A field study on the energy dissipation associated with step and touch voltage in earthing systems encased in earth enhancing compounds

An extensive field experiment was conducted to investigate the effects of earth-enhancing compounds on earthing systems. Four identical vertical earth rods were installed, each encased in concrete, Bentonite, and two commercial enhancing materials. Lightning impulse signals were injected into all electrode arrangements, and the measured responses were used to analyse the associated risks of step and touch voltages. The peak voltage values and the corresponding energy dissipation for each measurement were calculated and compared to those of a reference electrode. The analysis of step voltage measurements and the corresponding energy dissipation demonstrated that the use of concrete exhibited greater advantages than the use of Bentonite. However, touch voltage measurements and the associated energy dissipation indicated that despite the lower touch voltage exhibited by Bentonite, its energy dissipation exceeded that of the reference electrode. Consequently, the unique findings reveal that specific variations of earth-enhancing compounds can exhibit reduced earth impedance and lower step or touch voltages while also yielding higher energy dissipation, introducing an unforeseen risk of injury. © 2017


Introduction
Lightning is a fascinating yet dangerous atmospheric occurrence and significantly affects all grounding system designs.The purpose of a lightning grounding system is to safely channel lightning currents safely into the earth, minimizing equipment damage and risk of injury.While direct lightning strikes are commonly perceived as the main cause of human injury, other hazardous mechanisms exist, such as side flashes, upward leaders, and ground currents [1,2].This article primarily focuses on ground currents.
Upon striking, lightning significantly elevates the electric potential at the entry point, referred to as Ground Potential Rise (GPR).It is followed by a potential decrease along its dissipation path, creating a voltage difference along the current flow.This voltage difference can lead to electric shock risks for individuals in proximity, especially due to step and touch voltages.
Step voltage can route current from one leg and leave from the other, and touch voltage can create a current path from hand to feet, exposing vital organs.Therefore, GPR, step, and touch voltages raise significant safety concerns in lightning-related scenarios.
It is imperative to have an efficient low-resistance earthing system to mitigate such ground current injuries.This can be achieved by replacing the immediate soil surrounding the earthing system with Earth-Enhancing-Compound (EEC) [3].Theoretically, for a 3 m long vertical earth rod, approximately 50% of the earth resistance is concentrated within just 0.15 m radially from the rod [4].Bentonite and concrete show promising performance in challenging soil conditions [5][6][7][8][9] Bentonite can be further enhanced by mixing it with chemicals, and it slightly outperforms new formulations of natural EEC types in terms of impulse impedance [10,11].
The overarching goal of these grounding techniques is to minimize equipment damage and prevent injuries.Guidelines such as IEEE Std 80-2013 [12] provide comprehensive grounding grid design recommendations, including maximum allowable step and touch voltages during fault conditions.While the risk of injury is often related to maximum exposed voltage, calculating energy absorption during electric shocks could be a more suitable predictor of damage [2,13].A comprehensive study using live animals found that the minimum lethal energy level from a lightning strike is 62.6 joules per kilogram [14].However, energy-based calculations are less prominent in lightning earthing literature.

Experimental objectives
This article builds upon a sequence of lab and field tests [15,16] aimed at understanding lightning GPR and subsequent step and touch voltages when EECs are employed.Examinations of step or touch voltages often employ AC signals [17], which enables the utilization of a singular voltage value for analysis.Nevertheless, when impulse signals are used, the maximum value or a value at a designated time point is usually used [18].However, capturing the characteristics of an impulse signal through a single value does not provide a comprehensive understanding of its impact, particularly in relation to potential injury risks.Hence, the research introduces and compares energy-centric methodologies for assessing step and touch voltage characteristics.

Experimental arrangement
The experiment was conducted in a spacious, open area without major constructions nearby.Some tall trees and vegetation were present.Measurements were taken during the wet season, with care to avoid substantial rain within a week.Five 1.2-meter vertical earth rods were driven into the ground, spaced about 1 meter apart.All rods were 16 mm in diameter, featuring copper-bonded sheaths and steel cores.
Four rods were enclosed in different cylindrical EEC types, each around 100 mm in diameter.One rod served as a reference, directly buried in the natural soil.These included electrodes encased in concrete, Bentonite, and two rods encased in distinct commercial EECs referred to as EEC1 and EEC2.A simplified arrangement is illustrated in Fig. 1(a), while Fig. 1(b) depicts the four EEC-covered electrodes.

Measurements
A portable combined wave generator [19] was employed introduce impulse signals into the grounding system.The generator can deliver voltage signals of 1.2/50 μs at 2 kV and current signals of 8/20 μs.
Current measurements were taken using a current transformer with a 10 A/V ratio, while a differential probe with a 1000:1 ratio was utilized to measure potential differences.The waveforms were captured using a PicoScope (6000 series) connected to a laptop.

Summary of instruments and measurements
• 2D Soil resistivity profile (Fig. 2) using ZZ resistivity tester -Flash RES-universal, the 64-channel free configuration resistivity/IP exploration system • Earth resistance variation for each electrode using the Fall of Potential (FoP) method.Measurements were taken at one-meter intervals using KYORITSU digital earth tester model 4105A • Current and voltage data to measure the FoP for an injected impulse signal in order to calculate energy dissipation under touch voltage conditions • Current and voltage data are used to measure the step voltage for an injected impulse signal in order to calculate energy dissipation under step voltage conditions.

Resistivity measurements
Fig. 2 illustrates the 2D resistivity profile of the soil cross-section, as measured by the ZZ resistivity tester.The color bar represent the resistivity of the soil cross-section in ohm-meters (Ωm).The data was collected using the Wenner method, with measuring electrodes buried at one-meter intervals along a 50-meter straight line from the corresponding earth rod arrangement.The figure shows relatively constant resistivity values below three meters, but significant variation in the upper layers.
Table 1 shows the resistivity values used in calculating the earth resistance for EEC-encased electrodes.EEC1 and EEC2 values were obtained from the manufacturer's websites, and Bentonite and concrete

V
Voltage on the resistive load (V) V max Maximum value of the voltage signal (V) Earth impedance from FoP curve (Ω) E step/touch Electrical energy dissipated from step or touch-voltages (J) Human body resistance -taken as 1000 (Ω) L Length of the earth electrode (m) resistivity were taken from [20,21].The average resistivity of the surrounding area was taken as 811 Ω, taking the arithmetic average calculated from the resistivity of each meter depth.

Fall of potential and step-voltage measurements
Each electrode setup underwent a standardized FoP test [22] for both low-frequency signals generated by a digital earth tester and a lightning signal from the combined wave generator.The earth system resistance is determined at the point approximately 62 % of the distance between the two outermost electrodes.The same ground pegs utilized in the test were employed to measure the step voltage drop between each pair of pegs.Consequently, step voltage measurements were carried out at one-meter intervals along a straight line originating from each electrode configuration.A simplified representation of the FoP test and step voltage test is depicted in Fig. 3.

Impulse signal
A typical voltage and current impulse signal injected during the experiment is shown in Fig. 4 in a short time span.As and when required, all impedance calculations were performed considering the peak voltage and peak current values.Consequently, the impedance (Z) can be modelled as below [23];

Energy dissipation due to step and touch-voltage drop
The energy transferred during exposure to lightning current can be calculated by multiplying the action integral with body resistance [2], making standard electrical energy equations applicable, including Eq. (2) below;   In a step-voltage test, the voltage drop between two pegs can represent the voltage difference experienced by a person standing along the current flow path.This measured voltage can be used in Eq. ( 2) to calculate the energy dissipation due to step-voltage with human body resistance, typically taken as 1000 Ω [12].The energy calculated will be referred to as "step-energy" dissipation within the article.
Touch voltage is the difference in voltage between a person's hand and feet when the hand is in contact with an energized part of an earthed structure while standing on the ground.Meanwhile, the fall of potential is the voltage difference between two points on the ground, one on the earthed electrode and the other further away [22].Consequently, the results from the FoP assessment can be used to represent touch-voltage in the experiment.Subsequently, FoP recordings will be used to compute the energy dissipation during exposure to touch-voltage.The resulting energy dissipation will be denoted as "touch-energy" dissipation within this article.

Data analysis and investigations
An earthing system's performance is typically accomplished by evaluating its earth impedance, maximum step voltage, and touch voltage.This study introduces energy dissipation as an additional effective parameter for appraising earthing system performance.The analysis includes the determination of earth system impedance, step and touch voltage, and energy dissipation across four EEC-encased electrodes compared to a reference electrode.

Fall of potential test and earth impedance
The FoP test was used to calculate both the low frequency and impulse Impedance of each grounded electrode.The earth resistance values were directly measured using the digital earth resistance tester, while impedance was calculated from Eq. (1).Eqs.(3) and 4 [24] are used for comparative purposes to calculate each electrode's theoretical earth resistance.
For single vertical electrodes without EEC, For single vertical electrode with EEC; Table 2 compares theoretical and measured resistance and impedance values.The reference electrode displays the highest value in all categories.Like Bentonite, concrete is known to improve earthing performance [5,9], and concrete shows the lowest in both measured values and slightly higher value in the calculated resistance.

Step-voltage and step-energy dissipation
The highest step voltage was recorded in the first measurement for all five electrodes.To normalize the step voltage values, each value was divided by the maximum value of the respective injected voltage.The same step-voltage measurements were used to calculate the step-energy dissipation using Eq. ( 2).Since the step-energy dissipation is also related to the injected energy during the test, the calculations were normalized to unit injected energy before calculating the percentages.The percentage reduction of each outcome compared to reference electrode results is presented in Table 3.

Touch-voltage and touch-energy dissipation
The touch-voltage was investigated at the 15th measurement from the earth electrode.This measurement corresponds to approximately 62 % of the distance between the current injection and auxiliary electrodes.Table 4 illustrates the comparison between the reduction in touchvoltage and touch-energy dissipation as a percentage of reference electrode results.Similar to the previous calculation, all the values were normalized to the maximum relevant injected voltage and the injected energy dissipation before calculating the percentage reduction.

Discussion
While earth impedance or resistance is often used to estimate an earthing system's effectiveness, reaching a consensus between various measured or calculated values in real-world applications can be difficult.Although the impedance and resistance values outlined in Table 2 differ in terms of absolute values, their order of magnitude is consistent, excluding theoretical calculations.The primary cause of the discrepancy between theoretical and measured values lies in the linear dependence of the equations on resistivity values.These resistivity values may not accurately represent the true soil or EEC characteristics in terms of their impact on earth resistance/impedance.However, the findings are consistent with existing literature, indicating that concrete marginally outperforms Bentonite [5,6].The evaluation of two tested commercial products reveals similar earth resistance and impedance performance.Consequently, a more robust practical measurement technique was imperative to assess the effectiveness of an earthing system.Findings from both step and touch voltage studies demonstrated that earth resistance or impedance alone is insufficient for evaluating the efficiency of an earthing system.

Step-voltage and energy dissipation
Exposing to step voltage usually does not result in fatality for humans, as the current typically enters through one leg and exits through the other, safeguarding vital organs.Nevertheless, it could present a potential risk of electric shock and subsequent injuries due to indirect consequences.With respect to step voltage and step-energy dissipation, Table 3 exhibits a consistent order across all five electrodes.The reference electrode displays the highest values, and the two commercial EECs display the lowest values.Experimentally, it shows that step-voltage signal peak value and step-energy dissipation have an imperfect linear relationship.
While concrete exhibits superior performance in impedance, Bentonite outperforms concrete in terms of step voltage and step-energy dissipation.Specifically, Bentonite reduces over 50 % in step-energy dissipation compared to the reference electrode.Interestingly, two commercial EECs, which demonstrated moderate performance in impedance calculations, display a remarkable reduction of over 90 % in step-energy dissipation.This positions them as the safest option in mitigating step voltage-related hazards.

Touch-voltage and energy dissipation
Considering the preceding measurements and computations, incorporating any EEC results in an improved performance compared to using the electrode alone without any such enhancement.This pattern is similarly observed in the touch voltage calculations, as illustrated in Table 4, where using concrete and Bentonite as EECs leads to nearly equivalent reductions in touch voltage.However, in touch-energy calculations, the bentonite-encased electrode unexpectedly demonstrates higher energy dissipation, even surpassing the outcomes of the reference electrode by 100 %.
Despite this unanticipated behavior from Bentonite, both concrete and two commercial EECs still exhibit a significant reduction in touchenergy dissipation.No existing literature reports a similar phenomenon of EECs resulting in weaker earth system performance.Therefore, an additional 23 FoP measurements were conducted, employing similar calculations to explore this observation further.The results consistently revealed the elevated energy dissipation by the Bentonite-encased electrode.The dissipation of touch-energy from the first and tenthmeter measurements is illustrated in Figs. 5, using a bar graph.
To investigate deeper into the cause of this behavior, measured and injected voltage signals were compared across all electrode configurations.The rest of the analysis focused on the current and voltage signals collected at the 15th measurement.Fig. 6 presents both injected and measured touch voltages for all five electrodes.As shown in Fig. 6a, all five injected voltage signals exhibit near-identical waveforms, largely overlapping one another.Contrastingly, Fig. 6b reveals distinctive variations in the measured touch voltage signals.Notably, the areas under the bentonite curve are unmistakably greater, directly corresponding to the calculated higher energy levels.Figs.6c and 6d provide a closer examination of these signals within a shorter timeframe.

Frequency variation of earthing impedance
The slow decay of the voltage signal produced by the Bentonite encased electrode seen in Fig. 6b can be attributed to the high capacitive nature of the electrode system.All earthing arrangements exhibit slight capacitive behavior, as evidenced by the leading current signal relative to the voltage signal, as seen in Fig. 4. To further investigate this response, the complex earthing impedance Z(ω) was calculated by dividing the voltage phasor V(ω) by the current phasor I(ω) obtained after applying the Fast Fourier Transform (FFT) to the signals [25].Fig. 7 shows the complex earthing impedance and phasor plots for each electrode arrangement.All earth rods exhibit an approximately constant impedance at low frequencies.While having different values, the same order of magnitudes displayed in Table 2 was seen in the calculated complex impedance.With the increased frequency, a marginal impedance drop was seen in the Bentonite-encased electrode.Analysis of the phase angle plot indicates that the cause of the impedance decline with increasing frequencies could be due to the significant capacitive nature of the electrode, as evidenced by the substantial reduction in the phase angle.In addition to the Bentonite-encased electrode, the reference rod shows a significant decrease from a positive phase angle.This observation and the fact that both electrodes exhibit the highest energy dissipation in Fig. 5 suggests that the capacitive behavior probably plays a significant role in energy dissipation described in previous discussions.
The increased capacitive behavior can further be extended to the dielectric permittivity inherent in each material.In the case of pure Bentonite, the dielectric permittivity is approximately 600 at around 1 MHz frequency, progressively increasing with decreasing frequency to about 800 at 1 kHz [26].In comparison, the dielectric constant for concrete is roughly 150 at 1 MHz, as documented in [27].Furthermore, Bentonite exhibits superior moisture absorption capabilities compared  to concrete, further enhancing its dielectric constant [28].Consequently, this leads to increased capacitance in the Bentonite-encased earth rod configuration, subsequently slowing the voltage signal's decay and resulting in greater energy dissipation.
As observed with Bentonite encased electrode, lower earth resistance, and higher energy dissipation introduce a risk of potentially underestimating the risk of real exposure to touch voltage.In order to arrive at conclusive results, it is necessary to investigate deeper into the dielectric properties of these EECs and the distinctive attributes of the relevant part of the soil.However relying solely on a conventional touch voltage value might prove insufficient in evaluating the risk of touch voltage exposure.The unexpected outcomes seen in the touch-energy dissipation of Bentonite were undetected in the step-energy calculations and in time domain impedance measurements.

Conclusions
Four earth electrodes encased in concrete, Bentonite, and two commercial Earth-Enhancing-Compounds (EECs) were tested compared to a reference electrode responding to an impulse signal.The purpose was to assess the impact of these materials on earth impedance and the step and touch voltages.The results indicated that applying EECs notably decreased earth impedance for both steady-state and impulse signals.Concrete yields slightly better earth impedance than Bentonite under similar conditions, supporting the existing literature.
In terms of step and touch voltages, all EECs significantly reduce  Interestingly, using Bentonite as an EEC resulted in an unexpected outcome.The dissipation of energy calculated from touch voltage measurements for bentonite-encased electrodes was higher compared to that of reference electrode results.This counterintuitive result persisted even though Bentonite demonstrated notably improved earth impedance as well as step and touch voltage values.This finding was further confirmed by additional measurements.As a result, this field study suggests that Bentonite has the potential to increase the risk of touch voltage-related injuries.
An earthing impedance analysis in the frequency domain provides evidence of the strong capacitive nature of Bentonite which probably causing the higher energy dissipation.Consequently gaining a deeper understanding of the dielectric properties of materials and the effective soil properties at the site would contribute more insights into these outcomes.The introduction of energy-based calculations to step and touch voltage measurements presents a more dependable approach to analyzing potential risks associated with the performance of earthing systems.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 4 .
Fig. 4. Typical current and voltage signal used in the experiment.

Fig. 6 .
Fig. 6.Measured touch-voltage and injected voltage signals for all five earth rods for 15th FoP measurement -(a) and (b) for long time span, (b) and (c) for short time span.

Table 1
Resistivity values of selected EEC types.

Table 2
Resistance and impedance comparison of EEC-enhanced electrodes.

Table 3
Percentage reduction in step-voltage and step-energy dissipation compared to reference electrode results.

Table 4
Percentage reduction in touch-voltage and touch-energy dissipation compared to reference electrode results.