d electrical circuits

8/6/2019 d Electrical Circuits

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ELECTRIC CIRCUITS

The different types of wire used in blasting circuits can bedivided into three general categories classified by function:

i. Leading or leg wires. These insulated wires (usuallymade of copper, but sometimes iron) are integral

parts of an electric detonator unit. The insulation

has been removed from the free end of each lead

wire so that they can be connected to other wires to

make a circuit.

ii. Extension lead, connecting wire or buswire. Thiswire (the thickness varying between 21 and 25

SWG) is used to connect two detonators when their

lead wires are not long enough to meet. It is also

used to connect a circuit to the firing cable.

iii. Firing cable. This is used to connect the detonatorcircuit to the power source, which is generally a

portable exploder or mains electricity, and is of

heavy duty construction.


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Lead wires topower source



Lead wires to

power source

Various configurations of a series circuit

All electric blasting is done by using one of the three basic

circuits.

i. Series circuit

All detonators in a series circuit are connected together to

provide a single path for the current, and the same amount


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of current flows through each detonator. The figure above

shows various configuration of a series circuit.

This type of circuit is simple to connect and a convenient

way to fire a small number of detonators (say, a maximum

of 50). The series circuit has the unique advantage that

the entire circuit can be tested for continuity and

resistance.

It is stipulated that the minimum firing current is 1.5

amperes by direct current (DC) or 2.5 amperes by

alternating current (AC).

Worked example

Find the voltage required to fire 30 electric detonators

joined to 200 metres of firing cable with 50 metres of

connecting wire. The resistances are as follows: an

average of 2 ohms per detonator, 0.1 ohm per metre of

connecting wire, and 0.05 ohm per metre for two-core

firing cable. Suppose a DC power supply is available:


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Resistance of detonators: 30 X 2.0 = 60

Resistance of connecting wire: 50 x 0.1 = 5

Resistance of firing cable: 200 x 0.05 = 10 Total resistance of the circuit = 75

Hence the minimum voltage required is 1.5 x 75

= 112.5 volts DC

ii. Parallel circuitsIn this circuit, one lead wire from each detonator is

connected to one conductor, while the other lead wire is

joined to the second conductor.

A. Straight parallel B. Reverse parallel

C. Closed-loop straight parallel D. Closed loop reverse parallel


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Type A causes poor current distribution, whereas type D

produces the most even current.

Arcing in the detonator may occur in parallel circuit due to

heavy current being fed. This causes misfire.

Arcing can be prevented in a number of ways, one of which

is to select a firing cable and extension lead with high

resistance, so that the principal voltage drop will occur

within the firing line. Thus the voltage over the last

detonator to fire may be limited to between 50 to 60 volts.

The average current required is stipulated as 0.7 amps per

detonator in the circuit.

Parallel circuit is being used in shaft sinking operation.

Worked example

Find the power requirement and the energy input to a

detonator of a parallel circuit with 30 detonators (each with

a resistance of 1.5 ohms) with 300 m of twin core firing

cable (resistance of 0.0033 ohms m-1)


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Solution:

Total resistance of detonators = 1.5/30 = 0.05 ohms

Resistance of firing cable = 600 X 0.0033 = 2 ohmsHence total resistance of the circuit = 2.05 ohms

Now total current required is 0.7 X 30 = 21 amps

Voltage required = 21 X 2.05 = 43 volts

Power requirement = 43 X 21 = 904 watts

Total energy input (usually initiation time is 4 ms)

= 904 X 0.004 = 3.6 J

Energy input per detonator = 3.6/30 = 0.12 J = 120 mJ

iii. Parallel-series circuits


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d electrical circuits

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