Experimental Simulation of Lightning Current Discharge on Rocks

applied sciences

Article Experimental Simulation of Lightning Current Discharge on Rocks

Chiara Elmi 1,2,* , Nicholas S. Coleman 3, Karen Miu 3 and Edward Schruba 3

1 Department of Earth and Environmental Science, University of Pennsylvania, 240 S. 33rd Street, Philadelphia, PA 19104, USA 2 Now at Department of Geology and Environmental Sciences, James Madison University, Harrisonburg, VA 22807, USA 3 Department of Electrical and Computer Engineering, Center for Electric Power Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA; [email protected] (N.S.C.); [email protected] (K.M.); [email protected] (E.S.) * Correspondence: [email protected]; Tel.: +1-540-568-5586

Received: 6 October 2018; Accepted: 19 November 2018; Published: 26 November 2018

Abstract: Lightning is a transient, high-current discharge occurring within a thundercloud, between clouds, or between a cloud and the ground. Cloud-to-ground (CG) lightning is the most studied because of its impact on human life. The aim of this study is to elucidate the effects of lightning in Earth materials by simulating the lightning current discharges in a laboratory setting. Technical applications of this work include the study or development of customized materials used to prevent accidents, limit damage, or reduce interruptions in electrical power system owing to lightning strikes, such as lightning arresters or high-voltage fuses. High-voltage electrical arcs were discharged through rock specimens, and power, energy, and duration of discharge were estimated to provide a better understanding of the origin of naturally occurring fulgurites (shock-impact glasses) and the lightning/rock interaction. X-ray powder diffraction showed that the samples used for the experiment represent basalt (samples A0, A1–A4) and granite (samples B1, B2). Optical microscopy provides direct evidence that materials can be physically altered due to the heat generated by an arcing event. Optical microscopy observations showed that arcs passed through the target rocks and mimicked the effect of lightning strikes hitting the surface of the rock, melting the target rock, and passing to ground. Fulgurite glass observed on basalt samples shows the impact origin lining the surface of millimeter-size craters and a slash-like coating, whereas in the granite sample, the fulgurite was not observed because the arc passed directly to the laboratory ground. Signiﬁcant differences in the duration of the experimental electrical arcs that passed through dry and wet samples (A1 and A3; A2 and A4, respectively) were observed. This discrepancy can be ascribed to the variation of the electrical properties related to the distribution of the water layer on the rock sample and to the occurrence of magnetite grains, which may increase the local conductivity of the sample owing to its electromagnetic properties.

Keywords: electrical arc; lightning/rock interaction; rock fulgurite

1. Introduction Lightning is a transient, high-current discharge whose path is measured in kilometers [1]. Lightning is one of nature’s most spectacular sights, but unexpected and menacing at the same time. It can be extremely dangerous, presenting a major natural hazard in many different environments, from power utility companies to civil aviation, and more [2]. In recent years, with great interest in renewable energy, wind turbines have become extremely vulnerable to lightning damage [3].

Appl. Sci. 2018, 8, 2394; doi:10.3390/app8122394 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2394 2 of 13

Lightning impacts not only economical activities, but also daily life. Thousands of people are killed every year by lightning bolts, while tens of thousands are injured [4,5]. In the United States alone, damages due to lightning strikes amount to tens of millions of dollars annually [6]. Mason [7] showed that thunderstorm cells of 2 km radius would be able to produce a succession of lightning flashes at intervals of about 30 s and very intense lightning activity, with flashes at intervals of less than 10 s, would require storm cells exceeding 5 km in radius. Any cloud type can potentially cause lightning or some related form of electrical discharge, as can snowstorms, volcanoes, and dust storms [1]. Over half of flashes occur within the cloud and are called intra-cloud (IC) discharges. Cloud-to-ground (CG) lightning is the most studied because of their impact in human life [1]. Lightning strikes reach the ground on Earth as many as 8 million times per day or 100 times per second, according to the U.S. National Severe Storms Laboratory [8]. CG lightning strikes transfer about 109–1010 joules of energy. Most of the lightning energy is spent to produce thunder, hot air, light, and radio waves [9,10]. Lightning strikes reach a speed of 94,000 km/s [9–13]. The peak temperature of the lightning channel is 10,000–30,000 K, which is up to five times the temperature of the surface of the Sun (the temperature of the solar interior is 107 K) [14,15]. When CG lightning strikes an appropriate target material such as sand, soil, or rock, the current flows through the target, heating the material to temperatures that exceed its melting point, followed by rapid cooling that results in quenching to form a glass called fulgurite [16]. About 90% of lightning flashes occur over continental landmasses as opposed to the open ocean [10,17], with up to 10 fulgurites formed globally per second [16]. The lightning peak temperature on a target material is considerably higher than the melting point of silica (1600–2000 ◦C, depending on the moisture content [10,18]). Our research on lightning–rock interactions is motivated by the desire to prevent accidents owing to lightning strikes and by the need to protect advanced ground-based and air-borne systems that use low voltage, solid-state electronics. As lightning strikes are natural phenomena, it is difficult to investigate the electrical and physical properties of lightning strikes. Rock fulgurites can be used to study the properties of the lightning strikes that created them, and in turn, help to elucidate information such as the energy distribution of CG lightning. However, the mechanism of natural rock fulgurite formation is not yet clarified. The purpose of this paper is to contribute to the study of rock fulgurite formation by: (i) simulating the lightning current discharges to generate rock fulgurites in basalt and granite specimens; (ii) measuring electrical arc properties that are relevant to rock fulgurite formation, including power, energy, and duration of discharge; and (iii) exploring the lightning–rock interaction and the origin of shock-impact glasses.

2. Materials and Methods

2.1. Target Rocks Experiments were performed using four basalt samples (labeled sample A1–A4) from Pennsylvania (USA) and two granite samples (labeled samples B1, B2) from Spain. The samples’ dimensions were 15 × 15 cm (thickness = 1.1 cm). A testing sample (a basalt rock from India labelled A0) was used to verify that the measurement equipment was properly conﬁgured. In order to elucidate possible differences in the arc duration and electrical properties needed to form fulgurite glass, dry and wet experimental conditions were considered. In wet experimental conditions, 1 mL of water was used. The experiment was repeated several times to verify the consistency of data on each sample type.

2.2. X-Ray Powder Diffraction X-ray powder diffraction (XRPD) was used to identify the mineralogical composition of the samples used for the experiment. Fragments of each of the basalt and granite samples were finely powdered with an agate pestle and mortar before spinner mode analysis. X-ray patterns were obtained using a PANalytical X’Pert diffractometer (PANalytical Westborough, MA, USA) equipped with an X’Celerator detector and CoKα radiation (40 kV/40 mA) operating in Appl. Sci. 2018, 8, 2394 3 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 13 isothermalUniversity modeof Pennsylvania. (25 ◦C) at theData Department on all rock samples of Earth were and Environmentalcollected with scan Science range of between University 5–70° of Pennsylvania.2θ, step size 0.02°, Data divergence on all rock samplesslit of 0.125 were°, and collected anti-scatter with scan slits rangeof 0.5 between°. The fitted 5–70 peak◦ 2θ, width step size for 0.02collected◦, divergence data was slit about of 0.125 0.01°◦, and 2θ. anti-scatter A Co anode slits was of 0.5chosen◦. The to fitted avoid peak fluorescence width for collectedencountered data with was aboutFe-bearing 0.01◦ 2mineralsθ. A Co anodeand Cu was radiation. chosen to The avoid incident fluorescence beam encountered optical module with Fe-bearingPANalytical minerals Bragg- andBrentanoHD Cu radiation. was used The incidentto improve beam peak/background optical module PANalyticalratio and increase Bragg-BrentanoHD intensity in spectral was used peaks. to improveQuantitative peak/background mineralogical ratioanalyses and increaseand determination intensity in of spectral the content peaks. of Quantitative amorphous mineralogicalmaterial with analysesthe external and-standard determination method of the(the content K-factor of method amorphous after material O’Connor with and the Raven external-standard [19]) were performed method (theusing K-factor PANalytical’s method HighScore after O’Connor Plus version and Raven 4.6 software [19]) were [20]. performed Pure crystalline using PANalytical’s Al2O3 (NIST HighScoreSRM 676a) Pluswas versionchosen as 4.6 an software external [standard20]. Pure and crystalline was analyzed Al2O3 under(NIST the SRM same 676a) instrumental was chosen conditions as an external as the standardsamples. and was analyzed under the same instrumental conditions as the samples.

2.3.2.3. ExperimentalExperimental HardwareHardware SetupSetup TheThe experimentalexperimental simulationsimulation ofof thethe lightninglightning currentcurrent dischargesdischarges waswas conductedconducted inin thethe HighHigh VoltageVoltage Laboratory Laboratory at Drexelat Drexel University’s University’s Center Cen forter Electric for Electric Power EngineeringPower Engineering (CEPE). The (CEPE). controlled The experimentalcontrolled experimental environment environment described here described allowed here for allowed non-destructive, for non-destructive, repeatable testing.repeatable Data testing. were collectedData were using collected typical using electronic typical lab electronic equipment. lab equipment. AA highhigh voltagevoltage alternatingalternating currentcurrent (AC)(AC) powerpower sourcesource waswas connectedconnected acrossacross aa setset ofof electrodeselectrodes alignedaligned onon eithereither sideside ofof thethe targettarget rockrock specimen.specimen. TargetTarget rockrock specimensspecimens werewere placedplaced onon aa conductiveconductive brassbrass basebase belowbelow aa tungstentungsten weldingwelding tiptip (Figure(Figure1 ).1).

Figure 1. Circuit diagram for high voltage electrical arc discharge experiments. The 60 Hz AC voltage Figure 1. Circuit diagram for high voltage electrical arc discharge experiments. The 60 Hz AC voltage source magnitude was increased until an arc formed. This tripped the circuit breakers, which trip at source magnitude was increased until an arc formed. This tripped the circuit breakers, which trip at 28 A (referred to the portion of the circuit show). The 0.44:150 kV transformer was assumed to be ideal. 28 A (referred to the portion of the circuit show). The 0.44:150 kV transformer was assumed to be Measurements were taken on the primary side of the transformer. ideal. Measurements were taken on the primary side of the transformer. The AC equipment used in this experiment was rated for up to 15 kVA and 150 kVRMS. A non-conductingThe AC equipment vessel used was designedin this experiment for safety was and rated to contain for up anyto 15 fragments kVA and of150 the kVRMS. material A non that- mayconducting be blown vessel apart was during designed the test. for Figure safety1 andshows to acontain circuit any diagram fragments of the of electrical the material setup usedthat may in the be experiments.blown apart Theduring experiment the test. wasFigure conducted 1 shows at a atmosphericcircuit diagram temperature of the electrical and pressure. setup used Sample in A0the wasexperiments. used to determine The experiment the fixed was distance conducted between at atmospheric the electrodes temperature for all experiments. and pressure. With sampleSample A0A0 onwas the used base to electrode, determine the the applied fixed distance voltage between was gradually the electrodes increased for using all experiments. an autotransformer With sample until anA0 eitheron the (i) base an arcelectrode, passed the through applied the voltage sample was (i.e., gradually a “fault” occurred),increased using or (ii) an the autotransformer upper voltage rating until ofan 150either kV ( wasi) an reached. arc passed In thethrough latter the case, sample the laboratory (i.e., a “fault” was de-energized,occurred), or ( andii) the the upper distance voltage between rating the of electrodes150 kV was was reached. decreased In the (step latter length case, the = 10 laboratory cm). The was final de distance-energized, between and the electrodes distance was between fixed the at 30electrodes cm. Accounting was decreased for sample (step thicknesses, length = 10 the cm). vertical The final distance distance between between the weldingelectrodes tip was and fixed the target at 30 specimenscm. Accounting was fixed for sample at 28.9 cmthicknesses, for samples the A1–A4 vertical and distance 26.5 cm between for samples the welding B1 and B2.tip and the target specimensVoltage was and fixed current at 28.9 waveforms cm for samples were sensed A1–A4 on and the primary26.5 cm (lowfor samples voltage) B1 side and of B2. a 440:150,000-V single-phaseVoltage transformerand current (assumedwaveforms ideal were with sensed no losses) on the and primary referred (low to voltage) the secondary side of (higha 440:150,000 voltage)- sideV si ofngle the-phase transformer transformer (Figure (assumed2). ideal with no losses) and referred to the secondary (high voltage) side of the transformer (Figure 2). Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 13 Appl. Sci. Sci. 20182018,, 88,, x 2394 FOR PEER REVIEW 44 of of 13 13

Figure 2. Close-up of voltage and current waveforms around fault onset, referred to the secondary Figure 2. Close-up of voltage and current waveforms around fault onset, referred to the secondary Figure(150 kV) 2. Closeside of-up the of transformer,voltage and currentfor sample waveforms A2, trial around 5. The faultvoltage onset, ripple referred introduce to thed secondarynon-60-Hz (150 kV) side of the transformer, for sample A2, trial 5. The voltage ripple introduced non-60-Hz (150oscillations kV) side that of werethe transformer, used to identify for samplefault onset A2, usingtrial 5.a waveletThe voltage transformation. ripple introduce d non-60-Hz oscillations that were used to identify fault onset using a wavelet transformation. oscillations that were used to identify fault onset using a wavelet transformation. Measurement equipment included: (i) Tektronix DPO 3014 oscilloscope (Tektronix, Beaverton, Measurement equipment included: (i) Tektronix DPO 3014 oscilloscope (Tektronix, Beaverton, OregMeasurementon, USA); (ii) equipmentTektronix TCP included: 303 current (i) Tektronix probe; (DPOiii) Tektronix 3014 oscilloscope TCPA 300 (Tektronix, current amplifier; Beaverton, and Oregon, USA); (ii) Tektronix TCP 303 current probe; (iii) Tektronix TCPA 300 current ampliﬁer; and (iv) Oreg(iv) Tektronixon, USA); P5200A(ii) Tektronix differential TCP 303 voltage current probe. probe; (iii) Tektronix TCPA 300 current amplifier; and Tektronix P5200A differential voltage probe. (iv) Tektronix P5200A differential voltage probe. 2.4.2.4. ElectricalElectrical Measurementsmeasurements and Calculationscalculations 2.4. Electrical measurements and calculations VoltageVoltage andand currentcurrent waveformswaveforms werewere measuredmeasured onon thethe primaryprimary (440(440 V)V) sideside withwith aa samplingsampling Voltage ands current waveforms were measured on the primary (440 V) side with ap sampling frequencyfrequency ofoff sf == 250 kHz.kHz. With the ideal transformer assumption,assumption, thethe measuredmeasured currentcurrentI Ip asas referredreferred frequencyto the secondary of fs = 250 (rock kHz side). With of the idealtransformer transformer is =assumption,= −1 thewhere measured a = 341 wascurrent the Iturnsp as referred ratio of to the secondary (rock side) of the transformer is IR = Is = a Ip where a = 341 was the turns ratio to the secondary (rock side) of the transformer is = = −1 where a = 341= was the turns ratio of ofthe the transformer. transformer. The The measured measured voltage voltage V referredV referred 𝑅𝑅to the𝑠𝑠 to secondary the secondary𝑝𝑝 side is side is Vs =, anda V thep, and voltage the 𝐼𝐼 𝐼𝐼 𝑎𝑎−1 𝐼𝐼 thevoltageacross transformer. the across rock the isThe rock measured= is V = Vvoltage where− RI V whereR referred = 233.1R = 𝑅𝑅 tok 233.1Ω the was𝑠𝑠 secondary kΩ thewas 𝑝𝑝series the side seriesresistance is resistance= placed , and placed to the limit voltage to limitfault R s s 𝐼𝐼 𝐼𝐼 𝑎𝑎 𝐼𝐼 𝑉𝑉𝑠𝑠 𝑎𝑎 𝑉𝑉𝑝𝑝 acrossfaultcurrent. current. the rock is = where R = 233.1 kΩ was the series resistance placed to limit fault 𝑉𝑉𝑅𝑅 𝑉𝑉𝑠𝑠 − 𝑅𝑅𝐼𝐼𝑠𝑠 𝑉𝑉𝑠𝑠 𝑎𝑎 𝑉𝑉𝑝𝑝 current.DueDue toto equipmentequipment limitations,limitations, somesome currentcurrent measurementsmeasurements werewere clipped.clipped. CubicCubic splinespline 𝑉𝑉𝑅𝑅 𝑉𝑉𝑠𝑠 − 𝑅𝑅𝐼𝐼𝑠𝑠 interpolationinterpolationDue to wasequipmentwas usedused toto reconstructlimitations,reconstruct the thesome clipped clipped current portion portion measurements of of these these signals signals were (Figure (Figure clipped.3 ).3). Cubic spline interpolation was used to reconstruct the clipped portion of these signals (Figure 3).

Figure 3. Example voltage and current waveforms for dry and let experiments: (a) data from sample Figure 3. Example voltage and current waveforms for dry and let experiments: (a) data from sample A1 (dry), and (b) data from sample A3 (wet). The peaks of the current waveforms were reconstructed FigureA1 (dry) 3. ,Example and (b) data voltage from and sample current A3 waveforms(wet). The peaks for dry of andthe currentlet experiments waveforms: (a) were data reconstructedfrom sample using cubic spline interpolation. A1using (dry) cubic, and spline (b) data interpolation. from sample A3 (wet). The peaks of the current waveforms were reconstructed usingFrom cubic the voltage spline interpolation. and current waveforms, power and energy estimates were performed as follows: From the voltage and current waveforms, power and energy estimates were performed as follows:From the voltage and current waveforms,pR[k] = vpowerR[k]iR [andk] energy estimates were performed (1)as follows: Appl. Sci. 2018, 8, 2394 5 of 13

n e[n] = Ts ∑ pR[k] (2) k=k0 where pR[k], vR[k], and iR[k] are the power, voltage, and current at the rock specimen on sample k, k0 is −1 the estimated sample on which the strike occurs, Ts = fs is the sampling period, and e[n] is the total energy of the fault up to sample n. To compute the total energy dissipation for a given arc discharge event, it was necessary to use the data to estimate the instants of fault onset and of fault extinction. Fault onset occurs when the magnitude of the voltage across the electrodes is large enough for an electrical discharge to occur through a path connecting the electrodes. The physical discharge path is three-dimensional and random (i.e., current may not ﬂow in a straight line connecting the electrodes). Fault extinction occurs when the circuit breaker opens. The procedures for estimating fault onset and extinction times from the recorded current and voltage waveforms are explained next.

2.5. Fault Onset When flashover occurs, there is a near-instantaneous change from an open circuit condition to a short-circuit condition. In terms of the measured quantities, this is when the voltage drops to zero. A discrete wavelet transformation was leveraged to find events of this type within the discretely sampled voltage waveforms. Wavelet decomposition is a signal analysis tool that can be used to localize events/disturbances in a signal in both time and frequency [21]. The Daubechies family of wavelets in particular [22] is commonly used in the analysis of electric power system disturbances (e.g., References [23–25]). Here, the Daubechies-4 (db-4) wavelet was used to detect fault onset. Wavelet decomposition produces coefficients representing the strength of different frequency bands at different points in time within the signal. An arc discharge is a near-instantaneous event, thus the highest frequency components of the signal were of interest. Within each time-domain voltage signal, the data sample with the greatest high-frequency component (i.e., the sample associated with the largest first-level wavelet detail coefficients) was considered the sample of fault onset. This was verified by inspecting the portion of the voltage waveform around this sample to ensure that a short-circuit condition actually occurred at this time, and that the first-level detail coefficient strength could not be attributed to noise or truncation error.

2.6. Fault Extinction Fault extinction occurs when the circuit breakers open and replace a short-circuit condition (large current) with an open-circuit condition (zero current). Once the breakers open, the voltage across the electrodes remains at zero. Therefore, the fault extinction event must be detected within the current waveforms. Compared to the voltage waveforms, the current waveforms had a lower relative resolution and signal-to-noise ratio. As a result, the wavelet decomposition technique discussed above was less effective and more prone to error, thus an alternative method was used to detect fault extinction. A post-fault, open-circuit condition was considered to exist when the measured current amplitudes became indistinguishable from noise. For an event with an estimated fault onset sample k0, the procedure for identifying fault extinction was as follows: i. Characterize the pre-fault (open-circuit) noise level in the current measurement using the standard deviation (assume noise is Gaussian with mean µ estimated from the data): v u u 1 k0 σ = t (i [k] − µ)2 (3) − ∑ R k0 1 k=1 Appl. Sci. 2018, 8, 2394 6 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 13 ii. Find the post-onset sample for which all later data samples could be considered noise: With a 6σ threshold and the assumption of Gaussian noise, less than two in one-million samples of noise in the current channel will fallkF =outside{k|∀k ≥of ktheF, |ithreshold,R[k]| < 6σ }on average. This threshold was(4) selected through trial-and-error. Manual inspection of the waveforms showed that smaller thresholds resultedWith in a 6theσ threshold inclusion and of a the significant assumption number of Gaussian of post noise,-extinction less than samples. two in one-millionTheoretically, samples smaller of noisethresholds in the yield current higher channel false will alarm fall outsideprobability, of the which threshold, is problematic on average. because This threshold of the high was sampling selected throughrate (250 trial-and-error. kHz). Manual inspection of the waveforms showed that smaller thresholds resulted in the inclusion of a signiﬁcant number of post-extinction samples. Theoretically, smaller thresholds yield3. Results higher false alarm probability, which is problematic because of the high sampling rate (250 kHz).

3.3.1. Results Mineralogy of Target Rocks

3.1. MineralogyPrior to the of Targetexperiment, Rocks the testing sample A0, samples A1–A4, and samples B1–B2 were observed using optical microscopy. Samples A0–A4 showed a dark-colored, fine-grained texture, whilePrior samples to the B1 experiment, and B2 showed the testing light- samplecolored, A0, coarse samples-grained A1–A4, texture. and samples B1–B2 were observed usingX optical-ray powder microscopy. diffraction Samples (XRPD) A0–A4 was showed used to a identify dark-colored, the mineralogical ﬁne-grained composition texture, while of samplesthe rock B1samples. and B2 Testing showed sample light-colored, A0 was coarse-grainedcomposed mainly texture. of pyroxene, plagioclase, and magnetite, whereas the mainX-ray constituents powder diffraction of samples (XRPD) A1–A4 was were used pyroxene, to identify plagioclase, the mineralogical magnetite, composition and quartz of the (Figure rock samples.4). Testing sample A0 was composed mainly of pyroxene, plagioclase, and magnetite, whereas the main constituents of samples A1–A4 were pyroxene, plagioclase, magnetite, and quartz (Figure4).

Figure 4. X-ray diffraction pattern of samples A1–A4. Px = pyroxene; Plg = plagioclase; Qtz = quartz; Figure 4. X-ray diffraction pattern of samples A1–A4. Px = pyroxene; Plg = plagioclase; Qtz = quartz; Mag = magnetite; Bio = biotite; Act = actinolite. Observed spectra (red line), ﬁtted spectra (blue solid Mag = magnetite; Bio = biotite; Act = actinolite. Observed spectra (red line), fitted spectra (blue solid line), difference plot (below spectra), and Bragg peak positions (tick marks above difference plot) are line), difference plot (below spectra), and Bragg peak positions (tick marks above difference plot) are shown. The weighted R-factor, Rwp, was calculated using the observed and calculated intensities in shown. The weighted R-factor, Rwp, was calculated using the observed and calculated intensities in the powder diffraction patterns. the powder diffraction patterns. Accessory and alteration minerals include micas, talc, and amphiboles in sample A0; and micas and amphibolesAccessory and in samplesalteration A1–A4. minerals The include XRPD micas, data on talc, samples and amphiboles B1 and B2 revealedin sample the A0; presence and micas of quartz,and amphiboles plagioclase, in samples K-feldspar, A1 and–A4. micas The XRPD (Figure data5). on samples B1 and B2 revealed the presence of quartz,According plagioclase, to the K- XRPDfeldspar, results and andmicas optical (Figure microscopy 5). observations, the testing sample A0 and samplesAccording A1–A4 representto the XRPD a basalt results rock and type, optical whereas microscopy the samples observations, B1 and B2 represent the testing a granite sample rock A0 type. and Allsamples samples A1– usedA4 represent for the experiments a basalt rock were type, homogeneous whereas the sampl and isotropic.es B1 and QuantitativeB2 represent mineralogicala granite rock type. All samples used for the experiments were homogeneous and isotropic. Quantitative mineralogical analyses and determination of the content of amorphous material were performed on the test sample A0, samples A1–A4, and samples B1 and B2. Results are shown in Table 1. Appl. Sci. 2018, 8, 2394 7 of 13 analyses and determination of the content of amorphous material were performed on the test sample Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 13 A0, samples A1–A4, and samples B1 and B2. Results are shown in Table1.

Figure 5. X-ray diffraction pattern of the samples B1–B2. Qtz = quartz; Plg = plagioclase; Ksp = K-feldspar; Figure 5. X-ray diffraction pattern of the samples B1–B2. Qtz = quartz; Plg = plagioclase; Ksp = K- Bio = biotite. Observed spectra (red line), fitted spectra (blue solid line), difference plot (below spectra), feldspar; Bio = biotite. Observed spectra (red line), fitted spectra (blue solid line), difference plot and Bragg peak positions (tick marks above difference plot) are shown. The weighted R-factor, Rwp, (below spectra), and Bragg peak positions (tick marks above difference plot) are shown. The weighted was calculated using the observed and calculated intensities in the powder diffraction patterns. R-factor, Rwp, was calculated using the observed and calculated intensities in the powder diffraction Tablepatterns. 1. Mean mineralogical composition of the bulk rock samples. Standard deviation in parenthesis.

Weight % Samples A1–A4 Samples B1–B2 Table 1. Mean mineralogical composition of the bulk rock samples. Standard deviation in parenthesis. Quartz 10.2(3) 54.6(2) PlagioclaseWeight % Samples 49.6(4) A1–A4 Samples B1 15.7(4)–B2 K-feldsparQuartz 10.2(3) 54.6(2) 21.0(2) PyroxenePlagioclase 49.6(4) 18.5(4) 15.7(4) Biotite 5.6(2) 7.5(2) ChloriteK-feldspar 21.0(2) 1.2(4)

ActinolitePyroxene 18.5(4) 6.3(5) MagnetiteBiotite 5.6(2) 3.6(2) 7.5(2) AmorphousChlorite 6.2(4) 1.2(4) Actinolite 6.3(5) Pasek and Hurst [16] determinedMagnetite the energy3.6(2) at one bar of pressure required to promote the melting of quartz to form sandAmorphous fulgurite. Following6.2(4) Pasek and Hurst [16], considering the density of basalt is 3 g/cm3 [26], the enthalpy of basalt at 1 bar is 9.145 J/mol [27], the molecular weight of basalt is 125Pasek g/mol and [28 ],Hurst and the[16] internal determined diameter the ofenergy fulgurite at one observed bar of viapressure optical required microscopy to promote is 0.08 cmthe (thismelting work), of quartz the energy to form per sand unit fulgurite. length (E) Following required toPasek vaporize and Hurst basalt [16], to make considering the fulgurite the density glass isof 17.2383basalt is J/cm 3 g/cm3. 3 [26], the enthalpy of basalt at 1 bar is 9.145 J/mol [27], the molecular weight of basalt is 125 g/mol [28], and the internal diameter of fulgurite observed via optical microscopy is 0.08 cm 3.2.(this Arc work), Discharge the energy Experiment per unit Results length (E) required to vaporize basalt to make the fulgurite glass is 3 17.2383A photo J/cm camera. provided approximately the instant (static) tridimensional spatial image of the spark discharge on the target material (Figure6). Strikes were veriﬁed by macroscopically observing 3.2. Arc Discharge Experiment Results the surface of the samples after each lightning strike and labeling the arc entrance and exit location on the surfaceA photo of samples.camera provided approximately the instant (static) tridimensional spatial image of the spark discharge on the target material (Figure 6). Strikes were verified by macroscopically observing the surface of the samples after each lightning strike and labeling the arc entrance and exit location on the surface of samples. Appl. Sci. 2018, 8, 2394 8 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 13

Figure 6. StaticStatic tridimensional tridimensional spatial spatial image image of the of thespark spark discharge discharge on sample on sample A provided A provided by a photo by a photocamera. camera.

Table2 2 describes describes the the specimens specimens and and the the experimental experimental electrical electrical data data obtained obtained after after each each arcarc ﬂashflash event. RMS RMS voltage voltage and and current current data, fault durations, peak peak and average power, and total energy dissipation data was recorded andand calculatedcalculated asas describeddescribed inin thethe MaterialsMaterials andand MethodsMethods section.section.

TableTable 2.2. Summary of of the the electrical electrical data data for for each each strike strike on on sample sample A. A.Legend Legend:: TF T= Ffault= fault duration; duration; VR V= Rroot= root mean mean square square (RMS) (RMS) voltage voltage during during the thefault; fault IR ;=I RRMS= RMS current current during during the fault; the fault; P = meanP = mean fault faultpower; power; = peakpˆ = peakinstantaneous instantaneous power; power; E= totalE= fault total energy. fault energy.

Specimen Strike Number T (ms) V (kV) I (A) P (kW) pˆ (kW) E (kJ) Specimen𝑝𝑝푝 Strike NumberF TF (ms)R VR (kV) R IR (A) P (kW) ̂ (kW) E (kJ) AverageAverage A0 dry A0 dry 96.8696.86 85.2685.26 0.530.53 42.2542.25 116.28 116.28 4.09 4.09 𝒑𝒑 SampleSample A1 dry A1 dry 11 95.9795.97 85.1385.13 0.520.52 42.0742.07 124.65 124.65 4.04 4.04 Sample A2 dry 1 103.74 82.82 0.51 39.72 107.28 4.12 Sample A2 dry 1 103.74 82.82 0.51 39.72 107.28 4.12 2 88.04 83.01 0.52 40.18 102.62 3.54 32 96.2688.04 85.5783.01 0.530.52 42.2540.18 102.62 129.27 3.54 4.07 43 88.6196.26 85.6585.57 0.530.53 42.5242.25 129.27 121.36 4.07 3.77 54 96.0388.61 84.9385.65 0.520.53 41.6742.52 121.36 123.31 3.77 4.00 65 103.7896.03 79.6884.93 0.500.52 36.7841.67 123.31 117.00 4.00 3.82 Average A2 96.08 83.61 0.52 40.52 116.81 3.89 Sample A3 wet 16 87.04103.78 86.2679.68 0.530.50 43.0836.78 117.00 121.10 3.82 3.75 SampleAverage A4 wet A2 1 95.6796.08 83.2183.61 0.520.52 40.2840.52 116.81 122.78 3.89 3.85 Sample A3 wet 21 96.2487.04 86.3286.26 0.530.53 43.0243.08 121.10 121.48 3.75 4.14 Sample A4 wet 31 102.4295.67 81.2383.21 0.510.52 38.3840.28 122.78 122.67 3.85 3.93 4 96.56 85.22 0.52 41.91 121.36 4.05 Average A4 2 97.7296.24 84.0086.32 0.520.53 40.9043.02 121.48 122.07 4.14 3.99 3 102.42 81.23 0.51 38.38 122.67 3.93 4 96.56 85.22 0.52 41.91 121.36 4.05 Average A4 97.72 84.00 0.52 40.90 122.07 3.99

Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 13 Appl.Appl. Sci. Sci.2018 2018, 8, ,8 2394, x FOR PEER REVIEW 99 of of 13 13 4. Discussion 4. Discussion 4. Discussion 4.1. Lightning Simulation 4.1. Lightning Simulation 4.1. Lightning Simulation TheThe initiation initiation andand growthgrowth ofof aa conductiveconductive plasma channel (lightning leader)leader) betweenbetween aa cloud cloud andand theThe the ground initiationground (rock (rock and surface) surface) growth representsofrepresents a conductive thethe first plasma step channelto form (lightningfulgurite. TheThe leader) leaderleader between arises arises ain in cloud a a region region and thewherewhere ground the the electric (rockelectric surface) field field is is representsstronstrongg enoughenough the first toto ionizeionize step to the form air by fulgurite. electron The impact.impact. leader TheThe arises airair ionization inionization a region length length where thedoesdoes electric not not exceed exceed field is severalseveral strong hundreds enoughhundreds to ofof ionize voltsvolts the per air centimeter, by electron which impact. meansmeans The that airthat ionization therethere isis anan length intensiveintensive does notionizationionization exceed occurring severaloccurring hundreds in in its its tip tip ofregion, region, volts perchangingchanging centimeter, thethe neutral which air means to aa highlyhighly that there conductivconductiv is an intensiveee plasma plasma ionization[29]. [29]. This This occurringbecomesbecomes possible possible in its tip because because region, the changingthe lightning lightning the sparkspark neutral carries air to its a highlyown strong conductive electricelectric plasma fieldfield induced induced [29]. This by by the the becomes space space possiblechargecharge concentrated concentrated because the at lightningat the the lightning lightning spark tiptip carries andand transported itstransported own strong with electric it. In ourour field experiment,experiment, induced by the the the leader leader space electric electric charge concentratedfieldfield is is thatthat of atof a thea tungstentungsten lightning needleneedle tip and (approximate(approximate transported withdiameter it. In = our 1 mm) experiment, connectedconnected the with leaderwith aa electricwirewire toto field thethe issecondarysecondary that of a terminal tungsten terminal of needleof a a 440/150,000 440/150,000 (approximate VV transformer.transformer. diameter The = 1 strong mm) connected field region,region, with inin which awhich wire the the to theair air molecules secondarymolecules terminalbecomebecome ionized, ofionized, a 440/150,000 will will move move V dodo transformer.wnwn togethertogether The withwith strong the arc field flash. region, in which the air molecules become ionized,AlongAlong will the movethe pathpath down fromfrom together thethe neneedle withedle to theto thethe arc rock flash. surface, the lightning leaderleader tiptip carriescarries aa highhigh potentialpotentialAlong comparable comparable the path from with with the that that needle ofof thethe to needleneedle the rock at surface,the spark the start, lightning the potential leader tip differencedifference carries abeing being high potentialequal equal to to comparablethethe voltage voltage with dropdrop that inin the ofthe the leaderleader needle channel.channel. at the spark AA shortshort start, circuit the potential condition difference occursoccurs beingwhenwhen equal thethe spaspa tork therk goesgoes voltage toto droplaboratorylaboratory in the ground, leaderground, channel. resulting resulting A inshortin largelarge circuit currentscurrents condition thatthat heat occurs the when material the sparksurroundingsurrounding goes to thethe laboratory leader leader channel channel ground, resultingresulting in in the largethe formation formation currents ofof that fulguritefulgurite heat the glass.glass. material TheThe current surrounding magnitude the leaderdependsdepends channel onon thethe resultingpotentialpotential at inat the the formationneedleneedle and and of the the fulgurite composition composition glass. ofof The thethe current leaderleader magnitudechchannel (i.e., depends the composition on the potential ofof thethe airair at and/orand/or the needle rock rock andwithin within the compositionthethe channel). channel). of the leader channel (i.e., the composition of the air and/or rock within the channel).

4.2.4.2. Fulgurite Fulgurite Glass Glass FormationFormation ForFor the the ﬁrstf irstfirst lawl awlaw ofof thermodynamicstthermodynamicshermodynamics forfor closed systems, energy transferred transferred across across the the boundary boundary ofof a a system system in in the the form form ofof heatheatheat alwaysalwaysalways resultsresults fromfrom aa differencedifference inin temperaturetemperature between betweenbetween the the system system andand its its immediate immediate surroundings surroundings and andand a aa changechange ofof internalinternal energy ofof the the system. system. Due Due to to the the mechanical mechanical imbalanceimbalance betweenbetween between thethe the high-temperature highhigh--temperaturetemperature air airair around aroundaround the the lightning lightning channel channelchannel and andand the thethe air air surroundingair surroundingsurrounding the rock,thethe rock, rock, the formerthe the form form expands,erer expands expands vaporizing,, vaporizingvaporizing the rock thethe atrock its contactat its contact and producing and producingproducing a crater aa on cratercrater the onrockon the the surface. rock rock Thesesurface.surface. effects These These were effectseffects observed werewere via observedobserved optical microscopyviavia opticaloptical microscopy on testing sample on testingtesting A0 (Figure samplesample7) andA0A0 (Figure basalt(Figure samples 7)7) andand A1–A4,basaltbasalt samples samples which showA1 A1––A4A4 millimeter-size, ,which which showshow millimeter cratersmillimeter surrounded--size craters by glasssurrounded (Figures byby8 – glassglass10). ((FigFigureuress 8 8––10).10).

Figure 7. Optical microscopy images of fulgurite on dry testing sample A0. Figure 7. Optical microscopy images of fulgurite on dry testing sample A0.

Figure 8. Optical microscopy images of fulgurite on dry sample A1 (a) and wet sample A3 (b). Figure 8. Optical microscopy images of fulgurite on dry sample A1 (a) and wet sample A3 (b). Figure 8. Optical microscopy images of fulgurite on dry sample A1 (a) and wet sample A3 (b). Appl. Sci. Sci. 20182018,, 88,, x 2394 FOR PEER REVIEW 1010 of of 13 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 13

Figure 9. Optical microscopy images of orange-brown rims around a crater of dry sample A2. Figure 9. Optical microscopy images of orange orange-brown-brown rims around a crater of dry sample A2.

Figure 10. Optical microscopy images of bubbles in fulgurite glass of wet sample A3. These bubbles Figure 10. Optical microscopy images of bubbles in fulgurite glass of wet sample A3. These bubbles cancan be generated from from the vaporization of of water and incorporation of of air air during the the fast cooling of can be generated from the vaporization of water and incorporation of air during the fast cooling of rockrock melt on the wet sample surface. rock melt on the wet sample surface. An orange-brownorange-brown sprayspray rim rim was was observed observed via via optical optical microscopy microscopy around around the crater the ofcrater dry sampleof dry An orange-brown spray rim was observed via optical microscopy around the crater of dry sampleA2 (Figure A2 9()Figure and it 9) could and beit could ascribed be ascribed to the thermal to the decompositionthermal decomposition of magnetite of magnetite according according to XRPD sample A2 (Figure 9) and it could be ascribed to the thermal decomposition of magnetite according topatterns XRPD onpatterns samples on A1–A4. samples Octahedral A1–A4. Octahedral sites in the sites magnetite in the atomicmagnetite structure atomic contain structure ferrous contain and ferricto XRPD iron patterns species. on Different samplesT A1and–A4 total. Octahedral fault energy sites values in the observed magnetite on atomic sample structure A2 and A4contain after ferrous and ferric iron species. Differentf Tf and total fault energy values observed on sample A2 and ferrous and ferric iron species. Different Tf and total fault energy values observed on sample A2 and A4multiple after multiple strikes (Table strikes2) (Table could be2) relatedcould be in related part to in the part magnetite to the magnetite grains, which grains, may which increase may theincrease local A4 after multiple strikes (Table 2) could be related in part to the magnetite grains, which may increase theconductivity local conductivity of the sample of owingthe sample to its electromagneticowing to its electr properties.omagnetic The electronsproperties. coordinated The electrons with the local conductivity of the sample owing to its electromagnetic properties. The electrons coordinatediron species arewith thermally iron species delocalized are thermally and migrate delocalized within and the magnetitemigrate within atomic the structure magnetite [30]. atomic coordinated with iron species are thermally delocalized and migrate within the magnetite atomic structureThe electrical[30]. conductivity of a rock at room temperature is determined by the amount of water structure [30]. present,The theelectrical salinity conductivity of the water, of a androck the at room manner temperature in which is the determined water is distributedby the amount through of water the The electrical conductivity of a rock at room temperature is determined by the amount of water present,rock [31 ,the32]. salinity Therefore, of the the water, faster and process the manner involving in thewhich electric the water impulse is distributed discharge and through the total the faultrock present, the salinity of the water, and the manner in which the water is distributed through the rock [31,32].energy (Therefore,E) values observed the faster on process wet sample involving A3 and the A4 (Tableelectric2) couldimpulse be ascribeddischarge to and the variationthe total of fault the [31,32]. Therefore, the faster process involving the electric impulse discharge and the total fault energyelectrical (E properties) values observed for water-bearing on wet sample rock andA3 and the distributionA4 (Table 2) ofcould the waterbe ascribed layer onto thethe rockvariation sample. of energy (E) values observed on wet sample A3 and A4 (Table 2) could be ascribed to the variation of the electricalThe chemical properties interactions for water between-bearing the rock water and and the rock distribution material of at the porewater surface layer on can the also rock be the electrical properties for water-bearing rock and the distribution of the water layer on the rock sample.signiﬁcant in the bubble formation [18,33]. Optical microscopy of the wet samples (A3, A4) showed the sample. presenceThe chemical of bubbles interactions in fulgurite between glass (diameter the water of and bubbles rock clay minerals, and feldspars break down to a variety of clay minerals with plagioclase decomposing ﬁrst [35]. Elmi et al. [18] observed a rock fulgurite on natural chemically weathered granite from Baveno (Italy). These authors ascribed the dark brown-black color of the fulgurite powder to the lightning-induced burning of organic matter (e.g., lichen, leaves, roots, etc.) growing on the surface of the rock before lightning strike. Moreover, Elmi et al. [18] observed intense alteration of the minerals composting the bulk granite. As chemical weathering was not observed in samples B1 or B2, it can be assumed that the intense alteration of granite-forming minerals and the organic matter coating can create a local positive electrostatic charge to which the lightning is attracted.

5. Conclusions The randomness in time and space of lightning occurrences makes direct measurement of lightning difficult and consequently makes the studies on lightning processes generating rock fulgurite challenging. This paper reports the first attempt to measure the electrical conditions required to form a rock fulgurite. The data reported in this study provides a means of directly determining energy, voltage, current, and peak instantaneous power as parameters of an event mimicking the effect of the lightning current discharges hitting the surface of the rock and passing to ground. The textural features and electrical properties of basalt (samples A1–A4) reported in this paper showed that the series of events occurred to produce a rock fulgurite can be summarized as follow: (i) ultrafast mechanical impact of the leader spark with the rock surface. Consequently, a crater appears and an increase of pressure occurs around the hole; (ii) the sudden rise of temperature generated by the lightning channel melting of the rock; and (iii) fast cooling and generation of the glass (fulgurite) in the rock. Further investigations on the microstructure and texture of these samples will be carried out in future experiments in order to investigate the physical properties and thermal effects resulting from electrical arc discharge through a target material. The methods presented it this paper are accessible to a wide range of researchers studying the interaction of electrical discharges and rocks. In particular, methods for extracting fault onset and extinction times from time-series measurements taken with common electronic laboratory equipment have been described. These calculations enable power and energy estimations, which can be used to better understand how electric discharge properties relate to rock fulgurite properties in a quantifiable manner. Technical applications related to lightning effects on materials may potentially benefit from the results reported in this paper. Recognizing the paths that lightning can take through a material and the reactions occurring after a lightning impact are essential for mitigating thermal damage from lightning strikes and understanding the nature of the lightning environment near and at the point of a direct strike. Moreover, understanding how target materials interact with electrical arcs can support the development and testing of materials used in hazard protection devices such as lightning arresters and high-voltage fuses.

Author Contributions: Conceptualization, C.E., N.S.C., and E.S.; Data curation, C.E., N.S.C., and K.M.; Investigation, C.E. and N.S.C.; Methodology, C.E., N.S.C., and E.S.; Supervision, C.E. and K.M.; Validation, C.E. and K.M.; Writing—original draft, C.E. and N.S.C.; Writing—review and editing, C.E., N.S.C., K.M., and E.S. Funding: This research received no external funding. Appl. Sci. 2018, 8, 2394 12 of 13

Acknowledgments: Authors would like to thank Chika Nwankpa for his technical support and thoughtful advice in designing the experimental apparatus at Drexel University’s Center for Electric Power Engineering (CEPE). Three anonymous reviewers are acknowledged for their comments that greatly improved the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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