the end of the circuit from terminal "A." Then attach another connecting wire to terminal "B." Pick a point midway in the circuit and touch the connecting wire from terminal "B" to the bare connection of the detonator wires. If a reading is indicated on the instrument, the circuit is good between terminal "A" and the midpoint of the circuit. Continue moving the connecting wire from terminal "B" to connections further along the circuit until no reading is indicated on the instrument. In this manner the break in the circuit has been isolated and can be corrected.

ELECTRIC BLASTING HAZARDS

This section discusses some of the sources of extraneous electricity and some conditions that could present electrical hazards if precautions are not taken to ensure safe usage. Unwanted electrical energy that could potentially enter a blasting circuit must be kept at safe levels, or preferably excluded altogether. If it is not, such energy has the potential to cause premature detonations or detonator malfunctions. For this reason a thorough evaluation of any extraneous electricity or its potential should be made at the blast site before any explosives or detonators are brought onto the site. Sources of electrical blasting hazards are listed in table F.17.

The accepted "safe" level of extraneous electricity for electrical blasting is derived from the current required to detonate the most sensitive commercial electric detonators plus a safety factor. The minimum firing current for commercial electric detonators presently manufactured in the United States is approximately 0.25 amperes (250 milliamperes). The IME has established the maximum "safe" current permitted to flow through an electric detonator without hazard of initiation as one-fifth of the minimum firing current, or 0.05 amperes (50 milliamperes), which provides a current safety factor of five. Therefore, electric blasting must not be conducted in areas where extraneous currents are greater than 0.05 amperes (50 milliamperes).

When blasters-in-charge using electric detonators are alerted to the presence of extraneous current sources, they should measure for extraneous currents in the blast site area at frequent intervals to ensure that all extraneous currents are at a safe level. When extraneous currents exceed 0.05 amperes (50 milliamperes), the source of the current must be traced and eliminated before electric detonators can be safely used. If the source of the current cannot be traced and eliminated, a nonelectric initiating system must be used. However, it must be remembered that even nonelectric initiating devices can potentially be initiated by high-voltage sources such as lightning. Extremely high static levels also can be reached by the pneumatic loading of ANFO.

Table F.17 – Electrical Blasting Hazards

Table F.17 – Electrical blasting hazards.

Lightning

Lightning, undoubtedly, represents the greatest single hazard to blasting because of its high energy and erratic nature. A lightning strike can have voltage potential exceeding a million volts and discharge currents of over 100,000 amperes. If lightning strikes a blast area, all or part of the blast will probably detonate, regardless of the initiation system used. Because of this extremely high hazard potential even distant lightning strikes can be hazardous to electric initiating systems in both underground and surface blasting operations.

Therefore, whenever lightning storms are in the vicinity of the blast; all blasting operations should be suspended in the interest of safety, regardless of the initiation system used. All personnel should be immediately evacuated to a safe distance from the blast area. The thermal condition of the electric detonators in a blast seems to have little bearing on the susceptibility of detonators to premature detonation from lightning. The danger from lightning is considerably increased if there is a transmission line, water line, compressed air line, fence, stream, or other conductor available to carry the current between the storm and the deactivated round.

Receiving firing lines and electric detonators, as used, typically in underground operations, a 4.6 meter (15 foot) air gap should be provided to act as a "lightning break" between the conductors. Lightning storms tend to follow patterns. All possible benefit should be taken of personal recollections and the Weather Bureau records. Radio, satellite television, local forecasters, and the World Wide Web conditions that indicate the possibility of lightning) and be prepared to temporarily abandon all explosive loading activities until the threat passes.

Commercially available early detection equipment in widely used in the industries that use blasting and several companies around the world produce and market them in a common sensor option. Lightning detection equipment is commercially available that can warn of the approach of thunderstorms to distances of several tens of kilometers (miles). Such instrumentation can provide valuable lead time in hazardous atmospheres conditions even when they occur in the absence of thunder or visible lightning.

Regional tracking services for lightning storms that provide the course and arrival time of such events can warn subscribers well in advance of lightning storms. These services are available in many areas. A less defensive, but occasionally used, field expedient for lightning detection is an AM (not FM) radio tuned to a weak station or (preferably) between stations. Static crises on the radio indicate the presence of static charges in the air. Supplementing all lightning detection equipment should be one or more individuals located so they can spot the approach of thunderstorm activity. A specific procedure alerting all personnel who could be affected by an impending storm should be instituted.

Caution A common sense rule is to evacuate the blast area when thunderstorm activity comes within 8.05 kilometers (5 miles) of the blast site.

In wiring situations where some series are complete and shunted and some are incomplete or in the process of being wired and the approach of thunderstorm activity is noted, the blaster-in-charge should immediately order that the wiring process be abandoned and the area cleared and guarded.

Lightning is not the only hazard associated with the atmospheric generation of electrical energy. The atmosphere can build up dangerous charges of static electricity at distances well removed from the storm center. These static charges can be stored on any insulated and ungrounded conductive body, such as a person or truck, and can be discharged through detonator wires to ground causing premature initiation. The shunt and legwire insulation of electric detonators offer no assured protection under these conditions because the voltages can be sufficient to break down the insulation.

Stray Ground Currents

Electric detonators are potentially subject to premature detonation when exposed to extraneous "stray" ground currents. Electric current flowing through power lines to electrical equipment from a battery, a generator, or a transformer will always return to that source.

The preferred method of dealing with stray ground currents in the vicinity of electric blasting operations is to eliminate their source. If the return conductor between the load and the source is interrupted, the current will find another path and potentially result in dangerously high ground currents. This hazard can be minimized if continuous metal objects are kept away from the immediate vicinity of electric blasting circuits. In addition, measurements for stray ground currents should be conducted before electric detonators are introduced into a particular location. Generally, in homogeneous ground, AC or DC currents sufficient to detonate electric detonators rarely are found. That is because the resistance of the earth is usually high and the potential between two points close together is usually low.

However, dangerous currents can be found when electric detonators legwires contact separate conductive strata. Hazardous currents, greater than 50 milliamperes, can also reach electric detonators if the legwires contact rails, pipelines, or ventilating ducts. The earth offers such a huge cross section to vagabond extraneous currents that even high resistance earth draws currents out of rails or ground conductors.

Stray Current Test

The proper technique for measuring stray currents requires the use of a "blasters' " multimeter with sensitivity capable of reading less than 0.05 amperes. Most standard electricians AC-DC Voltmeters and VAO meters are capable of supplying enough current, under the right conditions, to detonate an electric detonator and should "not" be used in the vicinity of blasting circuits. "Blasters' " multimeters that include the multimeter function utilize a built in one ohm resistor that is used to shunt the input terminals to simulate the resistance of the electric detonator. Voltage and current are then numerically equal when making this test because of the meter circuitry modification with the one (1) ohm resistor (volts / 1 ohm = amperes). The method described is table F.18 is suggested for stray current checks using a "blasters' " multimeter.

Stray Current Test *

* Always use a "Blasters' " multimeter for this procedure.

** The built in one ohm resistor used in the stray current test typically has a power rating of 10 watts and can stand continuous application of voltages of up to 3 volts. Voltages between 3 and 6 volts may be safely tested in the milliamp range for short periods, 5 to 10 seconds. Voltages exceeding 6 volts must be eliminated. Do not attempt to test them for current output as the power rating of the instrument circuitry may be exceeded.

Table F.18 – Stray current test method.

Stray Current Test Frequency

Stray current from electrical equipment can vary during the startup or the application of a heavy load and testing. Therefore, the test must be made during this time. Electric blasting operations in highly conductive ground, in metallic rock formations, in salt water, and in slightly acidic or alkaline wet areas, as well as operations near electrical distribution facilities, warrant frequent tests for the presence of hazardous ground currents.

If AC and DC current readings exceed 0.05 amperes (50 milliamperes), electrical blasting should be suspended until the hazard is reduced to a safe level. The hazards that stray currents pose to electric detonators can be greatly reduced by isolating all electric power lines near the blast site from ground, and by providing a separate common bus wire bonded to the metal frames of all electrical equipment. All rails, pipes, removed cables, ventilating ducts, and other conductors not designed to carry power should be electrically bonded together at frequent intervals and connected to a single earth ground. This, in turn, should be isolated from power ground or neutral bus. Power line insulation and insulators should be kept in good repair.

Power and lighting circuits should be kept away from the blasting area during blasting loading operations. Exclusion of all lead-in wires and blasting circuits from ground and from possible current-carrying equipment are additional measures to reduce the stray current hazard. Lead lines should be checked for stray currents at periodic intervals. The insulated covers (some of which may be foil-lined) over the shunted legwires of some brands of electric detonators provide the final defense against stray current hazards. These covers should not be removed until the loading operation is completed and detonators are ready to be connected.

Radio-Frequency (RF) Energy

Intense high frequency radiation can accidentally initiate electric detonators. Therefore, an investigation of any potentially hazardous source of radio frequency (RF) energy near a blasting site should be conducted before any electric detonators are brought into the area. The intensity of RF current induced in a blast circuit depends on four factors including (1) radiated power, (2) distance from the transmitting source, (3) transmission frequency, and (4) wiring layout itself. Current limits a bridgewire independent of frequencies. Fifty (50) milliamperes will generate the same heat at DC, 60 Hz, TV, or microwave. The IME Safety Library Publication No. 20, Safety Guide for the Prevention of Radio Frequency Radiation Hazards in the Use of Commercial Electric Detonators, classifies sources of radio frequency (RF) and lists safe distances from three 82 transmitters sources in tables F.19 through F.21.

Caution Please refer to the IME Safety Library Publication No. 20 for a complete listing of radio frequency hazard tables.

Table F.19 – Recommended Distances For Commercial AM Transmitters

* Power delivered to antenna ** 50,000 watts is the present maximum power to U.S. broadcast transmitters in this frequency range

Table F.19 – Recommended distances for commercial AM transmitters. (Source: Reprinted by permission of IME: SLP 20, 2011)

Table F.20 – Recommended Distances for VHF TV and FM Broadcasting Transmitters

* Present maximum power channels 2 to 6 and FM: 100,000 watts ** Present maximum power channels 7 to 13 — 316,000 watts

Table F.20 – Recommended distances for VHF TV and FM broadcasting transmitters. (Source: Reprinted by permission of IME: SLP 20, 2011)

The IME periodically reviews its publications and these tables are subject to change. Often the radio frequency energy source consists of several transmitter stations of different power, thereby complicating the use of the tables. Under conditions a survey of the blast site must be made with a RF ammeter, which responds to frequencies up to 100 megahertz. Blasters-in-charge must make certain that they have sufficient separation, as outlined in the tables in IME SLP 20, between their blasting operations and any radio frequency transmitter(s), and that mobile transmitters are also kept at a safe distance from blasting areas.

Table F.21 – Recommended Distances from UHF TV Transmitters

* Present maximum power channels 14 to 83 1,000,000 watts

Table F.21 – Recommended distance from UHF TV transmitters. (Source: Reprinted by permission of IME: SLP 20, 2011)

The most critical test circuit is made up of a straight piece of wire exactly half the radio wavelength (at a multiple thereof) with the radio frequency ammeter positioned at the center of the wire. This is called a half-wave dipole and is best suited for FM and TV energy as shown in figure F.17 (top). When the wavelength is unknown, first use a dipole having a length of 0.7 meters (22 feet) (simulating a detonator with 4.3 meters (14 foot) legwires. After observing the radio frequency ammeter reading, cut it to 0.3 m to 0.6 meters (1 foot to 2 feet) from each end of the wire and observe the test both again. Repeat this process until the length of the dipole is 0.6 meters (2 feet). The dipole should be oriented broadside to the antennae for maximum pickup.

Near AM broadcast antennae the maximum current will be induced if one side of the radio frequency ammeter is grounded to earth and the other side is on a long wire deployed one meter (several feet) above the ground and extending several hundred feet as shown in figure F.17 (bottom). Change the orientation of the long wire to determine maximum pickup. The test should resemble the proposed blast circuit as nearly as possible. The radio frequency ammeter identifies a current intensity and if it is greater than 50 milliamperes, it is not safe to blast electrically. Electric detonators with iron legwires at least 24 feet (7.3 meters) long can be used safely at half the distances quoted in IME: SLP 20.

Iron legwires have DC resistance six (6) times higher than copper legwires as referenced in the IME SLP 20. As frequency increases, the RF resistance of iron wires increases more rapidly than that of copper wires. The high DC resistance and magnetism of iron wire limits the current. The ratio of the RF resistance of both copper and iron wire increases with the square root of the frequency. Copper legwires and iron wires give higher DC resistance with smaller gauges to be the neophyte. Copper legwires and iron wires whereas the bridgewire on the frequency.

All available evidence indicates that radio frequency energy is not normally a hazard in the transportation of electric detonators in their original containers. Coiled or folded wires provide effective protection against induced current. Metal truck bodies and freight cars also effectively prevent the penetration of RF energy. If vehicles equipped with radio transmitters are used in transporting electric detonators so to form a pair, the precautions listed in table F.22 should be strictly followed.

Table F.22 – RF Energy Precautions When Transporting Electric Detonators

Table F.22 – RF energy precautions when transporting electric detonators.

The general precautions listed as table F.23 increase safety and reduce RF hazards when conducting electric blasting operation near RF energy sources.

General Precautions To Reduce Radio Frequency Hazards

Table F.23 – General precautions to reduce radio frequency hazards.

Table F.24 illustrates the RF characteristics of an example iron wire electric detonator.

Table F.24 – Radio Frequency Resistance of a Typical 4.9-meter (16-foot) Iron Wire Detonator

* Values will vary depending on manufacturers' specifications.

Table F.24 – Radio frequency resistance of a typical 4.9 meter (16 foot) iron wire detonator.

Induced Currents

Alternating electromagnetic fields can induce current flow in nearby conductors. Such electromagnetic fields exist in the vicinity of power transmission lines, transformers, switches, and ground return motors. Such a situation makes a current directly in an electric blasting circuit. Electric detonator wires touching extended conductors could either intercept the induced current or physically complete a induction loop.

Induction loops need a well-defined closed circuit in order to establish a current flow. Such a circuit can be formed by a series of electric detonators and its connecting lines. Two or more series of electric detonators connected by a set of parallel bus wires can also form a closed circuit or loop, capable of intercepting induced currents, if they are located too close to an overhead power line or other alternating electromagnetic fields (see figure F.20).

To reduce the intensity of induced currents, the area enclosed by the loop of connected legwires and lead lines should be minimized. Measurements should always be made during the peak periods.

Caution When blasting near high-voltage transmission lines or any other high-voltage source, the public utility company or agency involved with the equipment should be consulted to determine the maximum power surge that can be expected.

INDUCED CURRENT TEST METHOD

Loop measurements for induced current must be supplemented by stray ground current measurements. The recommended loop configurations to measure induced current is shown in figures F.18. The recommended procedure for induced current testing is listed in table F.25. The configuration in figure F.19 is not recommended since it can lead to inaccurate readings—this configuration should be avoided.

Recommended Procedure For Measuring Induced Current

Table F.25 – Recommended procedure for measuring induced current.

Figure F.20 illustrates three induced current pickup configurations. The amount of voltage that can be induced by a power line into a blast circuit can be calculated using equation F.18 (provided for reference). The degree of pickup is summarized in table F.26.

Table F.26 – Induced current pickup for Figure F.20

Table F.26 – Degree of current pickup for loop to power line orientation in figure F.20.

INDUCED VOLTAGE CALCULATION

$$V = \frac{(0.00075) \times f \times Cos (A) \times l}{D}$$

Equation F.18

Where:

• V = Voltage induced into a loop simulating the anticipated blasting circuit (volts)
• f = Loop area (square meters)
• A = Angle of loop to power line (degrees)
• I = Largest power line current likely to be encountered (amperes)
• D = Distance of power line to loop (meters)

Caution If equation F.18 yields values of 50 milliamperes (0.05 amperes) or more, a nonelectric blasting system should be considered or measurements be made on a simulated blast circuit on site.

Equation F.18 assumes that the effective loop radius is small compared to the distance to the power line. It can be simplified by assuming that the loop is oriented parallel to the power line as shown in case 1 of figure F.20. This would be the most critical condition (where Cos (A) = 1, maximum value) and the equation becomes equation F.19.

$$V = \frac{(0.00075) \times f \times l}{D}$$

Equation F.19

Where:

• V = Voltage induced into a loop simulating the anticipated blasting circuit (volts)
• f = Loop area (square meters)
• I = Largest power line current likely to be encountered (amperes)
• D = Distance of power line to loop (meters)

After calculating V, the largest current in the loop can be determined using Ohm's Law, equation F.2. Based on the power line measurements in figure F.21 the induced voltage is calculated with equation F.20.

$$V = (0.00003875) \times I \times ln\left(\frac{A_2 \times A_4}{A_1 \times A_3}\right)$$

Equation F.20

Where:

• V = Voltage induced on blast line (volts)
• I = Current in power line (amperes)
• ln = Natural logarithm
• $A_1, A_2, A_3, A_4$ = Distances (any consistent units of length)

*The above equation is valid for 60 hertz only.

Mechanical Static Charges

Operating machinery can also generate static electrical charges. This charge potential must be considered if operated in the vicinity of electric blasting circuits. The recommended precautions in table F.27 should be taken for cases where static electricity is generated by mechanical means.

Table F.27 – Recommended Precautions When Mechanically Generated Electrostatic Energy Is Encountered

Table F.27 – Recommended precautions when mechanically generated electrostatic energy is encountered.

High-Voltage And Power Transmission Lines

Several potential hazards associated with electric blasting near high-voltage and power transmission lines are summarized in table F.28. Before conducting electric blasting operations in the vicinity of transmission lines, it is a good practice to check for stray ground currents. Care should be exercised when conducting the stray current test to avoid contacting the high-voltage lines with test wires or long metallic extensions.

Table F.28 – High-Voltage Power Line Hazards

Table F.28 – High-voltage power line hazards.

If hazardous stray or induced currents above 50 milliamperes are detected, or if the shot point cannot be relocated to ensure that the blast wiring will not be thrown over power lines, nonelectric initiating systems should be used.

Galvanic Action

Galvanic currents are generated when dissimilar metals are immersed in an electrolyte, such as wet ground or a conductive explosive composition. For example, an aluminum loading pole, designed to replace the heavier wooden loading pole in seismic shooting, experienced a cut-off load rate in the field. Not long after adopting the new pole, a spontaneous explosion over just two operations. Both were definitely traced to the battery effect developed by the aluminum loading pole, the steel casing in the borehole, and the alkaline drilling mud. It is obvious that metallic liners, metal loading poles, or any conductive devices should not be allowed to enter a borehole containing an electric detonator. Underwater blasting operators should be alert to the hazard of dissimilar metals in the borehole when loading, particularly in salt water or if making the loading hose bail part of the blasting circuit.

REFERENCES

International Society of Explosives Engineers (ISEE). 1998. ISEE Blasters' Handbook™ 17th Edition. ISEE, Cleveland, OH.