Chapter 20: Electric Initiation Systems

Learning Objectives

Describe the principles of electrical theory.
Explain the water tank analogy as it relates to current, resistance, and voltage.
Describe Ohm's law and the Ohm's law triangle.
Discuss the basics of electrical initiation.
List and describe the components of electric initiation systems.
Describe electric detonators, their purpose, and their components.
Explain how detonator delays work.
Describe leg wires and what they are made of.
Discuss the safety features of modern commercial electric detonators.
Describe generator and capacitor discharge blasting machines and how they work.
Explain the requirements for marking and maintenance of blasting machines.
Describe the different types of blasting wire used in blasting.
Explain how to secure connections between lengths of wire in a circuit.
Describe the types of equipment required for testing electric detonators and blasting circuits.
Describe how to test a circuit.
Explain blasting circuit configurations.
Demonstrate how to calculate blasting wire resistance for single and multiple series circuits.
Describe how to calculate the voltage required to successfully initiate a blast.
Discuss causes of extraneous electricity and their potential for causing premature detonation of electric detonators.
Describe general safety precautions to take when working with electric initiation systems.

Overview

Electric initiation systems use detonators designed to be initiated by the basic principle of electric current flow. When voltage is applied from a source, the current travels through a circuit and returns to the voltage source.

The main benefit of using electric detonators is the ability to test the circuit. The goals are to verify resistance and continuity (meaning there is no break or fault in the circuit) before initiating a blast. The main limitation of using electric detonators is the risk of initiation by stray current.

This chapter deals with the principles of electrical theory and the components and methods of electrical blasting.

Principles of Electrical Theory

Before using electric initiation systems, a blaster must understand basic electrical theory and Ohm's law.

Electricity is a type of energy that occurs in two forms: dynamic (current) and static (charge). Dynamic electricity involves the flow of electrons along a conductor. This flow is known as current, which is measured in amperes (amps).

Current flows more easily along a good conductor (e.g., copper wire) than a poor conductor (e.g., dry wood). The difficulty encountered by the flow of current is known as resistance, which is measured in ohms.

Current moves because of a difference of potential within the circuit. This is known as electromotive force, which is measured in volts. One volt is the amount of electromotive force necessary to move 1 ampere of current across 1 ohm of resistance.

Water Tank Analogy

A tank contains water. Water flows from a 25 mm (1 in.) diameter pipe at the bottom of the tank. The force of gravity causes the water to flow from the pipe, but the flow rate is restricted by the size of the pipe.

The analogy works as follows:

The flow of water represents current (in amps).
The size (diameter) of the pipe represents resistance (in ohms).
The gravitational pressure represents electromotive force (in volts).

An increase in current means more water flows through the pipe. A larger pipe will allow this to happen because it offers less resistance. Conversely, a smaller pipe offers more resistance. The greater the height of water in the tank, the greater the gravitational pressure, which is the force behind the water.

Just as water flows inside a pipe, electric current flows within the solid portion of a conductor. A conductor is any material capable of carrying this flow.

Electricity travels at the speed of light, 300,000 kilometres (186,000 miles) per second. As a result, it passes through an electric blasting circuit in a fraction of a second. And initiation of detonator bridgewires is virtually instantaneous.

Types of Current

Depending on the power source, the current is one of the following types:

Alternating (AC) - such as that found in household lighting or outlets
Direct (DC) - such as batteries in a flashlight or in automobiles

Most blasting machines produce direct current.

Factors Affecting Resistance

The composition of a conductor determines its resistance to the flow of electric current. Copper is a good conductor, so it offers little resistance.

The diameter (thickness) of a conductor also affects its resistance. As diameter is increased, resistance is decreased. Wire thickness is expressed in gauge or AWG (American Wire Gauge). Thicker wire has a low AWG number, and thin wire a high AWG number. Wires used for blasting range from 8 to 22 AWG. Leg wires on electric detonators are usually 22 gauge. Firing cables may be as thick as 8 gauge.

Length also affects conductor resistance. As the length increases, the current has a greater distance to travel. For this reason, the resistance increases. Resistance increases with each unit of length. The resistance of wire is usually stated in units of 305 metres (1000 feet).

The condition of a conductor can affect its resistance. Contamination, deterioration, oxidation of the bare metal exposed ends, or damage will usually increase resistance.

Ohm's Law

Ohm's law describes the mathematical relationship between voltage, current, and resistance. The blaster of record usually knows two of these variables and needs to calculate the third.

Resistance (in ohms): Ohms are the standard measure of resistance to the flow of current.

Current (in amperes): An ampere is a unit of electric current produced by 1 volt acting through a resistance of 1 ohm. A minimum of 1.5 amps DC is required to detonate each series of electric detonators.

Voltage (in volts): The power source must have the necessary voltage to reliably initiate all detonators in a circuit. To calculate this voltage, apply Ohm's law.

Ohm's law states that voltage equals current (amps) multiplied by resistance (ohms). This formula is expressed as:

V = I × R

Where:

V = Voltage (in volts)
I = Current (in amperes)
R = Resistance (in ohms)

Once the total resistance and total required current have been determined, multiply them to calculate the voltage. Then verify that the power source provides enough voltage to reliably initiate the blasting circuit.

Ohm's Law Triangle

The Ohm's law triangle is a memory aid for Ohm's law. To use the triangle, place a finger over one symbol. Then, multiply or divide the remaining symbols according to their relative position in the triangle.

For example, cover V and it equals I multiplied by R. Cover I and it equals V divided by R.

So: V = I × R and I = V ÷ R and R = V ÷ I

Basics of Electrical Initiation

All electric initiation systems use similar components, rely on electricity for initiation, and can be affected by electrical hazards.

Advantages

These systems offer the following advantages over other initiation methods:

They are easy to prepare and connect.
The circuit can be tested.
Initiation happens instantly.
Delay elements allow sequential blasting.
The blast can be initiated from a safe location.

Disadvantage

They have one disadvantage. Unwanted electricity can enter the circuit and damage the detonator or cause accidental detonation.

All electric detonators have protection from extraneous electricity (e.g., stray current), some more than others. If electrical hazards are identified and appropriate precautions taken, there is little danger in using an electric initiation system.

Components

The components of all electric initiation systems are similar in design and construction. Each consists of an electric detonator, a power source, and blasting wire. A testing instrument is used to verify the continuity of each system.

Electric Detonator

An electric detonator is an initiating device capable of detonating most high explosives. A detonator comes with pre-installed leg wires and a metal-foil shunt.

Power Source

A blasting machine is designed to produce an electric current.

Blasting Wire

Blasting wire refers to the conductors that transmit electric current within a blasting circuit.

Electric Detonators

An electric detonator is an initiating device capable of detonating most high explosives. It has an aluminum shell about 6 mm (¼ in.) in diameter. The shell's length can range from 33 mm (1¼ in.) for an instantaneous detonator to 100 mm (4 in.) for a detonator with a long delay period.

Components

Pressed into the base end of the shell is a composite charge of heat-sensitive lead azide (the primary charge) and high-explosive PETN (the base charge).

Two insulated leg wires enter the shell through a rubber plug. The plug holds the leg wires in position and forms a water-resistant seal.

The leg wires terminate in a bridgewire embedded in an ignition charge. When a minimum amount of current is passed through this filament, it becomes very hot and ignites the ignition charge. The ignition charge ignites the delay element, which sets off the primary charge and the base charge.

Electric detonators have a static short built in for protection against static charges. Each leg wire has a triangular compression slightly ahead of the second crimp known as the anti-static groove. The static short is designed to drain off static charges from the leg wires to the anti-static groove before they enter the bridgewire and cause detonation.

Detonator Delays

Most electric detonators available today have short-period delays. The delay elements are preset to increase at 25-millisecond (ms) intervals. Numbers on the detonators' labelling represent the intervals.

Detonator Number	Delay Time
0	0 ms (instantaneous; no delay element)
1	25 ms
2	50 ms
3	75 ms
4	100 ms
5	125 ms
Etc.	Increases by 25 ms

When delays are used properly, they can do the following:

Minimize cutoff holes
Reduce vibration and concussion
Improve fragmentation
Produce predictable amounts and throw of muck
Reduce overbreak

Leg Wires

Leg wires are solid metal conductors usually made of copper — a good conductor of electricity. Iron wire is used in operations where foreign materials are removed from blasted rock by magnetic separation.

Leg wires are two separate wires built into a detonator in the factory. These wires are covered with flexible plastic insulation that resists abrasion.

Electric detonators are available with 22-gauge leg wires in a number of common lengths.

Safety Features

Bypass or Path to Ground

All modern commercial electric detonators include an internal feature to prevent electrostatic energy from accidentally initiating the detonator. There are several designs for this feature. Some provide a bypass path around the bridgewire using a semi-conductive material. Others use a printed circuit that provides a controlled path to ground.

Shunts

All electric detonators produced in North America have shunts on the free end of the leg wires. A shunt is an intentional short-circuit that helps protect the detonator from stray current. A shunt usually consists of aluminum foil with an insulated layer on the outside. At the site of the shunt, the foil holds the bare leg wires together and they are shorted out.

Once removed, a shunt may be difficult to replace. The detonator can also be shunted (short-circuited) by twisting the bare ends of the leg wires together.

Blasting Machines

A blasting machine is a current-producing device used to initiate an electrical blast. Most blasting machines are small and portable. There are two types of blasting machines: generator and capacitor discharge. Both types produce electrical energy with enough current and voltage to "fire" the number and types of detonators for which they are rated.

Generator

In this type of device, a rackbar or twist spindle rotates the armature of a small generator inside the machine. Upon reaching full capacity, the generator automatically releases the current.

When connecting the circuit to a "push-down" machine, the rackbar must be in the down position. A deliberate up-and-down movement is required to fire the blast.

Capacitor Discharge

By pressing a button or a switch on this type of device, a high-voltage charge from a dry cell battery builds up on a bank of capacitors. A glowing light indicates a full charge. When the firing button is pressed, the capacitors discharge the current.

Capacitor discharge machines are also available in multi-circuit models. In these models, each series can be wired separately and fired with precise electronic delays between circuits. These are known as sequential blasting machines.

Warning: A dry (flashlight) or wet (car) battery should not be used. The power output from such batteries is unreliable, and a misfire could result. In addition, these batteries have exposed terminals. If the firing cables inadvertently touch the terminals, an accidental (premature) detonation will occur.

Requirements for Marking and Maintenance

A blasting machine must have its firing capacity clearly marked on it. The label must indicate the maximum number of electric detonators that can be initiated in a single series or in multiple series (series in parallel). This is known as the rated capacity of the blasting machine. In all cases, the manufacturer's specifications must be adhered to.

Blasting machines must be maintained in good condition. Do not make repairs or adjust them at the worksite. They should be serviced only by competent technicians. The battery of a capacitor discharge blasting machine must be replaced with a type recommended by the manufacturer.

Take precautions to prevent a premature blast. Keep the machine in a safe, secure location, and do not connect it to the circuit until immediately before use.

Blasting Wire

Blasting wire refers to the conductors that transmit electric current within a blasting circuit. The following types of blasting wire are commonly used in blasting.

Connecting Wire

Connecting wire is a light (18- to 22-gauge) copper wire available in simplex (a single wire) or duplex (two wires separately insulated in a common plastic covering). Both types have an insulating layer of plastic that is usually red or yellow. Connecting wire can be added to leg wires to reach the next hole. It is also used to connect the circuit to the lead wire or firing cable.

When used to connect detonators or multiple series of detonators in parallel, connecting wire is referred to as the bus line or bus wire.

Lead Wire

Lead wire is a medium (12- to 16-gauge) copper wire. It has an insulating layer of plastic that is usually yellow. Lead wire may be simplex or duplex. It is used to connect a detonator or a series of detonators to a firing cable or a blasting machine.

Firing Cable (Shot Line)

Firing cable is a heavy (8- to 12-gauge) copper wire, usually consisting of insulated duplex wires in a strong black or white cover. Firing cable may extend from the power source to the blasting area, and it is connected to the blasting circuit. If a firing cable isn't long enough, duplex lead wire may be used to extend it.

Bare sections of blasting wire, particularly connections, must be prevented from contacting the ground or conductive material. This is done by elevating them or insulating them with electrician's tape. Contact with the ground or other conductive materials can lead to current leakage and can adversely affect the blasting circuit.

Connections

Connections between lengths of wire in a circuit should be secure and offer little resistance to the current flow. Sections of wire to be joined should be clean and bare.

Similar gauge wires can be joined with the loop-twist or western union connection. The loop-twist is very effective for light (18- to 22-gauge) wire. The western union is used with heavier (8- to 14-gauge) wire. When joining wires of different gauges, the straight wrap connection is most effective.

Testing Equipment

Instruments specifically designed and manufactured for testing electric detonators and blasting circuits are required.

Testing equipment, such as blasting galvanometers and multimeters, will have the word "blasting" on their labels. These devices have special batteries and/or internal resistors to limit current output to a maximum of 25 milliamperes (0.025 amperes). That is less than one-tenth of the minimum current required to initiate an electric detonator.

Testing equipment must be maintained in good condition. Avoid exposure to cold temperatures. Exposure to cold can cause the battery to become weak and produce unreliable readings.

Readings will be inaccurate if the needle does not deflect to zero when the instrument's terminals are shorted. In some such cases, the instrument may be damaged or not properly calibrated (adjusted). In other cases, the battery may be weak and needs to be replaced. The battery must be replaced with a type recommended by the manufacturer.

All testing instruments are designed to verify the continuity of a blasting circuit. Continuity (meaning there is no break in the circuit) is determined by measuring the resistance.

In addition to continuity, blasting multimeters are designed to test for current leakage and stray current. Current leakage occurs when a bare wire contacts the ground or another conductor. Stray current is the presence of extraneous electricity in the blasting circuit.

Testing Capability by Type of Test Equipment

Type of Test Equipment	Continuity	Current Leakage	Stray Current
Blasting galvanometer	Yes	No	No
Blasting multimeter	Yes	Yes	Yes

Blasting Galvanometer

The blasting galvanometer, a special type of ohmmeter, is designed for testing continuity. It is commonly known as a "galvo" or "tester."

Analog galvanometers indicate resistance with a needle that points to a number on a scale. Digital galvanometers produce a numeric display of the resistance.

Some manufacturers call their instrument a blasting ohmmeter. All blasting galvanometers and ohmmeters have special batteries and/or internal resistors and two bare terminals.

Blasting Multimeter

Blasting multimeters are precision instruments designed to measure ohms, volts, and amperes (stray current). They have special batteries and/or internal resistors. They also have a switch to select the appropriate scale of measurement.

Blasting multimeters can be used in the following ways:

As a galvanometer (to measure resistance and test blasting circuits)
As a voltmeter (to measure voltage, current leakage, and stray current)
To measure the voltage output from a blasting machine

Most of these instruments are capable of measuring alternating current (AC) and direct current (DC).

Testing a Circuit

To test a circuit, press the bare ends of the blasting wire to the terminals of the testing instrument. Then compare the reading with the calculated value. This determines the continuity of the circuit.

What the Test Readings Indicate

Measurement	Indication
Within 10% of expected value	Circuit okay
No measurement	Open circuit or faulty tester
High resistance	Poor or loose connection
Low resistance	Short, current leakage, or detonators missing from the circuit

Blasting Circuit Configurations

An electric initiation system uses one or more electric detonators wired into a single series circuit or a multiple series circuit (also known as series in parallel).

The configuration of an electric blasting circuit is based on whether there is one or multiple paths for the current to flow.

Blasters most commonly use a single-path circuit to initiate a blast. This is known as a single series circuit.

A multiple series circuit is made up of two or more single series circuits that run parallel and are tied into the same bus line to be initiated simultaneously. This method requires more amperage because the flow of current is divided among multiple paths.

Single Series Circuit

A single series circuit has one or more electric detonators connected into one series. The total number of detonators in a single series circuit must not exceed the rated capacity of the blasting machine. Most electric blasting machines are limited to 50 detonators.

A single series circuit may be connected (wired up) in various patterns. However, it must include every detonator and should be laid out in a tidy way.

Single Series Calculations

The resistance for each series, and the complete circuit, must be calculated to ensure there are no breaks in the circuit. To do so, blasters must know:

The number and type of detonators
The length and type of blasting wire

The steps of the calculation are as follows:

Determine the resistance of each detonator connected into the single series.
Add the resistance of all the detonators in the series. If all detonators have the same type and length of leg wires, multiply the resistance of one detonator by the total number of detonators in the circuit.
Determine the resistance of all blasting wire used in the circuit. Duplex wire has two separate wires in a protective covering. Double the given length of duplex wire to obtain the total length of lead wire in the blasting circuit. (For example, 500 feet of duplex wire is equal to 1000 feet of lead wire.)
Total resistance equals the resistance of all detonators plus the resistance of all blasting wire.

Testing the Circuit

Each series and the complete circuit must be tested with a blasting galvanometer or multimeter. Once all the detonators are connected into a series, test the series before connecting it to the blasting wire or firing cable. Some blasters prefer to test the blasting wire separately before connecting it to the circuit.

The testing device should be tested prior to use. Touch a short length of wire to both terminals, and the galvanometer should read a resistance between 0 and 1 ohm. A high reading, or no movement of the needle from infinity (∞), indicates the galvanometer is damaged or out of adjustment, or the battery must be replaced.

If the test reading does not equal the calculated value, do not attempt to fire the circuit until the problem has been corrected.

Locating a Break in the Circuit

A testing instrument connected to a blasting circuit will indicate the possible source of the problem.

Test a detonator or section of blasting wire by touching the bare wire ends to the terminals of the testing instrument. Each detonator or section of blasting wire may be tested individually.

It is unnecessary to disconnect individual detonators from a circuit. That's because the instrument only measures the resistance in the portion of the circuit between its terminals.

When a circuit contains numerous detonators, it may be easier to locate a break using the following procedure:

Open the blasting circuit and hold one of the end wires to one terminal of the galvanometer.
Use a piece of spare blasting wire and touch one end to the other terminal of the galvanometer.
Use the other end of the piece of spare blasting wire to touch bare connections within the blast circuit. This creates smaller, closed circuits within the blast circuit for the purpose of finding a reading on the galvanometer. Start in the middle of the circuit to determine in which half the break is located.
Continue to touch the end to bare connections until the break is found.
After correcting a break, test the series and circuit again. There could be more breaks.

Multiple Series Circuit (Series in Parallel)

In British Columbia, a series-in-parallel circuit is commonly known as multiple series. A multiple series circuit may be thought of as two or more single series circuits joined together into one circuit.

It is very important that the resistance (in ohms) of each series in the circuit is the same or nearly the same. This is called series balancing. Balancing the circuit ensures that the total current travelling from the power source will be divided equally into each series.

Many blasters balance their series by placing the same number of detonators in each one. Sometimes extra detonators are added to balance one series with the others. However, the detonators themselves do not have ohms. The length of the leg wires determines the resistance. So in a series with varying hole depths and leg wire lengths, the resistance of the wires needs to be balanced, not necessarily the number of detonators.

Each series should be limited to a maximum of 40 detonators or 100 ohms. The number of series in a multiple series circuit is only limited by the rated capacity of the power source. Two series in parallel, or a "double series," would be the most common.

Best practice would have each series tied in side by side and shooting toward an open face. This way, if one series fails to shoot, it is not directly in front of a series that does shoot.

Multiple Series Calculations

To calculate the resistance of a multiple series circuit, first follow the procedures for a single series circuit for each series in the circuit.

Calculations must include the following:

The number (amount) of detonators in each series
The number of series
The lengths and types of blasting wire

The purpose of circuit calculations is to ensure each series will receive enough current to fire all of the detonators. Each series provides a separate path for the electric current. This means the resistance of the circuit decreases as more series are connected to it. When the series are balanced, the total resistance of a multiple series circuit equals the resistance of one series divided by the number of series in the circuit.

Total Resistance by Number of Series

Number of Series in the Circuit	Total Resistance of the Circuit
1 series	12.0 ohms
2 series	6.0 ohms
3 series	4.0 ohms
4 series	3.0 ohms
5 series	2.4 ohms
6 series	2.0 ohms

(Example assumes each series has a resistance of 12 ohms)

It is important to keep the resistance of each series balanced. If not, more electric current will flow through the series with the least resistance. Unbalanced series make it difficult to calculate and test the resistance of the complete circuit with conventional techniques and equipment.

The steps for calculating the resistance of a multiple series circuit are as follows:

Determine the resistance (ohms) of each detonator.
Calculate the resistance of all detonators connected into one series.
Calculate the resistance of all detonators that are connected into the multiple series circuit. (To do this, divide the resistance of one series by the total number of series in the circuit.)
Determine the resistance of all blasting wire used in the circuit.
Total resistance of the complete circuit equals the sum of steps 3 and 4.

Testing the Multiple Series Circuit

With a blasting galvanometer or multimeter, test each series individually before connecting it into a multiple series circuit. Unless each series is tested, the final circuit test may fail to reveal breaks in a series.

Record the test reading for each series. Because many instruments are not completely accurate, it may be impossible to obtain identical readings. However, the readings should be within 10 percent of each other. The greater the difference in resistance between series in a circuit, the greater the likelihood of a hangfire or misfire.

If the test reading does not agree with the calculated value, locate and correct the problem before connecting the entire circuit.

After all series are connected, a test reading of the complete circuit is unlikely to reveal a problem within a particular series. A faulty connection or a break in one series may not be detected by a test of the complete circuit.

Electrical Calculations

This section provides completed examples of single series and multiple series calculations. The examples are designed to help blasters make their calculations.

Resistance Reference Tables

Resistance of Copper Blasting Wire by Gauge

Gauge of Copper Blasting Wire	Resistance (in ohms per 305 m or 1000 ft.)
4	0.248
6	0.395
8	0.628
10	0.999
12	1.588
14	2.525
16	4.016
18	6.385
20	10.150
22	16.140
23	20.360

Resistance of Electric Detonators

Length of Leg Wire (Metres)	Length of Leg Wire (Feet)	Resistance of Copper Wire (ohms)
2	6.6	1.40
3	9.8	1.55
4	13.1	1.70
5	16.4	1.85
6	19.7	1.95
7	23.0	2.15
9	29.5	2.20
12	39.4	2.25
15	49.2	2.35
20	65.6	2.80
25	82.0	3.20
30	98.4	3.35

Blasting Wire Resistance Calculations

Simplex (Single Wire) Resistance Calculation

In the table, look up the gauge of the wire to determine the resistance per 1000 ft.
Multiply the resistance per 1000 ft. by the length of wire used.
Divide the result by 1000. This provides the total resistance of the simplex wire.

Duplex (Dual Wire) Resistance Calculation

Complete steps 1 to 3 above, then:

Multiply the answer for step 3 by 2. This provides the total resistance of the duplex wire.

Single Series Resistance Calculations

Example Calculation 1

Situation: A blaster is required to blast a single series circuit. The circuit contains two electric detonators with 6 m (19.7 ft.) copper leg wires and 305 m (1000 ft.) of duplex, 16-gauge blasting wire.