Chapter 7Blasting accessories

Mary Kassel T. Blanco

Outline7.1 INTRODUCTION

7.2 POWER SOURCE FOR ELECTRICAL FIRING7.2.1 Exploders7.2.2 Sequential blasting machine7.2.3 Mains firing

7.3 BLASTING CIRCUIT TESTING

7.1 INTRODUCTION

Blasting accessories

- Various products that are used in blasting, in addition to explosives and initiating devices.

- Few are consumed in the blast, but most of them are designed to be used in blasting repeatedly.

tools and equipment normally required include:

• Power source• Non-metallic measuring tapes equipped with

lead or non-sparking weights• Lowering ropes• Non-sparking lowering and retrieving hooks• Tamping poles

tools and equipment normally required include:

• Blasting knives• Connecting wire• Lead line• Blasting covering material• Crimper• Lightning detector

• The substitution of steel nuts or bolts for weights, steel pipe or length of drill steel for tamping poles, lengths of detonating cord as measuring tapes or lowering ropes, reclaimed leg wire as connecting wire, improper test instruments must not be permitted.

7.2 POWER SOURCES FOR ELECTRICAL FIRING

Electric blasting circuits are energized by exploders or by powerlines through special arrangements.

Storage in dry cells batteries are not recommended for blasting operations because they cannot be dependent on for consistent output.

7.2 POWER SOURCES FOR ELECTRICAL FIRING

• 7.2.1 Exploders

• 7.2.2 Sequential blasting machine

• 7.2.3 Mains firing

7.2.1 Exploders

Two types of exploders:

- Generator type

- Condenser discharge type

Generator typeRack bar:- Operated by vigorously pushing down a rack

bar which, through a set of gears, spins the rotor in a DC.

- The electrical energy from the generator is connected to output terminals when the rack bar reaches the bottom in its downward travel where it closes a switch.

Generator type

- capable of blasting 30-50 caps in single series

Generator typeKey-twist type:- A spring is first wound and with the help of key, it

is suddenly released. This through a set of gears spins the rotor in DC generator.

- Operated by a quick twist of the handle with one hand while the machine is held firmly in the other hand.

- Capacity of 250 to 400 shots

Condenser type

- The capacitor discharge (CD) machine a condenser is charged to a high voltage and discharged later through the external firing circuit.

- The basic source may be a low voltage dry battery or an electromagnetic generator.

Condenser type

• One button or switch is activated to charge the capacitors and a second button or switch is activated to fire the blast.

• Some are capable of firing 100 caps in parallel series.

• The capacity of the exploder must be more than sufficient to fire all the detonators, it should be too much higher than what is needed.

• A good rule of thumb is to use an exploder 1.5 - 2 times the needed capacity.

7.2.1 Sequential blasting

• SB machine is a unit containing 10 capacitor discharge machines that will fire up to 10 separate circuits with a preselected time interval between the individual circuit.

• This can be useful in improving fragmentation and in controlling ground vibration and airblast.

7.2.3 Mains firing

• Powerline blasting is often carried out with parallel circuits where the capacity of available blasting machine is inadequate.

• Power blasting should not be carried out unless precautions are taken to prevent arcing. Arcing can result in erratic timing or a misfire.

7.3 BLASTING CIRCUIT TESTING

• Blasting procedure can be successfully executed only if proper care is exercised in planning and connecting the blasting circuit.

7.3 BLASTING CIRCUIT TESTING

A list of possible circuit weaknesses is given below:

1. Discontinuity2. Current leakage3. Stray electricity

7.3 BLASTING CIRCUIT TESTING• Discontinuity or shorts in: – detonator circuit and– the total or any part of the blasting circuit;

• Current leakage– can occur when damage to a leg wire insulation

allows the bare wire to make a contact with the rock specially under conditions or conducting ore body.

7.3 BLASTING CIRCUIT TESTING

• Stray electricity– Current leakage from extraneous sources.

7.3 BLASTING CIRCUIT TESTING

Two type of blasting circuit testers:

- Blasting galvanometer

- Blasting multimeter

7.3 BLASTING CIRCUIT TESTING

• Blasting galvanometer– Is used only to check the circuit resistance.

• Blasting multimeter– Can be used to check resistance, AC and DC

voltage, stray currents, and current leakage.