Introduction
The 5G technology offers unprecedented possibilities for smart coal mining by enabling data transmission, information exchange, and scene perception through electromagnetic waves emitted by base stations [1–5]. However, since electromagnetic waves are carriers of energy, there is an inherent contradiction between explosion safety and communication efficiency when electromagnetic waves propagating in an explosive underground environment containing methane.
According to the IEC 60079-0 standard, the threshold power, also referred to as the equivalent isotropically radiated power (EIRP), must not exceed 6 W for underground RF equipment. The EIRP is defined as the sum of the transmitter output power at the port and the antenna gain, representing the effective radiated power in the direction of maximum antenna gain. This limitation ensures the safe operation of RF equipment in explosive atmospheres by restricting the total energy that could potentially ignite hazardous gases [6]. Under this standard, most underground 5G base stations use civilian fill-in stations rated at 250 mW or 500 mW, which are adapted for explosion-proof usage as primary communication stations. This approach leads to limited signal coverage, low edge transmission rates, high station density, and increased maintenance costs, severely restricting the potential application of 5G technology in underground mines.
Current standards are based on scientific research from the 1970s and 1980s, including studies by British scholars on electromagnetic fields near the Royal Navy’s power stations in Scotland [7–11]. Rosenfeld et al. [12] used a 50-Ω purely resistive circuit to simulate the characteristic impedance environment in RF transmission and tested the explosion-proof performance of circuits below 1.5 MHz. He conducted explosion tests on continuous waves at 10 MHz, measured the voltage across the spark gap when igniting methane, and hypothesized about conjugate matching with matched loads. These results initially established the boundary power for igniting a methane-air mixture at 12 W. During the adoption of these research findings into the British standard, a safety factor of 2 was applied to ensure a conservative margin of safety, thereby reducing the defined safety threshold to 6 W. Furthermore, although the original experimental investigations primarily focused on frequencies below 10 MHz, the standard expanded the applicable frequency range from 9 kHz to 60 GHz, thereby covering almost all underground RF equipment currently in use.
Given that the lowest 5G frequency band reaches as high as 700 MHz, low-frequency results should not be directly applied to high frequencies. Furthermore, considering that frequency variation can significantly affect the discharge mechanisms, the preliminary experimental methods, underlying assumptions, and resulting data have been carefully validated and confirmed to be applicable to high-frequency environments.
In specialized environments, such as underground coal mines, 5G communications primarily focus on the 700 MHz, 2.1 GHz, 2.6 GHz, and 3.5 GHz frequency bands. These bands provide bandwidths well-suited to meet the communication demands of complex subterranean settings and have already been widely implemented in underground coal mines in China. In contrast, higher frequency bands, such as those in the 25–40 GHz range, are generally unsuitable for these environments due to their shorter wavelengths, rapid signal attenuation, and limited transmission distances.
This research belongs to an interdisciplinary field, interdisciplinary nature of explosion protection with RF electromagnetic energy, integrating multiple scientific domains, such as plasma dynamics, electromagnetism, communication theory, explosion-proof technology, and thermodynamics. Since the 1970s and 1980s, international academic research in this field has been relatively scarce, leading to a paucity of relevant literature.
Maddocks and Jackson [13] observed that the combined operation of 20 RF sources, each delivering 16 kW, failed to ignite methane gas, and the spark power calculated by measuring stray capacitance and inductance was only 1.54 W, far below the ignition threshold of 6 W. Sun and his colleagues [14, 15] proposed a model of magnetic coupling resonance and electromagnetic wave radiation resonance energy coupling for the safety distance between RF sources and coupled structures, analyzing the influencing factors of magnetic coupling resonance and the impact of operating frequency on safety distance.
Feng et al. [16] and Peng [17] analyze the safety of electromagnetic wave energy but have not yet succeeded in igniting methane gas with RF energy in experiments. Liu et al. [18] used electromagnetic simulation software to analyze the safety distances at different frequencies, although this study relied on the assumption of conjugate matching between antenna impedance. Tian et al.’s [19] calculation model for the safety distance between underground RF equipment and detonators provides a reference for the safe use of electromagnetic waves below 10 MHz.
Although the aforementioned scholars have conducted research on igniting methane gas with high-frequency electromagnetic energy, they have not been able to precisely measure the boundary ignition power or successfully achieve the ignition of methane gas with high-frequency electromagnetic waves in their experiments. To address this issue, the present study designed specialized RF electromagnetic energy explosion-proof testing equipment and methods to precisely explore the safety thresholds for igniting methane within the 5G communication bands.
Materials and Methods
1. Design Principles
In-depth research has shown that the IEC spark tester is unsuitable for frequencies above 1.5 MHz, primarily because the energy is fully reflected and cannot reach the discharge electrodes [20–22]. Consequently, directly connecting RF energy to an IEC spark tester does not allow the observation of methane ignition.
Based on these findings, the following design principles were proposed for the RF electromagnetic energy explosion-proof testing apparatus developed in this study. 1) RF energy must be fully fed into the discharge electrodes. 2) The apparatus should be capable of simulating the ignition of methane gas at the transmitter end as well as in near-field and far-field scenarios. 3) The device should facilitate the observation of ignition phenomena and simultaneously record the instantaneous power of the RF source. 4) The discharge electrodes should possess the necessary conditions to form the optimal discharge gap. 5) Explosive gases must not be ignited by non-RF energy sources, such as mechanical sparks, motor sparks, or hot surfaces.
2. Simulation and Structural Design
Through an impedance matching design, this study successfully facilitated the feeding of RF energy into the discharge electrodes. We designed an RF electromagnetic energy ignition test apparatus based on a dipole antenna, as shown in Fig. 1(a). Each antenna arm was 71 mm long with a diameter of 15 mm, and the antenna flange had a diameter of 40 mm. The feeding electrodes had a diameter of 0.3 mm, and the notch on the antenna flange to form the discharge gap measured 9 mm in length and 5 mm in width.
In this design, the most concentrated energy was located at the feedline. Therefore, we structured a micrometer-scale discharge gap at this location. As depicted in Fig. 1(b), considering the frequency of 700 MHz, it was necessary to consider the impact of the gap between the two arms of the antenna on the S parameters. If the gap was too small, nearly all of the energy would be reflected. However, according to the dipole antenna theory, if the gap was too large, it would result in an imbalance in the currents of the two arms after feeding, losing the characteristics of a symmetric oscillator. Therefore, the maximum gap selected for simulation was 12 mm.
The simulation results showed that, at 700 MHz, with a 10-mm gap between the arms, the dipole antenna’s S11 parameter reached −28 dB, at which approximately 99.84% of the energy could reach the discharge electrode. Based on these results, we selected 10 mm as the optimal gap between the arms. For experimental apparatuses for other frequency bands, the design approach remains essentially the same; adjustments would primarily be based on the dimensional parameters of the key components.
3. Experimental Device Design
After constructing the model, additional designs were required for the actual experiment, including sealing the metal structure to create an enclosed ignition chamber specifically designed for controlled gas injection, ensuring a stable environment for accurate experimental results. The cadmium disc was thickened to enhance the balance of the rotating assembly and address issues caused by the mass of the copper tube. Plastic bearings were introduced to facilitate smoother rotation and reduce wear and tear on the moving parts. Valves for gas inflation and exhaust were meticulously installed to regulate the flow and composition of the gas within the chamber, which was crucial for maintaining precise experimental conditions.
The detailed construction depicted in Fig. 2(a) not only shows the physical layout but also highlights the integration of safety features, such as the explosion vent, and functional components, such as rubber rings for sealing and a 3D-printed casing for robust structural integrity.
The working principle of the experimental device is depicted in Fig. 2(b). The motor drives the left arm of the antenna to rotate, while the right arm remains fixed. The tungsten wire is welded onto the coaxial line feeding the antenna, and the cadmium disc is directly electrically connected to the left arm of the antenna, featuring a grooved structure. As the motor rotates the left arm, the tungsten wire slides relative to the cadmium disc. This interaction creates a contact-break-contact cycle where, when the tungsten wire reaches or leaves the interface of the cadmium disc, it forms a discharge gap that meets Paschen’s Law, thereby igniting the methane gas. Tungsten and cadmium were selected as discharge electrodes to prevent mechanical sparks caused by electrode friction, enhancing safety and effectiveness.
4. Experimental Circuit and Experimental Method
The experimental apparatus can theoretically conduct experiments at either the transmitter or the receiver end. According to the Friis transmission formula, electromagnetic waves experience significant energy loss during transmission in space, and this loss increases with frequency. To minimize energy loss during experiments and accurately measure the RF energy fed into the discharge electrodes, we designed the experimental circuit shown in Fig. 3.
The test circuit consisted of an RF source, a protection circuit, a measurement circuit, and an ignition testing device. The RF source, comprising a signal generator and a power amplifier, provided high-power RF signals with various modulation schemes for the experiment. When the tungsten wire and the cadmium disc created a discharge gap, the dipole antenna might have been in a state of impedance mismatch, resulting in a high voltage standing wave ratio and total reflection. The protection circuit prevented the reflected energy from damaging the RF source. The measurement circuit, composed of a bi-directional coupler and a power meter, measured the boundary ignition power of the RF signal, which is the safety threshold of electromagnetic waves. The explosion testing device, which was specifically designed for this study, is described in Section II-3. To prevent RF radiation from harming humans, the experimental setup was placed in a microwave anechoic chamber, with operators remotely conducting the experiment from the control room.
Results
1. Experimental Phenomenon
This study systematically explored the ignition of methane gas using RF electromagnetic energy across various frequency bands, effectively quantifying the boundary ignition power required under diverse conditions. Fig. 4 illustrates pivotal phenomena observed during these experiments. Fig. 4(a) displays the electrical sparks generated by RF electromagnetic energy, while Fig. 4(b) captures the decisive moment when the methane gas ignited, which was recorded with a high degree of precision by a high-speed camera.
It is crucial to understand that, although electrical sparks produced by RF energy are common, their mere presence does not guarantee the ignition of methane gas. Methane ignition critically depends on the energy level of the sparks. Specifically, our experiments confirmed that methane only ignites if the total discharge energy of the sparks, accumulated within a critical timeframe of 200 μs, exceeds the threshold of 0.28 mJ. This energy threshold is the minimum required to initiate a methane explosion, and it is in line with the quantitative energy parameters established through rigorous testing.
Furthermore, the relatively slow rotational speed of the cadmium disc and tungsten wire, which peaked at 400 rotations per minute, ensured that the sparks produced by the RF electromagnetic waves had ample time to ignite the methane gas within the crucial 200-μs timeframe.
Thus, the key determinant of whether RF electromagnetic energy can ignite methane gas shifts to the sufficiency of the electromagnetic wave’s power. This insight enables a precise assessment of ignition conditions and supports the establishment of reliable safety standards for environments potentially exposed to methane gas.
2. Experimental Data
In the boundary ignition power test experiment, the apparatus was filled with a methane/air mixture at a concentration of 8.3%, which is considered the most ignitable concentration for methane gas. In the test, Power Meter 1 measured the forward power of the electromagnetic waves, denoted as Wf, while Power Meter 2 measured the reverse power, denoted as Wr. The simulation results indicated that gap voltage is determined by forward power and is independent of reverse power. Therefore, when an explosion occurred, the reading of Power Meter 1, Wf, served as the indicator for the safety threshold.
Using the experimental apparatus and the methods described in Section II, 100 successful methane ignition experiments were conducted. As shown in Fig. 5, the boundary ignition powers measured at frequencies of 700 MHz, 2.1 GHz, 2.6 GHz, and 3.5 GHz were 49.7 W, 14.2 W, 13.5 W, and 12.4 W, respectively. Additionally, the minimal difference between the highest and lowest ignition powers indicated that this experimental method has a high degree of reproducibility.
In addition to presenting the boundary ignition powers for various frequencies, it is important to discuss the potential variability in ignition probability as a function of power. As electromagnetic energy interacts with the methane/air mixture, the ignition probability increases with rising power levels. This is due to the increased likelihood of energy reaching the critical threshold needed to ignite the methane, as more power provides a greater buffer against energy loss in the system. Therefore, even slight increases in power can significantly affect the ignition probability, explaining the differences observed at various frequencies.
In accordance with IEC 60079, a spark test was conducted on AC circuit, generating 8,000 sparks without causing an explosion, indicating the intrinsic safety of the equipment. To ensure the reliability of the data, the input powers for the 700 MHz, 2.1 GHz, 2.6 GHz, and 3.5 GHz bands were set to 49.6 W, 14.1 W, 13.4 W, and 12.3 W respectively, with each power input reduced by 0.1 W compared to the measured boundary ignition powers.
The experiment was conducted according to the following procedures. The rotation speed was set at approximately 200 rpm, with each rotation generating at least four sparks. The system was continuously rotated for 12 minutes, accumulating a total of 9,600 sparks to observe whether methane ignition would occur. After multiple repetitions, reducing the power by 0.1 W in each frequency band did not result in methane ignition. The reliability of the test gas was confirmed using an igniter, further proving the reliability of the data shown in Fig. 5(b).
Discussion
Current standards for safety in environments with RF devices, including 5G technology, typically use the EIRP as a safety threshold. This indicator stipulates that, to prevent hazardous discharges, the sum of port power and antenna gain must not exceed 6 W. Our study, however, challenges the necessity of strictly adhering to the EIRP for safety thresholds by conducting direct ignition experiments at the transmitter end and scrutinizing the potential for RF energy to ignite methane under controlled conditions.
In considering the potential mechanisms for RF electromagnetic waves to ignite methane gas in underground coal mines, we identified three primary scenarios. The first involved the production of electrical sparks due to direct discharge gaps on exposed transmission antennas. The second scenario involved direct collisions and ionization discharges in the air, which typically require extremely high electric field strengths (e.g., 3×108 V/m) that are unachievable with communication electromagnetic waves.
The third scenario involved a low-probability event in which incidental metal conductors in the mine might intercept and resonate with RF waves emitted by the base station. Although it is theoretically possible for metal conductors to resonate with electromagnetic waves and form discharge gaps, the probability of such occurrences is extremely low. According to the Friis transmission formula, the energy reception efficiency of these conductors typically does not exceed 1%. This implies that, even if such metal conductors existed and could discharge, the required output power from the base station would need to be hundreds of times that of the first scenario to achieve the necessary discharge energy. This requirement is impractical in operational terms, as such high power levels are not only difficult to achieve but could also lead to other severe safety issues.
In summary, our experimental design has considered all possible extreme conditions, including the low probability but theoretically possible third scenario. Thus, we deem the further use of EIRP as a safety threshold unnecessary in our experimental context. Our study provides empirical support for the safe use of RF electromagnetic waves in underground mines, establishing reliable safety standards to prevent accidental methane gas explosions.
Removing power constraints could significantly improve issues related to small base station coverage areas, low edge rates, and high maintenance costs. For instance, at 700 MHz, even with a safety factor of two, the port power nearly increased by 100 times. Assuming a transmission antenna gain of 6 dB (which is commonly used underground) and a reception antenna gain of 3 dB (which common for devices such as smartphones), according to the Friis transmission formula, this setup would expand the theoretical maximum transmission distance of the base station by approximately tenfold, and the number of base stations could be reduced to about 1/10th of the original.
Moreover, this experimental method is equally applicable to 5G technology in Class II and III explosive industrial sites, such as petrochemicals, similarly aiding the improvement of 5G technology applications in these industries.
Conclusion
Given the inapplicability of current standard safety thresholds to 5G technology and the limitations of the IEC spark test in high-frequency electromagnetic explosion-proof research, this study has developed a specialized experimental apparatus and method for RF electromagnetic energy explosion-proof testing, which is capable of efficiently and repeatedly igniting methane/air mixtures.
This research precisely measured the boundary ignition power for typical 5G frequency bands, establishing safety thresholds for 700 MHz, 2.1 GHz, 2.6 GHz, and 3.5 GHz at 49.7 W, 14.2 W, 13.5 W, and 12.4 W, respectively.
Through this study, we effectively alleviate the inherent contradiction between explosion-proof safety and communication technology advantages in underground coal mine communications. While ensuring safety, we fully utilize the technological benefits of 5G in the field of intelligent coal mining and provide references for the application of 5G technology and other RF devices in various explosive environments.
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