lable at ScienceDirect

Journal of Electrostatics 69 (2011) 200e205

Contents lists avai

Journal of Electrostatics

journal homepage: www.elsevier .com/locate/elstat

Transferred charge of brush discharges in explosive gas atmospheres e Averification of the ignition threshold limits

T. Langer*, D. Möckel, M. BeyerPhysikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany

a r t i c l e i n f o

Article history:Received 28 January 2010Received in revised form1 October 2010Accepted 25 March 2011Available online 9 April 2011

Keywords:Brush dischargeIncentivityTransferred chargeIgnition probabilityCoulombmeterCharged surfaceExplosion group

* Corresponding author. Tel.: þ49 531 592 3438; faE-mail address: tim.langer@ptb.de (T. Langer).

0304-3886/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.elstat.2011.03.010

a b s t r a c t

Awide series of experiments has been performed to check the incentivity of hydrogen/air, ethene/air andpropane/air mixtures due to brush discharges. Thereby, the transferred charge as a criterion to judge theignition potential is determined to verify the thresholds of transferred charge given in the standards IEC60079-0 and in EN 13463-1. These thresholds have never been examined directly in an experimentbefore. It is stated that the thresholds for explosion group IIA, IIB and IIC represent different levels ofsafety. Using adequate thresholds the criterion of transferred charge is suitable for a judgement ofpotential electrostatic ignition sources.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Charged surfaces in combination with explosive atmospherescan lead to dangerous situations, since a potential electricaldischarge could ignite the flammable mixtures. To prevent acci-dents due to this setting, standards have been developed in whichthe test procedure of products is described which will be operatedin hazardous areas where explosive atmospheres can occur - so-called zones.

One test described in the standards uses the transferredcharge which is measured when the accumulated charge of a testsample is systematically discharged into a charge measurementdevice [1,2]. The advantage of this test method is that there is noneed to test the samples in explosive atmospheres anymore. Infact, the test item is evaluated with a criterion (transferredcharge) which is supposed to be independent of the ambient gasand simultaneously gives the possibility to quantify the differ-ence between the measured transferred charge and the thresholdin the standard [3].

The relationship between the incentivity of brush dischargesand the transferred charge during the discharge event was first

x: þ49 531 592 3705.

All rights reserved.

mentioned by Gibson and Lloyd [4]. They performed experimentson the charging of insulated materials. The charging in dependenceon the humidity and the area of the insulating item and thedependence of the discharge on the electrode radius was analysed.Moreover, they tested the incentivity of brush discharges whichthey called insulating sparks in different hydrocarbon/air mixtures.They found that brush discharges can ignite hydrocarbon/airmixtures depending on the mixture concentration and the energyof the discharge. In these tests the transferred charge wasmeasured. However, the measurement of the transferred chargewas only possible when no ignition occurred since in case of igni-tion ionization in the flame prevented the measurement of thetransferred charge. The measurement uncertainty was not dis-cussed. Also, Glor found that brush discharges are able to ignitedifferent hydrocarbon/air mixtures [5]. Lövstrand examined thecritical charge density necessary to get incendive brush dischargesusing n-pentane/air mixtures [6].

Wilson performed ignition experiments with brush dischargesfrom positively and negatively charged fabrics in stoichiometricnatural gas/air and hydrogen/air mixtures varying the electrodesize, surface area and humidity [7]. It was stated that the minimumtransferred charge igniting hydrogen/air is about 20 nC and fornatural gas/air about 100 nC. But however, in these tests neither thecharge in fact leading to ignition could bemeasured directly nor themeasurement uncertainty was determined.

Fig. 1. Photographed brush discharge from a charged insulating surface to a groundedmetal sphere.

T. Langer et al. / Journal of Electrostatics 69 (2011) 200e205 201

Different gases, vapours and dusts are classified in explosiongroups according to their hazards (I, IIA, IIB, IIC, III) [1,2]. For eachexplosion group, a maximum transferred charge has been definedwhich should ensure that a discharge with a transferred charge lessthan the defined maximum does not ignite the representatives ofthe specific group. These thresholds are educed from an equiva-lence consideration of the minimum ignition energy and theminimum transferred charge necessary to ignite a given mixture[3]. However, the thresholds of the maximum tolerable transferredcharge during the ignition process have never been examineddirectly in a wide experiment. Since especially the threshold isextremely significant in judging the safety of the above mentionedproducts, this value surely needs to be validated in detail. Thereby,a direct measurement of the charge transferred in the brushdischarge in fact leading to ignition is essential as well as theconsideration of the measurement uncertainty.

Therefore, an experimental set-up was developed which allowsthe preparation of defined explosivemixtures in a test chamber andthe measurement of the transferred charge of a provoked brushdischarge from a PTFE surface charged previously in conjunctionwith the information as to whether the mixture explodes or not.Additionally, the distance between the discharged surface and theelectrode of the measurement device was determined.

Three different gases were examined - hydrogen as a represen-tative of explosiongroup IIC, ethene for IIB andpropane for IIA. In thefirst instance, the most ignitable gas/air compositions according toReference [8] were used. In the case of hydrogen, also a smallercharged surface areawas tested. The overallmotivationwas to checkif the thresholds of transferred charge given inReferences [1] and [2]ensure a confident judgement of the safety of the tested products.

This paper is structured as follows: Section 2 contains somebasic background information. The experimental set-up and thetest procedure are described in Section 3. The experimental resultsare presented and discussed in Section 4. Finally, the resultingconclusions are given in Section 5.

2. Background

It is well known that materials with a surface resistance of over1011 Ohm (at a relative humidity of less than 30% and ambienttemperature) can accumulate charge [1,4,7]. This so-called elec-trostatic charge can lead to different types of discharges e corona,brush and propagating brush discharges [9]. Brush and propagatingbrush discharges have been proved to be able to ignite flammablegas/air mixtures [9], whereas brush discharges in contrast topropagating brush discharges do not ignite explosive dust atmo-spheres [10]. There is no doubt that propagating brush dischargeseasily ignite all mixtures of the explosion group IIAeIIC and that thetransferred charge clearly exceeds the threshold values of theexplosion groups IIA, IIB and IIC. In contrast to this, the transferredcharge of brush discharges is in the range of the thresholds given inReferences [1] and [2]. As these thresholds have not been verifiedexperimentally, experiments with provoked brush discharges indefined explosive atmospheres were carried out and the trans-ferred charge was recorded with a commercially available hand-coulombmeter (HC) (Schnier Elektrostatik GmbH, Bayernstraße 13,72768 Reutlingen-Rommelsbach, Germany).

Brush discharges occur when a conductive electrode is intro-duced in an electric field of an electrostatic charge stored on aninsulating surface. Thereby, the electrode distorts the electric fieldand locally raises it above the breakdown value and, hence, enablesa brush discharge to occur [9]. The name brush discharge originatesfrom the brush-like shape of the discharge as can be seen in Fig. 1.

A brush discharge does not discharge the whole surface butparts of a more or less circular area of a few square centimetres

depending on the size of the electrode [4,7,9]. In general, brushdischarges can have enough energy to ignite the mixtures repre-sented in the explosion groups IIAeIIC [e.g. [4,5,7]].

There are different types of charge measurement devices. InReference [11] the metrological backgrounds of two differentmethods are described. The evaluation of the measurementuncertainty is based on the Guide to the expression of uncertainty inmeasurement [12]. The usage of unshielded probes is in discussionsince many years [e.g. [5,7,13,14]]. Thereby, the problem of inducedcharges on the earthed electrode is addressed which is supposed tocompensate a part of the transferred charge in a discharge eventleading to a too small measured value of the transferred charge.However, using spark discharges from a defined capacitor usinga defined charging voltage we have checked the suitability ofmeasuring the transferred charge when using the HC withunshielded probes. The results which will be published in the nearfuture do not show a falsification.

Themeasuring range, inwhich the HC can be used adequately, isbetween 10 nC and 200 nC. Based on the results in Reference [11],a calibration method for the HC has been developed. According tothe calibration of the HC, there is a deviation DQ between themeasured transferred charge Qmeasured and the reference value Qref.Moreover, an expanded measurement uncertainty has beendetermined according to Reference [11] with a coverage factork ¼ 2.0 [12]. Its dependence on the amount of transferred chargecan be seen in Table 1.

The measurement principle of the HC (see Fig. 2) is based on thedischarge current which charges a capacitor with defined capaci-tance C. This leads to an increase of the voltage U at the capacitor.The voltage rise is measured internally by a microprocessor whichscans the voltage to find increasing or decreasing edges. Thesampling rate is 100 kHz, so the voltage is scanned every 10 ms.The difference between the final and the initial value represents thevoltage rise at the capacitor due to the transferred charge Q.

Using

Q ¼ C$U; (1)

the transferred charge is calculated. Passing through an analogue-to-digital the measured transferred charge is displayed.

Table 1Results of the hand-coulombmeter calibration.

Qref/nC Qmeasured/nC DQ/nC Expandedmeasurementuncertainty/nC

k

12.7 9.4 3.3 0.8 2.031.1 29.6 1.5 1.0 2.062.2 58.6 3.6 1.8 2.093.2 89.5 3.7 2.6 2.0124.6 119.6 5.0 3.3 2.0155.3 148.5 6.8 3.9 2.0186.3 179.3 7.0 4.7 2.0

T. Langer et al. / Journal of Electrostatics 69 (2011) 200e205202

The occurrence of multiple discharges must be taken intoaccount [4,7]. Using the HC all discharges within 10 ms are recordedas one event. If the discharge is not finished within 10 ms which isevaluated by means of the voltage course measured at the capaci-tance of the HC another 10 ms are regarded for the measurementresult. Using spark discharges Gerlach and Wagner [15] showedthat more discharge events occurring in a sequence with certaintime interval between them can lead to ignition whereas a singledischarge in their tests could not ignite the test mixtures. The timeinterval leading to an accumulated discharge energy is found to be15 ms in their study but depends on the discharge energy (in case of15 ms they used a discharge energy of 7 mJ for a 21% hydrogen/airmixture). In our tests we did not recognize multiple dischargessince we took care to have a very slow approaching speed of theelectrode but we cannot exclude that multiple discharges mighthave occurred in particular cases. The time interval of 10 ms ensures,that multiple discharges occurring fast one after another (�10 ms)are recorded as one discharge which appear reasonable since theignition thenwill be influenced by both discharges according to thefindings in Reference [15].

The flammable gas/air mixtures prepared were in the mostignitable range since this is the most hazardous situation one canexamine in matters of explosion protection. Hence, a concentrationof 22.0% hydrogen in air, 8.0% ethene in air and 5.2% propane in airwas used for the experiments [8].

3. Test set-up and experimental procedure

For the experiments, a test chamber was designed which can beseen in a schematic view in Fig. 3. The chamber consists of glass andis gastight. Its volumewas 15.7 l. The openings on the left and rightwere covered with aluminium foil to release overpressure. On thebottom a PTFE disc (thickness: 1.6 mm, area: 0.027 m2) bordered ininsulating plastics was installed. The choice of an area of 0.027 m2

was in accordance with Gibson and Lloyd as they did not finda higher transferred charge with an increasing surface area begin-ning at 0.023 m2 [4]. PTFE was used due to its high surface resis-tance (more than 3 TU measured at 22 �C and 20% relativehumidity) and high chargeability. The PTFE surface was chargedusing a high voltage stick with a maximum voltage of �70 kV. Thestick was arranged outside the test chamber and supplied thevoltage to a so-called Fakir electrode, which could be moved up-

Fig. 2. Schematic view of the hand-held microprocessor-operated coulombmeter.

and downwards in the chamber and consisted of one hundredneedles. Supplying high voltage to the Fakir electrode causedcorona discharges, which charged the PTFE surface. Negativelycharged surfaces were used since the resulting discharges areshorter in time and higher in the amount of transferred charge [7].Moreover, brush discharges from negatively charged insulatorswere found to be much more incendive than from positivelycharged ones [9]. Additionally, using negatively charged surfacesmultiple discharges appear less often than in case of positivecharging [7].

According to Reference [16] the incentivity of brush dischargesincreases with increasing electrode diameter up to 10 mm. Gibsonstated that the transferred charge using a 10 mm and 20 mm elec-trode is very similar [4].Wilson [7] in contrast found that the chargetransfer increases with increasing electrode diameter from 2mm to20 mm and stated that the discharges to electrodes with a largerdiameter are therefore more incendive. Considering all thesereports a 20 mm electrode seemed most appropriate for our tests.The electronic parts of the HC were also mounted on the outside ofthe test chamber, whereas the spherical electrode with a diameterof 20mmwas inside the explosive mixture. Therefore, an extensionconnecting the electrode and the input of the HC conductively wasbuilt. Since thismodification slightly changed the capacitance of theHC, it was recalibrated with this modification (see Table 1). Like theFakir electrode, the HC was also moveable up- and downwards. Tovary and control the charge on the PTFE surface, it was backed witha slab of wood over a grounded metal layer while the surface wascharged. Through the top of the chamber an inlet for the flammablegas/air mixtures had been integrated. Themixture compositionwascontrolled using an Oxymat 6F (Siemens AG, Berlin and Munich,Germany). Thereby, the concentration of oxygen in the gas mixturein the test chamberwasmeasured. Since the ratio of oxygen and theother components of air (mainly nitrogen) keeps constant, even ifanother gas is blended, the mixture composition can be calculatedas long as only one gas is added.

The constancy of the concentration had been verified inpreliminary experiments for each gas/air mixture. Before each test,the chamber was rinsed for 15minwith a flow of 5 l/min in the caseof hydrogen and 10 l/min in the case of ethene and propane. Thiscorresponded to about 5 (hydrogen/air) - 10 (ethene/air andpropane/air) times of the test chamber volume. It was found thatthe concentration changes over time most likely because of diffu-sion to the outside. Therefore, an interval for each mixturecomposition was determined in which the experiments had to becarried out in a certain time. The intervals are shown in Table 2.

The testing procedure was as follows. The chamber was rinseduntil the upper interval of the mixture concentration according toTable 2 was reached and had become stable. The gas flow wasstopped. The Fakir electrode was then brought close to the PTFEsurface. At a distance of approximately 10 mm, the PTFE surfacewas charged by spraying electrons on it for 15 s. The Fakir electrodewas removed, panned into the side of the test chamber and thenthe high voltage was shut off. After this procedure, the sphericalelectrode of the HC was slowly brought nearer to the charged PTFEsurface until a discharge occurred. The transferred charge was readon the display of the HC and the distance between the electrodeand the PTFE surface was noted. The measurement uncertainty ofthe distance was about 0.5 mm.

If no ignition had occurred, another experiment was performedas long as the experimental time of Table 2 was complied with. If anignition had occurred, the test chamber was rinsed with dehu-midified air before it was filled with a gas/air mixture again.Although this procedure was very time-consuming, many experi-ments had been undertaken in order to have adequate statistics ofeach gas/air mixture.

Fig. 3. Experimental set-up.

T. Langer et al. / Journal of Electrostatics 69 (2011) 200e205 203

In the case of hydrogen, additionally a smaller PTFE surface area(0.008 m2) was tested.

Since the charging was done with negative polarity, allmeasured discharges were negative also. In the following, theamount is given even if the transferred charge was always negative.

4. Results and discussion

For all presented results the deviation DQ resulting from thecalibration of the HC is already regarded. 82 experiments wereperformed for the most ignitable composition of 22% hydrogen inair. 29 of these experiments ignited the hydrogen/air mixture. Theresults are presented in Fig. 4. The lowest measured value oftransferred charge (22.7 nC) which ignited the mixture, turned outto be about two times higher than the threshold value forhydrogen/air mixtures of 10 nC given in References [1] and [2]. Thedistance between the electrode and the PTFE surface was 4 mm forthis value. This value is in good agreement with the value of 20 nCgiven by Wilson [7] even if he used a stoichiometric hydrogen airmixture. The minimum ignition energy increases about 50% in caseof the stoichiometric mixture compared to 22% hydrogen/airmixture [17]. In Reference [7] the transferred charge during thebrush discharge finally igniting the mixture could not be measured.

Table 2Determination of the maximum experimental time.

Hydrogen/air Ethene/air Propane/air

Upper concentrationlimit/%

23.0 8.2 5.34

Most ignitable mixture/% 22.0 8.0 5.20Lowest concentration

limit/%20.4 7.8 5.05

Maximum experimentaltime/min

3 3 5

Therefore, a comparison is not justified. It is not possible to reliablymeasure transferred charges of less than about 10 nC using the HC.But there were 10 discharges between 10 nC and 20 nC which didnot ignite the mixture. However, based on the results in Fig. 4, it isvery unlikely that a discharge with a transferred charge of less than10 nC can ignite the mixture.

It is well known that the ignition process is very statisticallyaffected [18,19] as not every brush discharge ignites a mixture evenwhen the transferred charge seems sufficiently high. This behav-iour is analysed in Fig. 5. For the ignition frequency calculationa minimum number of 5 tests is taken into account for eachinterval. The points in Fig. 5 represent the middle of the interval,

Fig. 4. Experiments performed in 22.0% hydrogen in air with a charged PTFE surface of0.027 m2.

Fig. 7. Experiments performed in 8.0% ethene in air with a charged PTFE surface of0.027 m2.

Fig. 5. Ignition frequency of 22.0% hydrogen in air in dependence of the transferredcharge.

T. Langer et al. / Journal of Electrostatics 69 (2011) 200e205204

whereas the error bars represents the interval width. In the case ofhydrogen, every brush discharge with a transferred charge of over90 nC ignited themixture. Moreover, it can be seen that the ignitionof the mixture is a statistical process since not every brushdischarge with a transferred charge of over 22.7 nC ignited themixture. Even if the sample for the intervals is not that high theexpected course discussed in References [18,19] can clearly be seenin Fig. 5.

Another 37 experiments were performed using a PTFE surface ofa smaller area (0.008 m2). 7 of these experiments ignited themixture (Fig. 6). The lowest transferred charge at which an ignitionwas observed was 23.4 nC at a distance of 5 mm between theelectrode and the PTFE surface. This result is in agreement with thevalue of 22.7 nC given before. As expected, the maximum values oftransferred charge decrease with a decreasing charged PTFE area.

In the case of ethene/air mixtures, 184 experiments were per-formed. In 27 of the experiments an ignition occurred (Fig. 7). Thelowest measured value of a transferred charge which ignited themixture was 31.5 nC. This is almost the threshold value for ethene/air mixtures of 30 nC given in [1] and [2]. The distance between theelectrode and the PTFE surface for the ignition at 31.5 nC was

Fig. 6. Experiments performed in 22.0% hydrogen in air with a charged PTFE surface of0.008 m2.

11 mm. In 80 experiments, a lower transferred charge wasmeasured which did not ignite the mixture.

In Fig. 8 the statistical behaviour of the experiments withethene/air mixtures is presented. Since the verification of thethreshold was the main focus of this work not so many results areachieved for transferred charges higher than 100 nC. Therefore, thesample for these interval might be to small for a reliable statement.

Using propane/air mixtures (Fig. 9), a total of 606 experimentswere performed. In 27 of the experiments an ignition occurred. Ascan be seen in Fig. 10 the lowest transferred charge igniting themixture was 93.4 nC. This is about 1.5 times more than thethreshold value of 60 nC in References [1] and [2]. The distancebetween the electrode and the PTFE surface for the ignition at93.4 nC was 27mm. In 398 experiments, a lower transferred chargewas measured which did not ignite the mixture.

Since for propane/air most experiments were performed withinthis study the sample for a statistical analysis is the best here. Theresults are presented in Fig. 10. The ignition frequency is even incase of a transferred charge of 200 nC not 100% but in the range of70%.

Fig. 8. Ignition frequency of 8.0% ethene in air in dependence of the transferredcharge.

Fig. 10. Ignition frequency of 5.2% propane in air in dependence of the transferredcharge.

Fig. 9. Experiments performed in 5.2% propane in air with a charged PTFE surface of0.027 m2.

T. Langer et al. / Journal of Electrostatics 69 (2011) 200e205 205

In general, the expanded measurement uncertainty for theresults presented here is between 0.8 nC and 4.7 nC according toTable 1. For the reasons of clarity the measurement uncertainty isnot given in the figures.

Independent of the type of flammable gas/air mixture, it wasobserved that some discharges did not occur until the electrodealmost reached the PTFE surface. In these cases it cannot be excludedthat the discharge occurredwhen the electrode had already touchedthe surface. These discharges cannot ignite the gas/air mixture, sinceflame quenching of the electrode will prevent the ignition. Further-more a tendency of higher distances between the electrode and thePTFE surface forhigher transferred charge canbe seenmost clearly inFigs. 7 and 9. Moreover, it was found that no ignition occurred atdistances between the electrode and the PTFE surface of less than2 mm for all investigated flammable gas/air mixtures.

5. Conclusions

The transferred charge necessary to ignite the most ignitablehydrogen/air mixture of 22.0% hydrogen in air has been found to be

22.7 nC. This value is about two times higher than the thresholdvalue of 10 nC given in References [1,2]. In the case of an ethene/airmixture at the most ignitable composition (8.0% ethene in air), theminimum transferred charge to ignite themixturewas 31.5 nC. Thisis almost the threshold value of 30 nC given in [1,2]. For the mostignitable propane/air mixture (5.2% propane in air), the minimumtransferred charge to ignite the mixture was 93.4 nC. This is about1.5 times more than the threshold value of 60 nC given in Refer-ences [1,2].

Concerning explosion group IIA and IIC the results confirm thethreshold values given in References [1,2] to be sufficient to judgethe potential hazard of brush discharges, but in case of explosiongroup IIB the achieved results are close to the threshold value.Altogether, it can be concluded that the thresholds representdifferent levels of safety which should be discussed by the standardcommittees.

The ignition frequency increases with an increasing transferredcharge. The ignition process is very much affected by statistics.Therefore, not every charge having a certain charge transfer is ableto ignite the mixture.

Based on the presented results the method of transferred chargetogether with adequate thresholds seems suitable to review thepotential endangerment of materials with a surface resistance highenough to accumulate charge. A similar test procedure should becarried out in the future for spark discharges, since the incentivityof sparks might be even higher than that of brush discharges.

References

[1] International Standard IEC 60079-0 Ed 5.0, Explosive Atmospheres - Part 0:Equipment - General Requirements (2007).

[2] European Standard EN 13463-1, Non-electrical Equipment for Use in Poten-tially Explosive Atmospheres d Part 1: Basic Method and Requirements(2007).

[3] U. von Pidoll, et al., Determining the incendivity of electrostatic dischargeswithout explosive gas mixtures, IEEE Trans. Ind. Appl. 40 (2004) 1467.

[4] N. Gibson, F.C. Lloyd, Incendivity of discharges from electrostatically chargedplastics, Br. J. Appl. Phys. 16 (1965) 1619.

[5] M. Glor, Ignition of gas/air mixtures by discharges between electrostaticallycharged plastic surfaces and metallic electrodes, J. Electrostat. 10 (1981)327e332.

[6] K.G. Lövstrand, The ignition power of brush discharges - experimental workon the critical charge density, J. Electrostat. 10 (1981) 161e168.

[7] N. Wilson, The nature and incendiary behaviour of spark discharges fromtextile surfaces, J. Electrostat. 16 (1985) 231e245.

[8] M. Hattwig, H. Steen, Handbook of Explosion Prevention and Protection.Wiley-VCH, 2004.

[9] V. Babrauskas, Ignition Handbook. Interscience Communications Ltd., London,UK, 2003.

[10] M. Glor, K. Schwenzfeuer, Direct ignition tests with brush discharges,J. Electrostat. 63 (2005) 463e468.

[11] T. Langer, et al., Metrological characterization of electrostatic discharges (ingerman), Tech. Mess. 75 (2008) 515e524.

[12] ISO/IEC Guide 98-3:2008, Uncertainty of Measurement - Part 3: Guide tothe Expression of Uncertainty in Measurement (2008) ISBN 92-67-10188-9,Geneva, Switzerland.

[13] J.N. Chubb, Measurement of charge transfer in electrostatic discharges,J. Electrostat. 64 (2006) 321e325.

[14] H.L. Walmsley, Induced-charge errors in charge-transfer measurement: brushdischarges between charged, insulating discs and earthed, conductivespheres, J. Electrostat. 68 (2010) 5e20.

[15] U. Gerlach, S. Wagner, Feldbusanschaltung mit Wechselstromspeisung für dieZündschutzart Eigensicherheit, first ed. VerlagMainz, Wissenschaftsverlag,Aachen, 2000.

[16] E. Heidelberg, G. Schon, Explosionsgefahren durch elektrostatische Aufladungvon Kunststoffbehältern, Die Berufsgenossenschaft Betriebssicherheit (1960)pp. 265e267.

[17] H. Krämer, Minimum ignition energy of hydrogen, ethylene and methane, PTBJahresbericht (1988) 157e158 (in german).

[18] R.K. Eckhoff, et al., On the minimum ignition energy (MIE) for propane/air,J. Hazard. Mater. 175 (2010) 293e297.

[19] S.P.M Bane, et al. Statistical analysis of electrostatic spark ignition of leanH2-O2-Ar mixtures, 3rd International Conference on Hydrogen Safety, Ajaccio,Corsica, 2009.