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mS [ 5EFiRI\ 8989 1 PLE SE O NOT REMOVE FROM LIBR RY LBureau of Mines Report of Investigations/1985

Electrostatic Sensitivity Strengthand No-Fire Current of Short-DelayDetonatorsBy T S Bajpayee, R J Mainiero, and J E Hay

UNITED STATES DEPARTMENT OF THE INTERIOR O J 7 t ~MINES 75TH A

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Report of Investigations 8989

Electrostatic Sensitivity Strengthand No-Fire Current of Short-DelayDetonatorsBy T. S. Bajpayee R J Mainiero and J E Hay.

UNITED STATES DEPARTMENT OF THE INTERIORDonald Paul Hodel SecretaryBUREAU OF MINESRobert C. Horton Director

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Library of ongress Cataloging in Publication Data

Bajpayee, T. SElectrostatic sensitivity, strength, and no-fire current of short-delay

detonators.Report of investigations / Bureau of Mines; 8989)

Bibliography: p. 9Supt. of Docs. no.: I 28.23: 8989.1. Electric detonators-Safety measures. 2. -Coal mines and mining

Safety measures. I. Mainiero, Richard J. II. Hay, J. ,Edmuud. HI. Title. IV. Series: Report of investigations United States. Bureau ofMines) ; 8989.

TN23.U43 [TN279] 6228 [622 .334] 85-600245

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CONTENTS

Abstract....................................................................... 1I n t r oduc t i on 2Electrostat ic s e n s i t i v i t y 2Strength of detonation 2Shock energy............................................................... 3ubble energy.............................................................. 3No fire current.............................................................. 4Experimental setup and tes t p r o c e d u r e 5Electrostat ic s e n s i t i v i t y 5Strength of detonation 5No fire current 0 0 7Results and discussion 7Electrostat ic s e n s i t i v i t y 7Strength of detonation 7No fire current 0 . . . . . 9

References. 9

12.3.

1.2.3.4.5.

ILLUSTRATIONSSchematic diagram of electrostat ic sensi t ivi ty tes t apparatus Schematic diagram of experimental setup for detonation energy measurement Schematic diagram of no fire current measurement

TABLESStored energy levels of designated capacitors at lO kV potential Electrostatic sensi t ivi ty of short delay detonators Bubble and shock energies of short delay detonators Dispersion of bubble and shock energy data No fire current levels of short delay detonators

67

58889

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UNIT OF ME SURE BBREVI TIONS USED IN THIS REPORTmillisecondampere ms

t foot ]IF microfaradg gram ]l microsecondin inch Pa pascalJ joule pct percentkV kilovolt pF picofaradm milliampere s secondmJ milli joule V volt

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ELECTROSTATIC SENSITIVITY STRENGTH AND NO-FIRE CURRENTOF SHORTDELAY DETONATORS

By T. S Bajpayee, 1 R J Mainiero,2 and J E. Hay 3

ABSTRACTThe Bureau of Mines evaluated short-delay coal mine detonators manu-factured by three domestic companies for electrostat ic sensit ivity ,strength, and no-f i re current level . Electrostat ic sensi t ivi ty wasstudied in pin-to-pin and pin-to-case modes employing lO kV potential .Strength of detonation was measured in the Bureau s underwater tes t fa

ci l i ty No-fire current was measured using a constant-current powersupply uni t . Test procedures and experimental data are included.

Mining engineer.2Supervisory chemical engineer.3Supervisory research physicis tPittsburgh Research Center, Bureau of Mines, Pittsburgh, PA

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2

INTRODUCTIONData about safety characteristics ofshort-delay detonators are essential forsafe blasting practices in undergroundcoal mines. These characteristics are

broadly classif ied in three groups:1 Pyrotechnic characterist ics such asaccuracy of delays, absence of crowdingand out of sequence f ir ing, incendivity,thermal sensit ivity, and strength ofdetonation.2. Electr ical characterist ics such asohmic resistance of the bridge wire andleg wires, safe f ir ing current level andduration, sensit ivity to stray currentsand s tat ic elec t r ic i ty and sui tabi l i tyfor multiple blasting.3. Mechanical characteristics such asimpact sens i t iv i ty , resistance to t ract ion of leg wires, waterproofness, andresistance to abrasion of leg wires.The Bureau of Mines has been studyingthese characterist ics under i t s Development of Safer Blasting Procedures and Improved Explosive Hazard Control Techniques program. This paper reports theresul ts of a study of electrostat ic sens i t iv i ty strength of detonation, andmaximum no-fire current for three brandsof short-delay electr ic detonators typi

cally used in underground coal mines.ELECTROSTATIC SENSITIVITY

The potential danger of ini t ia t ion ofan electr ic detonator by electrostat icdischarge is an ever-present problem inmines. An electrostat ic charge is generated when two materials of differentdie lec t r ic values move against each other . The quantity of charge generateddepends on the degree of contact, freedom of electron movement, atomic structure of the materials, shape and size ofthe moving bodies, and other physicalparameters.The general sequence of events leadingto in i t ia t ion of a short-delay detonatorby electrostat ic charge i s - -

I Generation of an electrostat iccharge.2. Transfer of charge to a suitablecapacitor.

3. Accumulation of charge on thecapacitor.4. Transfer of adequate discharge energy to the detonator in pin-to-pinP-P) or pin-to-case P-C) mode from thecapacitor.During the las t few decades, the Bureauhas conducted extensive studies on theelectrostat ic sensit ivity of detonators.A standard test setup for conductingelectrostat ic tests was developed. Theunit is capable of delivering an electro

s ta t ic pulse at various energy levels upto 112.5 J at 30 kV. This setup is knownto give reproducible results .For testing short-delay elec t r ic detonators, capacitors are charged to apotential of 10 kV and subsequently discharged through the detonator to determine the energy values at threshold in i t ia t ion level TIL), the maximum energylevel that would cause no ini t ia t ions infive t r ia ls . In earl ier work conductedby the Bureau with a broad spectrum ofcommercial detonators, TIL values rangedfrom 16 mJ to 350 m in P-P mode, and 36mJ to 12.5 J in P-C mode. The energiesreferred to are stored energy in the capacitor. Commercial detonators provideantistat ic protection and, in general,are less sensit ive to electrostat ic in i t ia t ion in P C mode than in P-P mode.

STRENGTH OF DETONATIONThe effectiveness of a detonator in

ini t ia t ing an explosive charge dependsprimarily on the detonation energy of thebase charge. The pressure in the shockfront ahead of the reaction zone is considered as detonation pressure. I t isdifficul t to measure detonation pressuredirect ly , owing to i ts transient natureand exceedingly high magnitude. According to Fedroff 1),4 the detonation pressure can be computed as the productof the density, detonation velocity,and part icle velocity of any explosive

4Underlined numbers in parentheses refer to items in the l i s t of references atthe end of this report.

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material. For the base charge of a detonator, the particle velocity is about 5pct of i t s detonation velocity. Approximate values of detonation pressure aregenerally computed from the density anddetonation velocity of the base charge.For effective in i t ia t ion the detonat ion pressure of the base charge should

exceed the shock in i t ia t ion pressure ofthe main explosive charge. A low detonat ion pressure may cause a low order ofdetonation reaction in the main explosive charge. The base charge of a detonator must have sufficient energy to in i -t i a te the detonation reaction in themain explosive charge and support i t unt i l the reaction becomes self-sustaining.The available energy of a detonator is afunction of i t s mass, energy density, andother physical constraints.There are various ways of determiningthe energy of a detonator. The underwater method (2) was used because i t isequally applicable to explosives and detonators. The energy of the detonatorduring an underwater test is released asshock wave energy (shock energy) and alsoas energy expended by the gases in thereaction products, as they expand andperform mechanical work (bubble energy).

Shock EnergyWhen an explosive is detonated underwa

ter the shock produced by the detonationfront i s transmitted to the surroundingwater as a pressure discontinuity followed by an exponential pressure decay.The pressure front moves radially outward as a shock wave in the water unti li t s velocity decays to the sonic value.The shock energy of the detonator, Es, isdefined as the in tegral of pressure, P,over the duration, t of the shock pulse:Es = fp dt . 1)

The l imits of integration should besufficient ly wide to capture the entireshock front signature. The lower l imitof integration typically s tar ts from thebeginning of the shock pulse. The shockfront signature is generally manifested

3

within 10 e where e is the decay constant . The upper l imit of integrationseldom exceeds 10 e and in most casesintegrating beyond 6 e does not improveenergy computation. The pressure signature is reduced to digi tal data, whichare then squared and integrated to a timeequal to 6 e to obtain the shock energy.A computer program was developed to readthe data from the oscilloscope, derivethe equations for the exponential decaycurve, and compute the shock energy relat ive to that of the standard detonator.The shock energy of the sample detonator relat ive to that of a standard detonator, such as Hercules J2,5 is definedas the rat io of Es (sample) to Es (J2)in the shock front. This is also termedspecific shock energy.

Hercules J2 was selected as a standarddetonator for i t s consistency in strengthand wide acceptance as a military standard. However any other standard detonator may be selected.Bubble Energy

n underwater detonation, an expandinggas bubble composed of reaction productsunder high pressure follows the shockwave. According to Cole (3), at f i r s tth is gas bubble expands at velocity inexcess of that predicted by the in ternalbubble pressure, owing to the afterflowcharacteristics of the spherical wave.When the pressure in the bubble reacheshydrostatic equilibrium, the radius continues to expand because of the inert iaof the moving water. Later, as the gaspressure fal ls below the' equilibriumpressure (hydrostatic plus barometricpressures), the bubble radius begins todecrease, and this collapse continues unt i l the rapidly increasing pressure ofthe gases in the bubble acts abruptly toreverse the water motion. As this process continues, the result is a series ofpressure pulses. The time elapsing between the in i t i a l pressure peak and the

5Reference to a speci f i c product doesnot imply endorsement by the Bureau ofMines.

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4

second, arising from the collapse of thegas bubble, is referred to as the bubbleperiod. This period is related to thein ternal energy of the gas and the equilibrium pressure and i s proportional tothe cube root of the energy and the in verse five-sixths power of the pressure(3). For any given type of explosive,the bubble period, T, is

where

Eg 1 /3T = K - 5 /6Po (2)T = bubble period, s ,

Eg bubble energy,function ofweight, g,J , and is athe charge

Po = the equilibrium pressure(hydrostatic plus barometr ic , Pa,and K = a proportionality constant.

From equation 2 i t is evident that bubble energy is proportional to the cube ofbubble period E T3).The bubble energy of the sample detonator , with respect to the standard Hercules J2 detonator, is termed the specificbubble energy and can be computed fromthe following equation:

where

and

Bubblerently

E sampleEn

E sample

EJ2

T sample

TJ2

and shockavailable

= [ T sample ] 3TJ2 (3)

bubble energy of thesample detonator, J ,bubble energy of thestandard J2 detona-tor , J,

= f i rs t bubble period ofthe sample detonator,s ,= f i rs t bubble period ofthe standard J2 deto-nator, s .

energies of the cur-short-delay coal mine

detonatorsthose ofdetonator.werethe evaluated relat ivestandard Hercules

NO-FIRE CURRENT

toJ2

The bridge wire of a detonator i s avery fine resistance element that converts an electr ical impulse to thermalenergy (4). n application of a f ir ingcurrent,- the bridge wire heats up and ig nites the heat-sensit ive ignit ion compo-si t ion surrounding i t . This in turn in i t ia tes the primary charge, through thedelay element. The primary charge in i t ia tes the base charge. The thermal response of the bridge wire and the adjacent ignit ion material to the electr icalpulse is very complex. For successfulini t ia t ion, the magnitude of the f ir ingcurrent and i t s duration are cr i t ical .Depending on the magnitude and durationof the f ir ing pulse, four possibil i t ies(5) may arise: no-fire, uncertainty off ir ing, al l - f i re , and arcing. Recommen-dations for minimum, as well as maximum,f ir ing currents are supplied by manufacturers of detonators. Ten amperes is atypical maximum recommended current, especially for parallel circuits , to prevent a condition known as arcing, whichcan damage the detonators and cause themto malfunction.Becker (6) correlated delay times withf ir ing current for a variety of domesticshort-delay coal mine detonators. Results indicate an increase in delay timewith reduction in f i r ing current. Thetypical minimum f ir ing current level recommended by manufacturers for a singlecap i s in excess of 0.5 A (6). No-firecurrent is defined as the maximum levelof direct current that can be appliedto a detonator without significant probabi l i ty of causing an ini t ia t ion. No-firecurrent is a function of applicationtime. For short application periods, sayup to 30 ms, no-fire current varies inversely as the application time. Forlonger application periods, beyond 1 s ,the no-fire current does not vary withthe application time. For the purpose ofth is study, no-fire current was determined with an application time of 10 s .

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5

EXPERIMENTAL SETUP ND TEST PROCEDUREELECTROSTATIC SENSITIVITY

These tests were conducted in the Bureau's standard electrostat ic sensit ivitytes t apparatus (f ig. 1), which consistsof an energy storage unit and a detonatortes t chamber. Detailed descriptions ofth is apparatus are given by Mason (7) andBrown ~ ) . The energy storage uni t cons is ts of a 30-kV power supply unit, isolat ion resis tors , 10 storage capacitorsranging from nominally 200-pF to 0 . 2 5 ~ Fcapacitance, an electrostat ic voltmeter,and a vacuum contactor for transferringthe energy stored in the chosen capacitor to the detonator located in the testchamber.The electrostat ic sensit ivity tests areperformed at 10-kV potential, with var i able capacitance, and with no appreciableseries resistance in the. discharge unit.Many electrostat ic tests include a seriesresistance to simulate the f in i te res is t -ance of the human body, usually conceivedto be the capacitor in actual si tu-ations; this simulation i s not the funct ion of these tes ts Two types of testsare performed on short-delay detonators:(1) pin-to-pin (P-P), in which the discharge occurs across the detonator leg

TABLE 1. - Stored energy levels ofdesignated capacitors at 10-kVpotentialDesignatedcapacitor,

0 25 08

4 w02 01 005 002001 0005 0002

Stored energylevel, mJ12,5004,0002,0001,0005002501112156136116

wires, which are shortened to 3 in, and(2) pin-to-case (p-C) in which both3-in-Iong leg wires are twisted togetherand attached to the high-voltage (posit ive) side, and the detonator 's outermetall ic shel l is attached to the ground(negative) side. The results of thet es t s are expressed in terms of a TILvalue, representing the highest capacitorenergy level a t which at least five consecutive noninitiations were observed.The stored energy levels at 10-kV potent ia l ranged from 16 mJ to 12.5 J for capacitor settings of 200 pF to 0.25 asshown in table 1. At some of the lowertest energy levels, an adjustment wasmade in the energy to include the contr ibution from stray capacitance.

STRENGTH OF DETONATIONStrength of detonation i s determined byf ir ing one detonator at a time under wa

ter and measuring the bubble and shockenergies. The experimental setup, asshown in figure 2, consists of an underwater cylindrical tes t chamber two digita l oscilloscopes, two water-resistanthigh-frequency dynamic pressure transducers, and a permissible blasting unit .The test chamber i s f i l led with water,and a detonator is suspended in water ata depth of 4 f t Pressure transducersare positioned 22 in from the center ofthe detonator. Weights are used to keepthe pressure transducers and the detonator suspended in the water. Pressuretransducers are four-element tourmalinegages, 3/8 in in diameter and designedfor underwater use. Each tourmaline gageis connected to one digi ta l oscilloscopehoused in an instrumentation t ra i lerthrough insulated coaxial cable and anamplifier unit. The oscilloscopes aretriggered by the shock wave and thedynamic pressure is displayed on the oscilloscope screen. The bubble energy iscompared with that of the standardHercules J2 detonator having an averagebase charge of 0.935 g PETN using equat ion 3. The relat ive shock energy i s a l -so computed using equation 1. Standard

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6

115 Vac

Input

T

Meter0}E illumination------- lampsHigh-voltage unit

SC J - - - - - - - .

Cdist-- . . ,1 LTest Ilj \ 1-11-....I II 1L _____ J

Charging resistorsand storage capacitors

115 VacFIGURE 1 - Schematic diagram of electrostatic sensitivity test apparatus.

Output

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Permissible Variableblastingunit resistorFiring line to detonator

I - - - - - - - I

IAmpl ifierunit

I 22 22 Amplifier J_unit

I Detonator test tankFIGURE 2 - Schematic diagram of experimen

tal setup for detonation energy measurement.

Hercules J2 detonators are fired at thebeginning and end of each working day,and the pressure profiles are recorded ondigi tal oscilloscopes.

NO-FIRE CURRENTThe experimental setup shown in figure3 consists of a constant-current regulated power supply unit, a digi ta l amme

te r , and a manual on/off switch. Onedetonator at a time is connected to thepower supply unit , through the digi talammeter, and f i red. The manual on/offswitch is kept in the on position for

115 Vaeinput

Regulatedpower supplyunit

Test chamber

Detonator

Digitalammeter

FIGURE 3. - Schematic diagram of no fire cur-rent measurement.

7

10 s to energize the detonator with thef i r ing pulse. After 10 s the switch isturned to the off position. A new detonator is used in each t r i a l . To s tar t ,a high f i r ing current setting is used,which is capable of ini t ia t ing the sampledetonator under tes t . The current se t -t ing is changed in 10-mA steps. If anin i t ia t ion occurs, the tes t is continuedat the next lower current set t ing, and soon. The highest value of the currentset t ing at which at least five consecut ive noninitiations are observed i s reported as the no-fire current.

RESULTS ND DISCUSSIONELECTROSTATIC SENSITIVITY

Table 2 shows the TIL values in P-P andP-C modes for the three domestic manufacturers. The energies are reported asstored energy in the capacitor. TIL values in the P-P mode are generally lowerthan TIL values in the P-C mode. Formanufacturer C the TIL ranges from 36 mJto 250 mJ in the P-P mode, and from 56 mJto 2,000 mJ in the P-C mode. Manufacturer CIS detonators show a higher TILthan manufacturer A's detonators in bothmodes. Manufacturer Bls detonators showa similar TIL in the P-P mode, but ahigher TIL in the P-C mode, compared withmanufacturer A's detonators.

STRENGTH OF DETONATIONTable 3 l i s t s the specific bubble andshock energies expressed as percent ofstandard Hercules J2 detonator. The energies represent the average of at leasttwo t r ia ls . Manufacturer C's detonatorsexhibit the greatest bubble and shock

energies. For any given type of detonator, as seen from tables 3 and 4, thedispersion of shock energy data points iswider than that of the bubble energy datapoints. In table 4 the quantity 100 c r ~expresses the dispersion of data pointsrelat ive to their average. This valueranges from 1.60 to 4.20 for specificbubble energy and from 9.06 to 9.57 for

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specif ic shock energy. Owing to widedispersion of shock energy data points,bubble energy is generally used tocompare the strength of differentdetonators.Bubble energy also correlates well withthe detonation energy measured in theclassical Sand Bomb Test Apparatus.Tests so far conducted by the Bureauindicate a coefficient of correlation of97.25 pct.

NO-FIRE CURRENTTable 5 shows the no-fire current levels of short-delay detonators for thethree manufacturers. The highest valueof the current sett ing at which at leastfive consecutive noninit iat ions were ob

served is reported in th is table. Detonators of manufacturer A consistentlyexhibit the highest values of no-firecurrent, in the range of 440 to 590 mAoThis represents good s tabi l i ty againststray currents. Detonators of manufacturer B indicate no-fire current rangingfrom 240 to 270 mA which is generallyadequate against stray currents normallyencountered in underground blasting s i tu-ations. Detonators of manufacturer Cshow a no-fire current ranging from 320to 350 m t again th is is generally adequate against stray currents.

TABLE 5. - No-fire current levelsof short-delay detonators

9

Delay [ N o ~ i n a l d e l a ~ l No-fire current,period ms rnAMANUFACTURER A

3456789r34567123456789

25 - 590100 500175 500250 520300 490350 500400 440450 480500 560M A N U F ~ A ~ C ~ T ~ U ~ R E ~ R ~ B ~25100170

240320400500MANUFACTURER C25100175250300350400450500

270240240260260250240330320330330340350340340350

REFERENCESand O. E. ShefExplosives andArsenal, Dover,

1. Fedroff, B. T.,f ield. Encyclopedia ofRelated Items. PicatinnyNJ, v. 4, 1969, 484 pp.2. Condon, .J. L., J N. Murphy, andD. E. Fogelson. Seismic Effects Associated With an Underwater Explosive Research Facil i ty. BuMines RI 7387, 1970,20 pp.3. Cole, R. H. Underwater Explosions.Princeton, 1948, 437 pp.4. E. I du Pont de Nemours Co.,Inc. (Wilmington, DE). Blaster s Handbook. 16th ed. , 1980, 494 pp.

) (-U.S. GPO: 1985505019/20,116

5. Atlas Powder Co. Handbook of Elect r ic Blasting. Dallas, TX, rev. ed.,1976, 93 pp.6. Becker, K. R., and J. E. Hay. Effect of Direct-Current Firing Levels onDetonator Delay Times. BuMines RI 8613,1982, 13 pp.7. Mason, C. M., and E. G. Aiken.Methods for Evaluating Explosives andHazardous Materials. BuMines IC 8541,1972, 48 pp.8. Brown, F. W., D. J . Kusler, andF. C. Gibson. Sensit ivity of Explosives to Init iat ion by Electrostat icDischarges. BuMines RI 5002, 1953, 7 pp.

INT. BU.O F MINES PGH. PA. 28158