lightning rod

After checking this standard, it seems that the requirement for ground resistance is a bit high; please take this into consideration! Code for Lightning Protection of Petroleum and Petroleum Facilities GB 15599-95 4.13 Metal oil tanks must be provided with a circular lightning protection grounding system; there should be no fewer than two grounding points, and the arc distance between them should not exceed 30 m. The distance between the grounding electrode and the tank wall should be greater than 3 m. When a lightning rod is installed on the tank top or the tank itself is used as a lightning receptor, the impulse grounding resistance at each grounding point should not exceed 10 Ω.

I have some information on oil depots (from HaiChuan) for reference. 2) Lightning protection design for the oil tank area: In accordance with Chapter 14.2 of GB50074-2002 \"Code for Design of Oil Depots\" regarding lightning protection requirements, metal oil tanks must be grounded in a circular pattern; there should be no fewer than two grounding points, with the arc distance between them not exceeding 30 m. The grounding electrodes should be at least 3 m away from the tank walls. When the thickness of the steel oil tank roof is <4 mm, lightning protection measures against direct strikes should be installed; when the roof thickness is ≥4 mm, such measures are not necessary. However, for oil tanks and oil tanks with aluminum roofs located in thunderstorm-prone areas (with an average annual number of thunderstorm days exceeding 40), separate lightning rods should be installed as protection against direct lightning strikes. The horizontal distance between the independent lightning rod and the oil tank to be protected should be no less than 3 m, and the protection range should extend at least 2 m above the breather valve. In accordance with Chapter 3 of GB 50057-94 \"Code for Design of Lightning Protection of Buildings\" – Lightning protection facilities for buildings, and Chapter 14.2 of GB 50074-2002 \"Code for Design of Oil Depots\" – Requirements for lightning protection: The lightning protection category for oil tank areas should be set as Category I. The protection area for conventional oil tank areas is calculated using the ball-movement method; when the ball radius R is 30 m, the height of the equidistant lightning rods arranged along the diagonals is generally 9.5 m, and their cross-sectional area for conducting electricity should be greater than 100 mm2. These lightning rods can be welded or fastened to the top of the lightning rod tower as required by installation needs. 3) Design of downconductors: The lightning rods and lightning conductors in the station area can use the rebar inside the building as downconductors. The roof lightning conductors should be connected to the four corners in accordance with standard requirements, and these lightning conductors should be linked to the rebar within the building’s concrete. The lightning rod in the oil tank area can use an iron tower as the down conductor; since the iron tower is already properly grounded, it is sufficient to ensure a good electrical connection between the lightning rod and the tower during installation, as well as to apply anti-corrosion treatment. 4) Design of the grounding grid: The grounding grid of a gas station consists of four components: protection grounding against direct lightning strikes (with a required grounding resistance of ≤10Ω), anti-static grounding (also with a required grounding resistance of ≤10Ω), power supply grounding (with a required grounding resistance of ≤10Ω), and DC grounding for signal lines (with a required grounding resistance of ≤4Ω). In accordance with Chapter 3 of GB 50057-94 \"Code for Design of Lightning Protection of Buildings\" regarding lightning protection facilities for buildings, and Chapter 14.2 of GB 50074-2002 \"Code for Design of Oil Depots\" on lightning protection requirements, gas stations should adopt a unified grounding system, with equipotential bonding implemented in all areas. The tank bodies of oil tanks, their metal components, as well as metal accessories such as vent valves and sampling ports; the outer covers of power cables and the iron feet of insulators; the outer covers of wiring cables for information systems installed on steel oil tanks; and the anchor screws of fuel pumps—all of these must be electrically connected to the grounding system in a reliable manner. The required value for the unified grounding resistance is ≤4Ω. Considering the long-term use and corrosion resistance required of grounding grids, it is recommended to use non-metallic grounding modules for constructing such grids. The layout of the ground grid is designed based on the terrain. Horizontal grounding electrodes are made of 40×4 mm galvanized flat steel, buried at a depth of 0.6 meters ； Vertical ground elements use L50×50×5×2000mm galvanized angle steel ； Non-metallic grounding modules are used between vertically connected grounding bodies. The ground grid leads the test electrodes of the ground grid to the ground surface, so as to enable future inspection of the condition of the ground grid. The iron tower should be connected to the ground grid through four feet, and the rebar in the foundations of the machine room and substation should be connected to the ground grid at the four corners. 4. Lightning protection design for the power distribution system 1) Layout of external conductors External conductors include metal water pipes, communication cables, as well as the armor covering of power cables or the metal tubes of cables. All water pipes and cables should be buried and led into the machine room. The armor covering of the water pipes and cables, as well as the protective metal tubes, should be grounded upon entering the machine room. Armored cables or cables enclosed in metal tubes should be used for laying them underground in the machine room, and the phase wires and neutral wires of these cables should be grounded through surge protectors. 2) Layout and selection of surge protectors in the power supply system: A. Principle of surge protector layout is as shown in the figure below: a) This layout is based on the standards GB 50057-94 (2000 edition) and IEC 61312. At the boundary between LPZ0 and LPZ1: U2 = U1 – I2R2. It can be seen that U2 can have its residual voltage gradually reduced through multiple clamping stages, thereby effectively suppressing the intrusion of external lightning waves and the hazards caused by lightning electromagnetic pulses. b) The lightning current passing through the surge protector is reduced step by step, which also facilitates the installation of such protectors. As shown in Figure 3, we always use wires for connections when installing surge protectors, and the inductance of these wires cannot be ignored at the frequency of lightning waves. Thus, we have: Uc = UL1 + Us + UL2; Uc = Is(ZL1 + ZL2) + Us. This residual voltage includes an additional term of Is(ZL1 + ZL2). If there is only one stage of surge protection, most of the lightning current will flow into the ground through this single stage, resulting in a very large value for Is. To ensure that U remains within acceptable limits, Is(ZL1 + ZL2) must be kept small, which means the wires need to be very short. This is often difficult to achieve during installation, making the installation conditions quite stringent. A multi-level arrangement enables the resolution of this issue. c) SPD4 must be as close as possible to the equipment, as GB 50057-94 (2000 edition) and IEC 61312 state that if the distance between the surge protector and the equipment it is protecting is too large, the reflection effects of lightning waves can cause high-frequency oscillations in the protected equipment, resulting in voltages on that equipment exceeding the residual voltage of the surge protector and thus damaging the equipment. This distance should be less than 10 meters. B. Selection of surge protectors a) Selection of operating voltage The three-phase voltage of the surge protector on the low-voltage side of the transformer is the operating voltage ； U0 = 400V. b) Selection of the communication capacity of surge protectors: Regarding the selection of the nominal discharge current for primary surge protectors, GB 50057-94 (2000 edition) and IEC 61312 state that for Class II protection requirements, the selection of surge protectors should be based on a total lightning current of 150 KA (10×350 μS waves). According to the recommended distribution method for lightning currents, 50% of this amount, namely 75 KA, flows directly into the ground through the grounding system (such as water pipes, the outer sheaths of armored cables, or metallic protection conduits for wires) ； The other 50% is grounded through surge protectors installed on the phase and neutral wires. Based on the above criteria, taking into account that 50% of the lightning current is distributed to the most severe conditions in the power supply system, and according to the lightning current parameters provided in Table 6.1 of GB 50057-94 (2000 edition), the lightning current on each phase of the surge protector is approximately as follows: when the line is unshielded, Iimp = ÷4 = 18.75 KA; when the line is shielded, Iimp = ÷4 = 5.625 KA. Given the characteristics of the power lines in this system, in accordance with Article 6.4.7 of Section 4 in Chapter 6 of the Code for Design of Lightning Protection of Buildings, the nominal discharge current for each line should not be less than 15 KA. The nominal discharge current per phase of the primary surge protector should be greater than 15 KA (10/350 μS). The selection of the nominal discharge current for secondary surge protectors is based on Article 6.4.8 of the national standard GB 50057-94: When the voltage protection level provided by the 10/350μs SPD installed in front of the device in accordance with Article 6.4.7, together with the induced voltage across its leads and the effects of reflected waves, is insufficient to protect equipment located further away from it, an SPD must still be installed at that protected equipment. Furthermore, the voltage protection level of this SPD, together with the induced voltage across its leads, is less than 80% of the voltage tolerance level of the equipment being protected. Depending on the characteristics of the protected equipment (such as high-resistance or capacitive types), or when it is open-circuited, the reflected wave effect can double the intruding surge voltage. In accordance with Article 6.4.9 of the national standard GB 50057-94: When there is a distribution panel between the SPDs installed as required by Articles 6.4.7 and 6.4.8, if the voltage protection level of the first-stage SPD, together with the induced voltage protection from its leads, is not sufficient to protect the equipment within that distribution panel, a second-stage SPD should be installed within the panel. When selecting the discharge current In for the SPD in the downstream circuit, the residual voltage in the circuit after the SPD in the upstream circuit is activated, as well as the induced voltages at its terminals and the effects of reflected waves, must be taken into consideration. For the unshielded cable circuits used in this system, the nominal discharge current per phase of the secondary surge protector should be greater than 20 KA (8/20 μS). To protect precision equipment, lightning-proof sockets should be used; they are small in size and can be placed very close to the equipment. Wiring diagram of a gas station equipped with an submersible pump and a level gauge. 3) Design scheme for the power supply system of the gas station: In accordance with the requirements regarding lightning protection zone division and lightning and overvoltage protection for various levels of power supply systems as specified in IEC 61312 \"Protection against lightning-induced electromagnetic pulses\", GB 50057-94 \"Code for design of lightning protection of buildings\", GB 50074-2002 \"Code for design of oil depots\", and GB 50058-92 \"Code for electrical design in explosive and fire-hazardous environments\", the distribution system of a car gas station can be divided into three lightning protection zones, each to be considered separately. As mentioned earlier, single-stage lightning protection may lead to excessive residual voltage after current discharge due to high lightning currents, or equipment damage resulting from insufficient protective capacity. Therefore, a multi-level protection system for the power supply is employed to defend against overvoltages of all types, from direct lightning strikes to operational surges. A. Primary lightning protection zone for power supply]: In accordance with Chapter 6 of the Code for Design of Lightning Protection of Buildings: Protection against lightning electromagnetic pulses ； Section 3: Requirements for shielding, grounding, and equipotential bonding: Article 6.3.4; Section 4: Requirements for surge protectors and other devices: Article 6.4.7 stipulates that SPDs should be installed on the lines coming from outside at the boundary between LPZOA or LPZ0B and LPZ1 zones. When the lines are shielded, the lightning current passing through each SPD is considered to be 30% of the amplitude of the lightning current. Car fuel stations are classified as Category II lightning protection buildings, with a peak lightning current amplitude of 150 KA. The power supply lines use a non-shielded, buried TN distribution system. Therefore, the current distributed to each line in the low-voltage distribution network during a direct lightning strike is as follows: After appropriate lightning protection measures have been installed in the building, 50% of the lightning current flows into the grounding system through the down conductors; thus, the current per line is In = 150 KA ÷ 4 = 18.75 KA. According to Article 6.4.7 of Chapter 6, Section 4 of the Code for Design of Lightning Protection of Buildings, the nominal discharge current for each line should not be less than 15 KA. At the same time, in accordance with Article 6.4.4 of Section 4 in Chapter 6 of the \"Code for Design of Lightning Protection of Buildings\" and the requirements for surge protectors in Part 3 of IEC 61312 \"Protection against lightning electromagnetic pulses\", surge protectors can limit the overvoltage caused by induced lightning strikes, which can reach tens of thousands of volts, to below 4 KV. In summary, a switch-mode modular power surge protector with a nominal current-carrying capacity of 25 KA and a 10/350 μs waveform should be installed in the 380V low-voltage main distribution panel to provide primary power protection for all electrical equipment in the gas station. The author recommends the use of switch-mode modular power surge protectors with a 10/350 μs waveform that employ multi-layer graphite gap technology and special material processes. The advantages of such SPDs over spark-gap type SPDs are as follows: 1) They have a stronger capacity to discharge lightning energy ； 2) Its pulse response time is shorter than that of the spark-gap type SPD ； 3) Its pulse ignition voltage is lower than that of spark-gap type SPDs, with a protection level of less than 2000V, whereas the protection level of spark-gap type SPDs is typically 4000V ； 4) The multi-layer graphite gap-type SPD has no power-frequency freewheeling current, which avoids the freewheeling current and arc-quenching problems of the spark-gap type SPD, resulting in a more stable operating condition. B. Secondary lightning protection zone for power supply]: In accordance with Chapter 6 of the Code for Design of Lightning Protection of Buildings: Protection against lightning-induced electromagnetic pulses ； In Section 4, Articles 6.4.1 to 6.4.12, the requirements for surge protectors (SPDs) in LPZ1 area are specified, as well as the relevant provisions in Chapter 4 of GB 50054-95 \"Code for Design of Low-Voltage Distribution Systems\". Based on the theory of lightning current distribution, an 8/20μs waveform with a current-carrying capacity of 20 KA is required. Chapter 6 of the Code for Design of Lightning Protection of Buildings specifies that for distribution panels, circuit breakers, fixed-mounted motors, etc., which are subject to Category III impulse overvoltage, the voltage withstanding capacity is 4 KV. To prevent a short circuit from the power supply to ground after the surge protector is damaged by a lightning strike, an air switch needs to be installed in front of the surge protector as a short-circuit protection device. Fire-resistant power surge protection boxes with an 8/20μs waveform and a current-carrying capacity of 20KA can be installed separately in the control cables for submersible pumps, submersible pump fueling units, tax-controlled fueling units or ordinary fueling units’ power distribution boxes, as well as in the power distribution boxes located in the sales areas. For the power cables, RVV-type insulated wires with a plastic sheath and good oil resistance, with a cross-sectional area of 4×2.5mm2, should be used. C. Power supply third-level lightning protection zone]: In accordance with IEC 61312-3 Protection against lightning-induced electromagnetic pulses – Part 3: Requirements for surge protectors, in the LPZ2-LPZ3 zones, surge protectors can limit surge voltages to over 1,000 volts, and their current-carrying capacity for (8/20 μs) impulses is ≥10 KA. Socket-type surge protectors can be used at the power switches of computer management equipment, UPS systems, ticket printing devices, fuel pump data transmission equipment, and other precision equipment in the business hall. 5. Signal system protection scheme: When a lightning strike occurs, it generates a massive transient electromagnetic field. Metal loops within a 1 KM range, such as those in networks, signal systems, and communication connections, will be affected by this field; this can disrupt the normal operation of network, signal, and communication systems or even completely destroy them. Lightning protection for networks, signals, and communications is often overlooked; it is usually only when the system suffers severe damage and significant data loss that people realize the need for preventive measures. Regarding the protection of network and signaling equipment in this plan, in accordance with the requirements for lightning and overvoltage protection of signaling systems specified in GB 50174-93 \"Design Code for Computer Rooms\", YD/T5098 \"Design Code for Lightning Overvoltage Protection Systems in Communication Bureaus (Stations)\", and GB 2887-89 \"Safety Requirements for Computer Facilities\", high-power special signal surge protectors with a rated load current of 1–1.5A should be installed on the level gauge control lines that originate from the level gauge detectors in the service halls, afin of protecting the signal lines of these detectors. A precision control signal surge protector should be installed on the main control circuit of the fuel pump in the service station, to protect that circuit. Surge protectors for telephone lines should be installed before the MODEM in the PSTN dial-up network connection and at the incoming end of the telephone communication system, respectively, to provide lightning protection for the network cards of various devices as well as the telephone communication lines.