Electric detonation system

ELECTRIC DETONATION SYSTEM
Mukteswar Padhan

BACKGROUND
• Initiating devices are those which provide sufficient amount of shock to
detonate and explode the high explosives.
• These are of various types such as detonators(plain, electric, non electric ,
electronic),detonating cord , safety fuse etc..
• Detonators can be chemically, mechanically, or electrically initiated, the latter
two being the most common.
• The commercial use of explosives uses electrical detonators or the capped
fuse which is a length of safety fuse to which an ordinary detonator has
been crimped. Detonators have primary explosive generally ASA compound
which is formed from lead azide, lead styphnateand aluminiumand is pressed
into place above the base charge, usually TNT in military detonators
and PETN in commercial detonators. All these put inside a cylindrical metal
case usually a aluminium or copper case.

HISTORY
Black powder first used to fragment rock in mining in early 1600s which was
extremely dangerous due to its unreliable burning speed
This problem was solved by the invention of ‘Miners Safety Fuse’ by William
Bickford which is a rope with a strand of yarn infused with black powder.
After this discovery of Nitroglycerine (1,2,3-tris-nitroxy -propane)in 1846 by
Ascanio Sobrero which is more powerful than black powder . But due to the
fact that it generates toxic fumes in hgh volumes which causes headache.
Results in hazardous ignition in mines . This problem was overcome by the
development of ‘detonator’ by Alfred Nobel.

EVOLUTION OF DETONATORS
• Plain Detonator
- Primary charge of ASA
- Base charge of PETN
• Instantaneous Electric Detonator
- First introduced in 1880s
- Safety fuse is replaced by electric leg wire connected to fusehead
- Initiation via electric current passed through leg wires
• Delay Electric Detonator
- Same as instantaneous electric det but includes a delay element
- Delay time based on length and composition of delay powder
- Half-second delay early 1900s, millisecond delay 1943

EVOLUTION OF DETONATORS
• Detonating Cord
- Strong, flexible, continuous detonator
- Developed in 1907 in France and called Cordeau
- Uses PETN cotton core surrounded by various textile
combinations, plastics and waterproofing materials
- Burning speed in excess of 7000m/s
• NONEL
-NONEL tube (shock tube) transmits shock wave to
NONEL detonator
- Shock wave results from tube coating of reactive powders
(HMX-CYCLONITRAMINE TRINITRAMINE) and travels at
2100m/s
- Minimal noise and cord movement
• Electronic
- It is the latest development which provide increased
accuracy, improved blast results
- costly as compared to other detonators

ELECTRIC DETONATOR
• Basically electric detonators are of two types which are
1. Instantaneous Electric Detonators
2. Delay electric detonators
• Detail construction of an instantaneous electric detonator:
-it consists of a cylindrical metal tube of aluminium or copper.
-inside the tube starting from the bottom part contains PETN as base
charge then after this ASA a prime charge.
- inside the tube a fuse head is inserted . Fuse head consists of nichrome
wire which is covered by flashing composition such as potassium
chlorate(oxidizer),charcoal(fuel),lead thiocyanide (sensitizer).
-A pair of leg wires (TWC/GI) connects the fuse head which passes current
to the fuse head which heats the nichrome wire which when ignited burns
the flashing composition. This ultimately initiate the prime charge which
then initiate base charge and then the detonator get blasted

• To make it water resistant the leg wire is first inserted into a sealing
plug then soldered to the fuse head . The whole set is inserted into
the cylindrical metal shell containing PETN(base charge) and
ASA(prime charge) and crimping is done with the help of a collet.
• In the case of delay electric detonators the only difference is that a
delay element is placed in between prime charge and fuse head
which provide the necessary delay timing .
• The delay timing is predetermined by the length,composition and
burning speed of delay element.
• The delay time and length of leg wire is tagged to the leg wire

CONNECTION OF ELECTRIC DETONATOR
1.SERIES CONNECTION
2.PARALLEL CONNECTION
3.PARALLEL SERIES CONNECTION

eq
SERIES CIRCUIT:
equivalent resistance of series circuit =
where R(L)=firing line resistance(Ω),R(P)=bridge wire resistance(Ω),n= no. of
detonators, m=length of connecting wires(m), r(l)=resistance per lineal meter of
connecting wires,R(D)= total resistance of detonators
-> the circuit is widely used due to its simplicity and possibility of checking it out by
simple continuation. This is due to the fact that th current from power source has
only one path to follow. So if no. of detonators is high more current is required by
the blasting m/c so resulting amperage is low.
RRrRR DLlpL
nmn *)*2( +=++

Total Resistance=
-> parallel connection system is
mostly used in underground and is
recommended where ther is high
risk of current leakage
PARRALLEL CIRCUIT:
n
R
RR D
LT
+=

Total resistance(R(t))
=
n(s)=no. of detonator in series
n(p)=no. of detonator in parallel
-> this type of connection is used
where it is necessary to the total
resistance of the circiut to fit it to
the capacity of blasting m/c.
PARALLEL-SERIES CIRCUIT:
n
nR
R
p
sD
L
∗
+

Worked example(series circuit)
Q. Find the voltage required to fire 30 electric detonators joined to 200 meters of
firing cable with 50 meters of connecting wire. The resistances are as follows: an
average of 2 ohms per detonator, 0.1 ohm per meter of connecting wire, and
0.05 ohm per meter for two-core firing cable. Suppose a DC power supply is
available:
Solution
Resistance of detonators: 30 X 2.0 = 60 Ω
Resistance of connecting wire: 50 x 0.1 = 5 Ω
Resistance of firing cable: 200 x 0.05 = 10 Ω
Total resistance of the circuit = 75 Ω
Hence the minimum voltage required is 1.2 x 75 (minimum firing current for
supreme det=1.2 A)
= 90 volts DC

Worked example(series circuit)
Q. 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).
Solution:
Total resistance of detonators = 1.5/30 = 0.05 ohms
Resistance of firing cable = 600 X 0.0033 = 1.98 ohms(let 2 ohms)
Hence total resistance of the circuit = 2.05 ohms
Now total current required is 0.7 X 30 = 21 amps(avg. current required is 0.7
amps per detonator the circuit.)
Voltage required = 21 X 2.05 = 43.05 volts
Power requirement = 43.05 X 21 = 904.05 watts
Total energy input (initiation time = 4 ms) = 904.05 X 0.004 = 3.616J
Energy input per detonator = 3.616/30 = 0.120 J = 120 mJ

TESTING OF ELECTRIC DETONATORS
Following tests are done for electric detonator before using:
1. SNATCH TEST
2.DROP TEST
3.UNDERWATER TEST
4.SHAKE TEST
5.X-RAY TEST
6.IMPULSE TEST
7.NO FIRE CURRENT TEST
8.MINIMUM FIRE CURRENT TEST
9.SERIES FIRE CURRENT TEST
10.LEAD PLATE TEST
11.DELAY TIMING TEST
12.INCENDIVITY TEST
13. SAND BOMB TEST

SNATCH TEST:
In this test the detonator is placed inside the test box and a 5 kg weight
is attached to the leg wires.Now the weight is dropped from 50 cm height.The leg wires must be
remained attached to the the crimped detonator after the test to pass.
DROP TEST:
In this the leg wires are cut and the detonator is dropped inside an elevated shell
made for the purpose from height 1.5-2.0m. The detonator should not explode.
UNDERWATER TEST:
Detonators samples are placed inside water under pressure of 4 kg/sq.cm for 4 hrs.
SHAKE TEST:
Done with the help of a shake test apparatus.It simulates the transporting goods in the
carriers and ensures safety and reliable initiation of detonators.
X-RAY TEST:
An x-ray m/c physically takes a picture of explosive inside the shell to ensure the
proper crimping of shell and filling of explosive.
IMPULSE TEST:
It ensures the limits of energy for reliable initiation and non initiation of electric
detonators

LEAD PLATE TEST:
It confirms the strength of detonator in delivering the expected
energy when fired. A 5 mm thickness lead plate is used on which a
detonator is placed just at the middle and fired. The detonater should fire
making a hole in the lead plate with diameter greater than the outer
diameter of the detonator shell.A testing gauge is available to test the
strength of detonator.
DELAY TIMING TEST:
Done with the help of delay firing m/c and delay time
measuring meters
It ensures the required delay timings of the electric delay detonators.
INCENDIVITY TEST:
Done with the help of Buxton Chambers, gas analyzers,
methane gas.to evaluate the incendivity of Copper detonators to methane
gas for underground coal mines.
SAND BOMB TEST:
It physically verifies the strength and side brisance of
detonatorS.

NO FIRE TEST:
• This test is done to measure the ability of the detonators for not
getting initiated when the prescribed level of current(180 mA for 300 sec)is
passed through it for a specified time. It is a measure of safety against stray
currents , static electricity which can initiate accidentally the fuse head and
thus the detonator.
• This test is done on all batches of fuse heads manufactured for use in
Electric Detonators.

NO FIRE CURRENT
• The bridge wire of a detonator is a very fine resistance element that
converts an electrical impulse to thermal energy . On application of a firing
current, the bridge wire heats up and ignites the heat-sensitive ignition
composition surrounding it.
• This in turn initiates the primary charge, through the delay element the
primary charge initiates the base charge .
• Depending on the magnitude and duration of the firing pulse, four
possibilities may arise: no-fire, uncertainty of firing, all-fire, and arcing.
• Recommendations for minimum, as well as maximum , firing currents are
supplied by manufacturers of detonators.10 amperes is a typical maximum
recommended current, especially for parallel circuits, to prevent a condition
known as arcing, which can damage the detonators and cause them to
malfunction.
• No-fire current is defined as the maximum level of direct current that can be
applied to a detonator without significant probability of causing an initiation .
For SUPREME electric detonators the value of no fire current is 180 mA
for 5 min..

MINIMUM FIRE TEST
• Effect of current strength on detonator ignition
It is also necessary to know how the minimum firing current varies with the
time of current application . The results of a typical series of observations
are shown in the Figure 2 which shows that the minimum application time is
about 2 ms when the amount of current required is 2 amps. To avoid
misfire, manufacturers recommended firing currents of 1.2 to 2.5 amps in a
blast.
Consider a series of electric detonators to which a uniform
direct current is applied .The fuse head must be traversed by this current
for a certain length of time, during which the bridge wire heats up to a
temperature at which the sensitive composition of the fuse head ignites.
->The period between the application of the current and ignition
of the composition is called the excitation time
->The time it takes for the bridge wire to be broken is the lag time.
->The interval between the first application of the current and the
detonator firing is the bursting time.
->The induction time is the interval between the breaking of the
bridge wire and the bursting of the detonator.

* The bursting time may be equal to or slightly more than the lag time.
*For all the electric detonators in the circuit to fire successfully , the
shortest lag time of any of them must be more than the longest excitation
time of any of the others, otherwise misfire will occur.

EVENTS IN FIRING OF ELECTRIC DETONATOR

SERIES FIRE TEST:
This test is a measure of the firing ability of Electric Detonators, when
connected in series and minimum firing current of 1.2 amperes DC for 4 ms
is applied so that all detonators should fire. All Electric Detonators are
subjected to this test.

Use in Underground
• In conjunction with permitted explosives only approved electric
detonators with copper tube are permitted for use in coal blasting.
• Two types
1.CED(Inst. copper electric detonator)
2.CDD(Copper non-incendive short delay
detonator)
• In pre-cut blasting all the detonators shall be fired simultaneously in
series.
• In solid blasting in longwall and development faces only permitted P-
5 explosives with Copper non-incendive short delay detonator shall
be used

Ohmmeter
• Ohmmeter is an electronic instrument used to check a complete
circuit or to measure the resistance of a circuit element.
• Micro Ohmmeter, Mega Ohmmeter and Milli- Ohmmeters are used
to measure extreamly low ,,large ,low resistance respectively.
• An Ohmmeter consists of a DC ammeter and few added
characteristics:
A DC source of potential (generally a 3V battery)
One or more resistors (one of which is variable)
• Two types of ohmmeter available
series type for measuring high resistance
shunt type for measuring low resistances

Measuring Blast Circuit with an Ohmmeter

Measuring Blast Circuit with an Ohmmeter
• Before calculating the resistance of blast circuit short the test leads
of ohmmeter ;needle of ohmmeter comes to zero position which
confirms zero resistance between test leads
• When the test leads of the Ohmmeter are connected serially to the
circuit, this causes the current to flow through the circuit being
tested. If the meter’s test leads are connected at a point a & b in the
circuit, then the amount of the current in the meter coil will depend
on the resistance of the meter and the total resistance of the
resistors R1&R2.
• The addition of R1 and R2 raises the total series resistance,
decreasing the current and therefore decreasing the needle
deflection. The needle then comes to rest at a scale for indicating
the combined resistance of the R1 and R2.

EXPLODERS
• There are three types of exploder used in Indian mines i.e.
1. Magneto (or Dynamo) Exploder
2. Battery condenser Exploder and
3.Condenser dynamo Exploder
• In general there are two types of exploders namely, permitted type and
general (or non-permitted ) type used in the blasting operations.
• The permitted type exploders are used in underground locations where
flammable gas hazard may be present. Such locations are coal mines, non-
coal mines or underground civil works. Such exploders shall be approved by
statutory authority before use. The general type exploders are used in the
open areas where gas hazard may not be present.It is essential that the
exploder must fire the specified number of detonators.

CONSTRUCTION OF EXPLODERS
The exploder can be so constructed that:
• It shall be operated by a removeable handle or key
• The firing circuit is made or broken either automatically or by the
operation of handle or key or a push button switch, and
• No residual energy is left at the terminals after the completion of
operation sequence. ’
The permitted type exploders shall have suitable devices so as to
terminate the output energy pulse within four milliseconds of the
operation of the firing switch/key.
All exploders must incorporates circuit testing facilities to be used in
mines.

REQUIREMENTS FOR CIRCUIT
CONTINUITY TESTER
• Before firing the detonators with exploder, it is required to check the
continuity of the firing circuit with a resistance measuring device. It is better
to measure the circuit resistance to be of the order for the particular number
of shots. If the resistance is high and the firing is continued there may be
misfire. To avoid this, there should be a check over the measurement of
continuity and circuit resistance.
• While doing so the testing current in the circuit should be as low as
possible not exceeding 50 mA so that the detonators may not be fired while
checking the continuity of shots. The circuit of the continuity tester shall be
intrinsically safe.
• The instrument shall be so designed and constructed as to be incapable
of firing even a single detonator in the circuit.
• The construction and the enclosure of the continuity tester shall meet
the similar requirements as applicable to exploders.

Trouble Shooting
• Before firing, the resistance of the coupling is checked once more. In most
cases of error, the resistance is too low (detonators have not been
connected) or infinite, ∞ (circuit broken).
• For locating the fault, the circuit is divided into two parts. Both parts are
measured, and the faulty section is divided again. This is continued until the
fault is localized.
• For Earth Leakage check for any uninsulated part of the detonator
cables.When heavy-duty blasting mats are used, earth leakage must be
checked with a special earth leakage detector after each mat is set in place.
• In underwater blasting or other blasting that involves the risk of earth
leakage, control
measurements must be conducted with an earth leakage detector.
• If a leak is detected in the round, the fault is localized similar to a resistance
measurement.

MARKING ON EXPLODER
* Each exploder shall be marked with the following information on its
rating plate:
a) Name of the manufacturer;
b) Type of exploder;
c) Rated load resistance;
d) Current in amperes;
e) The rated number of detonators, the exploder would fire in
series and/or in parallel
f) Any other marking indicating the approval from the statutory
authority.
* Each exploder shall be provided with an instruction manual giving
necessary operating instructions, illustrations, testing instructions, and
safety precautions to ensure proper operation and maintenance of the
exploder.

Shunting and Testing
Requirement of shunting:
All the electric detonators after manufacturing are shunted at the free
end by joining the leg wires.This is done due to the fact that by joining the
leg wires a low resistance path is developed which prevent the flow of
current through the bridge wire of fuse head. This is because by joining the
wires both wires are at same potential which doesn't allow etraneous current
to flow into the detonator.This shunt is removed at the blasting site when the
detonator is connected to the blasting circiut.
Testing before blasting:
The continuity and resistances of all the individual detonators and the
blasting circiut as well are to be checked with the help of a blasting
galvanometer.The testing should be done before the priming and after the
stemming.While initiating electric detonators current leakage from the
blasting circiut must be prevented.The bare wires should not be come in
contact with each other and it is adviced to provide insulation at such places
to avoid short circuit and leakage to the ground.

Firing The Circuit
• Check the blasting machine's type and plate to verify that the no. of
detonators in the circuit or the total resistance does not exceed the
values specified for the machine.
• Follow the instructions for use of blasting m/c given on its plate.
• Clear the blast site ,connect the main to the terminal screws, and
insert the key to the exploder .The neon light will glow deep orange
• Sound the necessary warning signal and press the fire button.
• Disconnect the key from blasting m/c after firing.

Regulations relating to electric shot firing:
• all the shots shall be fired with the help of asuitable shot firing apparatus
• shot firing apparatus should be so constructed that it can only be operated
by a handle or plug and the plug shall be removed as soon as the shot has
been fired.
• if the apparatus fails to fire the shots in a properly connected circuit,the
blaster shall return that apparatus to the manager.
• no current from from signalling ,lighting or power circuit shall be used for
firing the shots
• the cable to the shot firing apparatus shall be connected last.
• only competent persons or certified shot firer should fire the shots.

Electric detonation system

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