CLC/TR 50426:2004
(Main)Assessment of inadvertent initiation of bridge wire electro-explosive devices by radio-frequency radiation - Guide
Assessment of inadvertent initiation of bridge wire electro-explosive devices by radio-frequency radiation - Guide
This standard provides guidance in assessing the potential ignition hazard from the inadvertent extraction of energy from electromagnetic fields, propagated from communication, radar or other transmitting aerials to plant where a potentially flammable atmosphere may be present. The standard is not concerned with similar hazards arising from electromagnetic fields generated by other means, such as electric storms, electricity generating installations or other radiating electrical equipment, nor is it concerned with any hazard arising within telecommunications or oter electronic equipment
Leitfaden zur Verhinderung des unbeabsichtigten Auslösens einer Zündeinrichtung mit Brückendraht durch hochfrequente Strahlung
Evaluation de la création par inadvertance de dispositifs électro-explosifs par pont métallique, par rayonnement de radiofréquence - Guide
Ugotavljanje nenamernega radiofrekvenčnega proženja elektroeksplozivnih naprav - Vodilo
General Information
Standards Content (Sample)
SLOVENSKI SIST-TP CLC/TR 50426:2005
STANDARD
april 2005
Ugotavljanje nenamernega radiofrekvenčnega proženja elektroeksplozivnih
naprav - Vodilo
Assessment of inadvertent initiation of bridge wire electro-explosive devices by
radio-frequency radiation – Guide
ICS 13.230; 29.260.20 Referenčna številka
© Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno
TECHNICAL REPORT CLC/TR 50426
RAPPORT TECHNIQUE
TECHNISCHER BERICHT December 2004
ICS 13.230; 29.260.20; 33.060.20
English version
Assessment of inadvertent initiation of bridge wire
electro-explosive devices by radio-frequency radiation –
Guide
Evaluation de la création Leitfaden zur Verhinderung
par inadvertance de dispositifs des unbeabsichtigten Auslösens
électro-explosifs par pont métallique, einer Zündeinrichtung mit Brückendraht
par rayonnement de radiofréquence – durch hochfrequente Strahlung
Guide
This Technical Report was approved by CENELEC on 2004-08-28.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia,
Spain, Sweden, Switzerland and United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2004 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TR 50426:2004 E
Foreword
This Technical Report was prepared by the Technical Committee CENELEC TC 31, Electrical apparatus for
explosive atmospheres - General requirements.
The text of the draft was submitted to the formal vote and was approved by CENELEC as
CLC/TR 50426 on 2004-08-28.
___________
– 3 – CLC/TR 50426:2004
Contents
Introduction. 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Symbols and abbreviations. 8
4.1 Modulation codes . 8
4.2 Polarization codes . 9
5 General considerations . 9
5.1 Radio-frequency hazard . 9
5.2 Philosophy of the systematic method of approach . 10
5.3 Responsibility for making the hazard assessment. 10
5.4 Recommended practices for radio silence in offshore operations. 11
6 Transmitters and transmitter output parameters. 11
6.1 Types of transmitters. 11
6.2 Frequency range . 11
6.3 Transmitter output power. 12
6.4 Antenna gain . 12
6.5 Modulation factors . 12
6.5.1 General . 12
6.5.2 Frequency modulation (FM) . 12
6.5.3 Amplitude modulation (AM). 12
6.5.4 Single sideband (SSB) operation . 13
6.5.5 Pulsed radar. 13
7 Circuits for blasting and well perforation . 13
7.1 General . 13
7.2 Typical blasting circuit layouts. 14
7.2.1 General . 14
7.2.2 Extended line blasts . 14
7.2.3 Benching or 3-dimensional blasts . 15
7.2.4 Multipattern blasts . 16
7.2.5 Shaft sinking. 16
7.2.6 Demolition work. 17
7.3 Circuits formed during well-perforating using wireline . 18
8 Electro-explosive devices . 21
8.1 General . 21
8.2 Commercial EED . 21
8.3 EED hazard threshold . 23
8.4 Common mode operation . 24
9 Methods of assessment for determining potential RF ignition hazards on a site
where EED are used . 24
9.1 General . 24
9.2 Basis of the theoretical assessments for land sites. 28
9.2.1 General . 28
9.3 Initial assessment for land sites. 36
9.3.1 Initial assessment of the risk from a particular transmitter site . 36
9.3.2 Initial assessment for a particular site using EED . 36
9.4 Full assessment procedure for land sites. 37
9.4.1 Procedure. 37
9.4.2 Information to be obtained . 37
9.4.3 Calculation of effective field strength . 38
9.5 Safe field strength . 45
9.5.1 General . 45
9.5.2 Single EED. 45
9.5.3 Single EED with extended leading wires . 46
9.5.4 EED in typical quarry/demolition firing circuits . 46
9.5.5 EED in well-perforating drilling operations . 61
9.6 Multiple transmissions . 63
9.6.1 General . 63
9.6.2 Multiple-transmission assessments for quarry/demolition sites . 64
9.6.3 Multiple transmission assessments for well-perforating drilling operations . 65
9.7 Assessments for offshore drilling operations. 68
10 Practical on-site testing . 68
11 Safety procedures . 69
11.1 General procedures. 69
11.2 Alternative means of firing. 69
12 Special applications . 69
12.1 Semi-permanent installations . 69
12.2 Flammable hazard situations . 69
12.3 Use of transmitters in mines and quarries. 70
12.4 Safety in transit . 70
Annex A (informative) Extraction of energy from the electromagnetic field . 71
Annex B (informative) Measurement of electromagnetic fields. 72
Annex C (informative) Sources of information and addresses of advisory bodies - UK ONLY . 76
Annex D (informative) Electromagnetic radiated fields and examples of radiating antennas
and unintended receiving antenna characteristics. 78
Annex E (informative) The effective resistance of the leading wires of an EED. 86
Annex F (informative) Derivation of minimum distances of safe approach for Table 2 and Table 3 . 92
Annex G (informative) Ground-wave propagation (vertical polarization): calculation
of field strength. 94
Annex H (informative) Worked examples to demonstrate the effects of antenna gain. 96
Annex I (informative) The effects of leading wire resistance, safety resistors and the use of
EED with different characteristics . 97
Annex J (informative) Derivation of Figure 12a) to Figure 12g) for EED alone incorporating
the resistance of leading wires and safety resistances. 100
Bibliography. 101
– 5 – CLC/TR 50426:2004
Introduction
Electromagnetic waves produced by radio-frequency (RF) transmitters (e.g. radio, television and radar)
will induce electric currents and voltages in any firing circuit including leading wires of the electro-
explosive device (EED) on which they impinge. The magnitude of the induced current and voltages
depends upon the configuration of the firing circuit and leading wires relative to the wavelength of the
transmitted signal and on the strength of the electromagnetic field. If the induced current which is
transferred to the EED is in excess of the no fire current then the EED could initiate. This European
Technical Report provides a systematic approach to assist transmitter operators, quarry managers and all
others concerned with a logical method for the assessment and elimination of the initiation of EED by RF.
The assessment procedures contained in this European Technical Report are based on measurements of
the powers and current that can be extracted from typical firing circuits and leading wires and on the
physical electrical parameters of various types of EED.
1 Scope
This European Technical Report provides guidance on assessing the possibility of inadvertent extraction
of energy from an electromagnetic field propagated from radio frequency (RF), radar or other transmitter
antennas and the coupling of this energy to an electro-explosive device (EED) in a manner capable of
causing initiation. The frequency range covered by this European Technical Report is 9 kHz to 60 GHz.
This European Technical Report only applies to bridge-wire devices which are directly initiated by radio
frequency current and does not apply to special detonators, for example, electronic detonators. It does not
cover the similar hazard arising from electromagnetic fields generated by other means, for example
electric storms, electricity generating plant or power transmission lines.
This European Technical Report does not apply to the following equipment:
− air bag igniters for automotive applications (including the igniters before they are fitted);
− special pyrotechnic devices;
− pyromechanisms;
− igniters for fireworks;
− special military devices;
− special safety equipment.
NOTE The methods of assessment from 9 GHz to 60 GHz are based on extrapolation of data for frequencies below 9 GHz.
2 Normative references
No normative references are made in this standard.
3 Terms and definitions
For the purposes of this European Technical Report the following terms and definitions apply.
3.1
duty cycle
product of pulse duration (in seconds) and the pulse repetition frequency (in pulses per second)
3.2
electro-explosive device (EED)
one shot explosive or pyrotechnic device initiated by the application of electrical energy
NOTE EED is used to refer to either a single electro-explosive device or several devices, to comply with general practice within the
industry.
3.3
hazard
potential source of danger to life, limb or health, or of discomfort to a person or persons, or of damage to
property
3.4
safe distance
distance outside which it is considered that there is no potential hazard
– 7 – CLC/TR 50426:2004
3.5
no-fire energy/power/voltage/current
maximum energy or steady state power/voltage/current that will not cause initiation of the most sensitive
EED of any particular design
NOTE The manufacturing tolerances permitted during the production of EED will cause normal statistical variation in their firing
characteristics. The most sensitive EED permitted by this variation sets the appropriate no-fire level, which is generally accepted as
a probability no greater than 0,01 %, with a confidence level of 95 %.
3.6
round of charges (shot)
one or more primed explosive charges or shots, for example main charge, primer (if used) and detonator
3.7
toe shot
shot designed to clear the foot of a face, for example a quarry face
3.8
hazard area
area, of any shape, containing the transmission source or sources and within which the radiation
magnitude exceeds the designated hazard threshold
3.9
hazard threshold
mean power flux density or field strength that would permit only a negligible probability of EED initiation
3.10
exploder
means whereby a round of charges (shot) is fired electrically
3.11
equivalent isotropically radiated power (EIRP)
product of the power supplied to the antenna and the antenna gain in a given direction relative to an
isotropic antenna (absolute or isotropic gain)
3.12
effective field strength
value of electric field strength due to a single transmitter which is derived from the transmitter
characteristics, modulation factors (see 6.5) and distance, and is used for the calculation of extractable
power
3.13
antenna gain
gain produced by an antenna concentrating radiation in a particular direction
NOTE 1 The gain of an antenna is always related to a specified reference antenna.
NOTE 2 The gain, G, of an antenna in a particular direction is given by the equation:
R
G =
A
where
R is the power in Watts, W, that should be radiated from the reference antenna;
A is the power in Watts, W, that should be radiated from the given antenna to give the same field strength at a fixed distance in that
direction.
NOTE 3 The gain, which is often expressed in logarithmic form, is stated in decibels.
3.14
far field
region, distant from the transmitter, in which the field strength is inversely proportional to distance in the
absence of ground reflection
NOTE The inner limit of the far field is generally regarded as the distance d from the transmitter defined as follows. For frequencies
up to and including 30 MHz, d = 8H /λ where H is the height of the top of the antenna above ground and λ is the wavelength. At
d W λ W
frequencies above 30 MHz, = 2 / where is the width of the antenna.
3.15
near field
region close to the transmitter, which lies within the far field region
NOTE In the near field region the dependence of the field strength on distance is complex and mutual coupling effects can also
affect the value of extractable power.
3.16
leading wire resistance
total d.c. resistance of the leading wires excluding that of the EED itself
3.17
bridge wire resistance
internal d.c. resistance of the EED alone
3.18
safety resistor
resistor or resistors placed within the casing of an EED in order to desensitize it to the external electrical
environment
4 Symbols and abbreviations
4.1 Modulation codes
AM Amplitude-modulated speech or music transmission. Carrier power quoted
MCW Amplitude-modulated tone transmission. Carrier power quoted
TV Amplitude-modulated video transmission. Peak power quoted
R ( ) Pulse-modulated radar transmission. Peak power quoted. The number in brackets indicates
the pulse duration in s where known
FM Frequency modulation
FSK Frequency shift keying
GFSK Gaussian frequency shift key modulation
SSB Single sideband transmission. Peak envelope power quoted
CW Continuous wave
MSK Minimum shift keying
GMSK Gaussian minimum shift keying
– 9 – CLC/TR 50426:2004
CDMA Code division multiple access.
PCM Pulse code modulation
PSK Phase shift keying
PM Phase modulation
DQPSK Differential quadrature phase shift keying.
4.2 Polarization codes
V Vertical polarization.
H Horizontal polarization.
V/H Either vertical or horizontal polarization, or both simultaneously.
5 General considerations
5.1 Radio-frequency hazard
For radio-frequency hazard assessment detailed consideration should be taken of the conditions that have
to be satisfied simultaneously for the hazard to exist. These are as follows:
a) an electromagnetic field of sufficient intensity;
NOTE 1 An electromagnetic field of sufficient intensity may be generated by a fixed/mobile or portable transmitter, the
magnitude of the field depending upon the transmitted power, the antenna gain and the proximity of the site under
consideration.
NOTE 2 Intense electromagnetic fields are also generated by the intentional radio frequency sources in industrial, scientific
and medical (ISM) equipment. Field strengths in the order of 10 V/m may be present in the near vicinity of the equipment.
Typical characteristics of industrial equipment are:
2,5 GHz to 10 kW
915 MHz to 100 kW industrial microwaves
27 MHz to (10 to 50) kW
welding or drying techniques
13,56 MHz to (10 to 50) kW
b) a means of extracting power from the electromagnetic field;
c) an EED in a situation such that it can accept power or energy from the extracting device.
The procedures contained within this European Technical Report for assessing the presence of a potential
hazard are based on a number of reasonable “worst-case” assumptions. They are based upon both
experimental evidence and engineering judgement. Taken together these give a substantial margin of
safety due to the extremely low probability of concurrence of all the worst-case factors. No extra, arbitrary,
safety factors are included.
All conducting materials behave as receiving antennas, but the magnitude of the induced current and
voltage depends upon the circuit configuration, that is, whether the EED is in a firing circuit or by itself.
Experience gained indicates that for frequencies below 7 MHz it is the loop firing circuit which is the most
sensitive whereas for higher frequencies it is the EED and its leading wires alone. The behaviour of these
circuit configurations is described in Clause 7.
5.2 Philosophy of the systematic method of approach
A potential hazard only exists in relatively few locations, with only a small number of incidents reported
that are possibly attributable to this cause.
This European Technical Report is based on a series of graded assessments, each requiring a
progressively more detailed analysis.
The initial assessments are designed to eliminate from further consideration those locations where it is
highly unlikely that a hazard exists. They are based on “realistic worst-case” estimates of the minimum
distance of safe approach around different classes of transmitter within which a hazard might exist from
the presence of a particular circuit configuration in this area.
For land based operations, if the initial assessments given in 9.3 indicate that a hazard might exist, the full
assessment procedure given in 9.4 should be followed. For offshore operations the assessment in 9.7
should be followed. These provide a method of computing the field strength available from the
transmitters, based on detailed information about the transmitters and their location relative to the site.
The calculated field strength should then be compared to those that are required to initiate an EED in
various circuit configurations, whether in a loop firing circuit or with the leading wires acting as a dipole.
When this systematic assessment procedure is followed, it will quickly become apparent whether the
available information is adequate for an assessment to be made with a high degree of confidence or
whether additional information is required from practical on-site measurements (see Clause 10 and
Annex B). If doubt exists, then expert opinion should be sought (see Annex C).
The assessment procedures recommended in Clause 9 apply generally to most circumstances. For
offshore and land based well-perforating operations the special considerations described in 7.3, 9.5.5 and
9.6.3 should be taken into account.
5.3 Responsibility for making the hazard assessment
The radio-frequency (RF) environment is becoming increasingly severe, with the proliferation of
transmitting sources and increased transmitter powers and the exploitation of new techniques.
NOTE 1 Legislation, (for example in the UK see [1]) requires that employers safeguard both their employees and others who may
be placed at risk by their activities. Hence, both operators of RF transmitters and users of EED have a responsibility to ensure safe
operation.
NOTE 2 Particular locations such as mines and quarries may exist where additional responsibilities are placed on the owners and
managers.
Operators of a proposed site in which EED are to be used should request details from the transmitter
operators about relevant transmitters in the locality of the site. The transmitter operators should include
details of transmitters for broadcast, commercial, military, air traffic and emergency services such as
police, fire and ambulance. The site operator should then use the assessment procedures given in this
European Technical Report, if necessary in consultation with the transmitter operators concerned.
– 11 – CLC/TR 50426:2004
Similarly, an operator of a proposed new (or altered) transmitter should contact all operators of sites
where EED are used within the minimum distance of safe approach for the transmitter, and use the
procedure given in this European Technical Report to assess the potential hazard at each location.
Where both the site and the transmitter already exist but an assessment is required, the site operator
should be held responsible for ensuring that the assessment is made. If for some reason relevant
information cannot be made available to the body responsible for the assessment, the responsibility for
having the assessment carried out should be assumed by the body unable to release the necessary
information.
NOTE 3 As an aid to those who need to make a hazard assessment but do not have the necessary technical resources, a list of
sources of information and specialist organizations capable of providing consultation or test facilities is given in Annex C.
5.4 Recommended practices for radio silence in offshore operations
The position adopted by many offshore operators has been to switch off all transmissions from the
installation during the surface preparation of the explosive tool until its immersion in the well at 70 m below
sea bed level. At this point services would be restored until the explosive tool was returned to a similar
depth on the upward journey when all services would again be cut off. Following its removal from the well
and inspection to ensure its safe condition, services would be restored provided no further explosive
handling was to take place.
However, too great a reliance on all-embracing curtailment of services can itself present a potential
hazard to structures which employ radio communication for safety reasons and as an integral part of
product transportation systems (pipelines). The identification of these difficulties has highlighted the need
for the hazard to be more accurately quantified in order to minimize the disruption of other necessary
operations and to avoid the creation of further potential hazards.
6 Transmitters and transmitter output parameters
6.1 Types of transmitters
This clause provides information on various types of transmitter and transmitting systems. This information
is necessarily rather brief for certain types of radar and other military equipment but basic details are given
and further information may be sought from the specialist organizations listed in Annex C. Typical types of
antenna installations are shown in Figure D.1.
6.2 Frequency range
The main frequency range covered is 9 kHz to 60 GHz. The types of transmitter considered include the
following:
a) radio and television broadcast transmitters in specific bands in the range 0,15 MHz to 1 000 MHz;
b) fixed and mobile transmitters for communication purposes, private, commercial and amateur, in
specific bands above 0,4 MHz and for military use above 0,15 MHz;
c) radar, in specific bands at 220 MHz, 600 MHz and above 1 GHz;
d) navigational equipment, non-directional beacons, etc., from 9 kHz upwards.
6.3 Transmitter output power
Transmitter output power from several watts up to megawatts may be encountered depending on the
frequency range and the application. The method of specifying the power varies, which is significant for
the hazard assessment when highly directional antennas are in use and when considering different
modulation systems. In general, values are specified in the technical documentation for either the carrier
or peak power output from the transmitter together with the antenna gain, although the product of the two
is often quoted in the technical documentation to give the equivalent isotropically radiated power (EIRP).
6.4 Antenna gain
The reference antenna is often an isotropic antenna that radiates uniformly in all directions. Although this
is a purely hypothetical concept it is nevertheless very useful for reference purposes. When the gain of an
antenna relative to an isotropic reference antenna is stated in decibels it is denoted by dBi. In practice, the
gain of an antenna is often expressed relative to a half-wave dipole which itself has a gain of 1,64 (or
2 dB) relative to an isotropic antenna. In special circumstances other reference antennas may be used, for
example a short monopole. It is therefore important that the reference antenna is correctly specified. The
maximum gains for typical antennas are included in Table D.1, Table D.2 and Table D.3.
6.5 Modulation factors
6.5.1 General
Most transmissions are modulated in order to convey information or to enable them to carry out specific
tasks (for example, radar). The characteristics of the principal modulating systems are described in 6.5.2
to 6.5.5. A modulation factor, m, is necessary for calculating the effective field strength E (see 9.4.3).
Modulation factors are listed in Table 4 for different types of modulation.
NOTE Radio-frequency transmissions may be unmodulated, in which case the radiated power is constant and the quoted power in
the technical documentation should be used for assessment purposes. Such transmissions are sometimes referred to as continuous
wave (CW).
6.5.2 Frequency modulation (FM)
The frequency of the transmission is varied to carry the information but the output power remains constant
as with CW transmissions. Frequency shift keying (FSK) and gaussian frequency shift keying (GFSK),
minimum shift keying (MSK) and gaussian minimum shift keying (GMSK), are forms of frequency
modulation. Phase shift keying (PSK) and phase modulation (PM) are treated similarly in that no
allowance for modulation is necessary since the output power is not affected by the modulation.
6.5.3 Amplitude modulation (AM)
6.5.3.1 General
The amplitude of the transmission is varied to carry the information. The latter may consist of speech or
music or coded transmission or it may be a television picture waveform.
6.5.3.2 Speech and music
When speech and music are transmitted, the power quoted in the technical documentation is that of the
unmodulated or carrier transmission but the mean power at the modulation peak may be up to 50 %
greater when the speech or music is at its loudest volume. Since EED respond to mean power, it is
necessary to make some allowance for the power increase when making an assessment; this may be
done by assuming an effective field strength which is greater than that due to the unmodulated
transmission. As a result of experience gained with amplitude modulated broadcast transmitters, a
modulation factor, m, of 1,15 should be used for calculations of effective field strengths.
– 13 – CLC/TR 50426:2004
6.5.3.3 Coded transmissions
A version of amplitude modulation used for Morse and other coded transmissions is known as modulated
continuous wave (MCW). For modulated continuous wave, whereby the carrier is fully modulated by a
continuous tone, a modulation factor, m, of 1,22 should be used.
6.5.3.4 Television transmissions (TV)
For television transmissions the peak power is usually quoted in the technical documentation but in this
case no allowance for modulation is necessary, because the mean power is approximately equal to the
peak power.
6.5.4 Single sideband (SSB) operation
For single sideband (SSB) operation the peak envelope power is usually quoted in the technical
documentation and a value of 0,71 for the modulation factor, m, should be used.
6.5.5 Pulsed radar
Pulsed radar consists of pulses transmitted at frequent intervals. The transmissions are characterized by a
P P t n
peak power, , a mean power, , a pulse duration (in s) and a pulse repetition rate of pulses per
o m
second. These are related by the equation:
P
m
P =
o
nt
where
nt is the duty cycle.
Since the interval between pulses is less than the thermal time constant of bridge wire EED, the mean
power, P , should be calculated and used for assessment purposes.
m
7 Circuits for blasting and well perforation
7.1 General
Voltage and current may be induced in a metal object or wire in an electromagnetic field. When sufficient
voltage and current is transmitted to the initiating device of an EED such as the bridge wire, the EED
might initiate. The source of RF power for the EED is in effect an antenna situated in the incident field.
The antenna configuration may be formed by the leading wires of the EED itself or by the blasting circuit.
For analogy with simple antenna types, the leading wires of an EED may be considered as a dipole
antenna when these wires are separated to form a blasting or well-perforating circuit. After connection the
circuit may be considered as a loop antenna.
For the optimum transfer of voltage and current it is essential for the antenna to be tuned. However, even
for large loop circuits, untuned loops can carry voltage and current.
Experience gained indicates that below 7 MHz it is the loop which is the most sensitive circuit, whilst at
higher frequencies it is the EED and its leading wires alone.
7.2 Typical blasting circuit layouts
7.2.1 General
Whilst in theory an infinite number of blasting circuit layouts are possible, in practice they may be divided
into five broad types:
a) extended line blasts;
b) benching or 3-dimensional blasts;
c) multipattern blasts;
d) shaft sinking;
e) demolition.
These layouts are briefly considered in terms of methods of connecting up the shotholes one to another,
of connecting up the whole blast to the shotfiring cable, and of overall dimensional variations that might
reasonably be expected to occur in the field.
Figure 1, Figure 2, Figure 3 and Figure 4 show the extremes of circuitry that are known to occur, although
the vast majority of practical circuits lie between the two extremes.
7.2.2 Extended line blasts
The simplest version of this blast pattern is a row of single holes along a quarry face. For example,
between two shotholes and 100 shotholes the layout would be as in Figure 1.
2 m to 200 m
2 m to 200 m
Key
1 Shot firing cable to exploder (30 m to 100 m)
a) Shotfiring cable connected at end of a row b) Shotfiring cable connected anywhere along the row
Figure 1 — Typical single extended line blasting circuits (plan view)
In Figure 1 the holes, from 0,5 m to 60 m deep, may be either top or bottom primed, with essentially one
or two detonators on the surface or from one to 10 detonators down the holes. In the case of Figure 1b)
the connection to the shotfiring cable may be made anywhere along the line of holes. Other situations
might arise where more than one row of holes might be connected up to form a single blasting circuit,
giving rise to patterns such as those shown in Figure 2.
0 m to 2 m
0 m to 100 m
– 15 – CLC/TR 50426:2004
0 m to 300 m
0 m to 200 m
2 rows to 10 rows
1 1
Key
1 Shot firing cable to exploder (30 m to 300 m)
a) Quarry-face blasting b) Trench blasting
Figure 2 — Typical multiple extended line blasting circuits (plan view)
NOTE 1 Figure 2a) corresponds to a situation where retreating faces are being blasted, and Figure 2b) to trench blasting.
NOTE 2 All the configurations shown in Figure 1 and Figure 2 may be on horizontal ground or on ground inclined at angles up to
about 30°.
In blasts of the type shown in Figure 2a) the number of rows can vary from two to 30. The circuits will be
series connected but different connecting configurations may be adopted. The detonators may be on the
surface (one or two per hole) or down holes from 0,5 m to 60 m deep (one to 10 detonators per hole).
7.2.3 Benching or 3-dimensional blasts
Possible situations in which a benching or 3-dimensional blast pattern may be used are those in which a
single line quarry blast is coupled with one or more toe shots on the quarry floor, or those in which two
single line blasts are being done on “benches” at different levels. A composite pattern is shown in
Figure 3.
1 m to 100 m
0 m to 15 m
Key
1 Shot firing cable to exploder 4 Face
2 Toe shots 5 Crest
3 Floor 6 Top
Figure 3 — Quarry blast including toe shots
As shown in Figure 3 the floor and top may be in the horizontal plane, the face in the vertical plane, or the
whole system may be set on an inclined plane, for example on a hillside. Again, top or bottom priming
may be used in the quarry face part of the blast with one or two detonators per hole for top priming and
from one to 10 for bottom priming.
7.2.4 Multipattern blasts
This situation might arise when more than one of the previously described blast patterns are set up in
close proximity to each other and so may conveniently be connected together and fired as a single round.
A typical case might be where two faces at right angles to each other are being blasted. Where separate
blast patterns are being connected together to form one circuit, the overall distances of wiring involved
might correspond to roughly a square or rectangle with sides up to about 400 m in length. Again the
terrain may be level or inclined.
7.2.5 Shaft sinking
Blasting does not begin in shaft sinking operations until the soil has been removed and the bedrock
exposed, normally about 10 m below the surface. The shaft diameter can vary from 1 m to 10 m and the
depth can be down to 1 200 m. The blasting system commonly used is one in which the detonators are
connected in parallel to busbars which form concentric circles. Up to five such circuits might be formed,
each within the other in the form of pairs of decreasing diameter circles. The busbars are connected
together to form one parallel circuit which is then connected to two hanging cables. These hanging cables
in turn are connected to two cables on the surface which lead to the firing point, between 50 m and 200 m
from the shaft. Both the surface cables and the hanging cable may be some distance apart, the only
constraint on their relative positioning being the connections at the ends.
0 m to 200 m
1 m to 50 m
1 m to 10 m
– 17 – CLC/TR 50426:2004
7.2.6 Demolition work
The pattern of the blasting circuit to be used in demolition will depend on the structure being demolished.
Any circuit is likely to contain both vertical and horizontal components. Where large buildings are to be
demolished, it is probable that horizontal series connected circuits will be laid on more than one level and
then connected to vertical wires leading to the shot firing cable. The sub-circuits may be connected using
series or parallel connections. The vertical heights involved may range from 0 m to 100 m and the
dimensions of the horizontal loops may range from 0 m to 50 m by 0 m to 100 m, treated as being
rectangular. An example of a multilevel blast in which the series connected loops on the various levels are
connected in parallel to the vertical wires is shown in Figure 4. The horizontal loops may be at risk from
horizontally-polarized transmissions, especially as field strength usually increases with increasing height
above ground. The vertical wires leading to the shot-firing cable may be susceptible to vertically-polarized
transmissions, but the risk can be minimized by spacing them close together as shown in Figure 4. Expert
advice should always be sought when demolition blasting.
0 m to 100 m
0 m to 50 m
P
P
P
P
Key
1 P , P , P and P are the perimeters of the loop circuits
1 2 3 4
2 Shot firing cable to exploder
3 busbar
Figure 4 — Multilevel series-in-parallel demolition blast
0 m to 100 m
7.3 Circuits formed during well-perforating using wireline
Explosives initiated by EED are used for well perforation in both offshore and land based operations. The
EED are initiated by electric current transmitted through the wireline which connects the explosive
downhole tool to the controlling logging cabin. The method is well tried and convenient. However, a
potentially hazardous situation exists since there is a risk of premature detonation caused by stray
currents induced from sources other than the intended means of providing the firing current impulse.
Currents may be induced by RF radiation which may emanate from local radio equipment (on board or
within the site) or from external sources such as transmitters based on land, ship or aircraft. It should be
noted that the number of potential hazard sources is much greater with land based operations.
Figure 5 shows the layout of the wireline cable when it is connected to the explosive tool and about to be
lowered into the well. It runs from the cable drum up to a sheave in the derrick and down to the well head.
Under fault conditions the EED could be connected in series with the cable as shown in Figure 5. It will be
seen that the arrangement of a land-based drilling rig is similar to an offshore platform, the main difference
being that the site is level, with the metal deck replaced by ordinary ground. The size of the loop formed
by the wireline cable is not very different from that of an offshore platform but the loop is completed by an
earth wire between the derrick and the cable drum. The presence of a single wire rather than a metal
platform tends to increase the RF resistance of the loop. This in turn introduces a margin of safety if the
methods used for offshore rigs are applied as they stand.
– 19 – CLC/TR 50426:2004
5 6
20 m to 50 m 10 m
10 m
20 m to 50 m
Key
1 Cable drum 5 Deck
2 Wireline cable 6 EED
3 Sheave 7 Wellhead
4 Derrick 8 Earth connection
Figure 5 — Layout of wireline cable for offshore and land based operations
2 m to 10 m 2 m to 10 m
15 m to 30 m 15 m to 30 m
It has long been recognized that a hazard may occur during such operations and two stages have been
identified during the operations when such a hazard may exist. The first is when an EED is removed from
its package and is prepared for connection to the firing circuit. During this time, the lead wires are splayed
out and may form a dipole receiving antenna which can accept sufficient RF energy from incident radiation
to initiate the EED. The second is when the EED is connected to the firing circuit. Under fault conditions,
the wireline itself, which forms a loop or monopole receiving antenna, may similarly deliver sufficient RF
energy to the EED to cause premature initiation. The various stages of a typical explosives operation and
the extent of the hazard presented are as follows.
Stage 1: Preparation of the explosive
The assembly of the explosive material into its carrier may take place at the
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