Lightning protection grounding for buildings

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Discussion on the Structure of Lightning Protection and Grounding Systems in Modern Buildings
Publication date: November 5, 2005, 18:22:28
Source: Focus Real Estate Network
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1. Relevant concepts
Connecting electrical equipment to a grounding system is referred to as grounding. The grounding of electrical equipment is an important aspect for ensuring personal safety and the proper operation of electrical equipment; it is also the most crucial element in lightning protection technology. Based on their function, grounding can be divided into three categories: (1) Protective grounding, which refers to the grounding of the enclosures of electrical equipment and its non-live metal parts under normal conditions. Such as the grounding of the enclosures of electrical equipment like generators and transformers. 　　(2) Working grounding refers to the use of the earth as a conductor in systems such as power and communication, or grounding carried out to ensure their proper operation. Such as the ground wire in the three-phase four-wire system of power supply systems, and the grounding of the neutral point of certain transformers. 　　(3) Lightning protection grounding refers to the grounding of the metal structure of overvoltage protection devices or equipment. Such as the grounding of lightning arresters and the grounding of lightning rod frameworks, which is also known as overvoltage protection grounding. 　　The grounding device consists of a grounding electrode and a grounding wire. 　　Ground resistance refers to the resistance encountered by current as it flows through the grounding device into the ground. Numerically, the grounding resistance is the ratio of the voltage between the grounding electrode and a point at infinity to the grounding current, that is, Re = Uj/Ie. Where: Re represents the grounding resistance (Ω), Ie represents the grounding current (A), and Uj represents the voltage between the grounding electrode and a point at infinity (V). The main factors affecting grounding resistance include soil resistivity, the size, shape, and burial depth of the grounding electrode, as well as the connection between the grounding wire and the grounding electrode. 2 Structure of the lightning protection grounding system Whether it is to protect against direct lightning strikes or induced lightning, in both cases the lightning current is ultimately conducted into the ground through the grounding system. Therefore, a proper grounding system is essential to carry out lightning protection tasks. 　　In the design, construction, and acceptance of lightning protection systems, people often tend to focus solely on the value of the grounding resistance, considering it as the most important indicator of the quality of such systems. It is believed that the lower the grounding resistance, the better the lightning protection effect, and the safer the objects being protected will be. It is undeniable that there are certain requirements regarding the grounding resistance value of the grounding devices in lightning protection systems; indeed, the lower the grounding resistance, the faster the current can be dissipated, the shorter the time during which the high voltage remains on the object struck by lightning, and thus the less dangerous it becomes. As a result, the step voltage and contact voltage also decrease. However, theory and practice have shown that the structure of the grounding grid deserves more attention than the grounding resistance. 　　With the rapid advancement of science and technology, people are no longer unfamiliar with the term modern buildings. A so-called modern building is one that features systems such as power supply, computers, and communication systems in operation within it. For the safe operation of these systems, various types of grounding devices are often required, and determining how to handle their interrelationships in a rational and scientific manner becomes an issue that cannot be avoided. 3 Independent grounding has been largely replaced. Independent grounding refers to the situation where systems that require grounding have their own separate grounding networks, with sufficient distance required between these various grounding networks. This grounding method was widely used in the 1950s and 1960s because it prevented interference between different grounding systems, a feature that is particularly important in communication systems. However, in recent years, this independent grounding method is used only in particularly hazardous environments; in the vast majority of cases, the common grounding method is adopted. This is because: (1) The grounding of various communication systems, computer systems, and power supply systems is done to obtain a zero potential point. If each system is grounded separately, the potentials at their grounding points can differ significantly when a lightning strike occurs. It is assumed that “1” represents the grounding of the AC power supply, “2” represents the logical grounding of the computer system, and “3” represents the safety grounding of the chassis. Assuming that current shock waves originate from \"1\", and since lightning-induced voltages can reach hundreds of thousands of volts, this means that the components on the same computer that are connected to the power supply, communication lines, and the chassis must withstand such high voltages between different networks, resulting in breakdown and damage to those components. In computer networks, modems and network cards will be the first to fail. It is understood that in computer communication networks, systems with independent grounding have a much higher probability of being damaged by lightning strikes compared to those with common grounding. 　　(2) It is quite difficult to create several separate grounding grids within a single building or complex that are not electrically connected to each other, especially in modern large cities. Therefore, when an independent grounding method is used, there must be a distance of at least 20 meters between different ground grids, and sufficient distance must also be maintained from various underground metal pipes, the metal shielding layers of cables, and various metal components. These requirements are difficult to meet in actual design and construction. Even if these requirements are met when a new system is built, they are very likely to be violated during subsequent system maintenance and other urban renovations. 　　The above are the fundamental reasons why independent grounding is gradually being replaced. 4 Single-Point Grounding and Interference Analysis A major reason for implementing an independent grounding system is to avoid signal interference and eliminate \"noise\". Theory and practice have shown that the aforementioned problems can also be **solved** without using an independent grounding method. 　　In the 1960s and 1970s, \"interference\" was also referred to as radio interference, as the vast majority of electronic noise and interference signals occurred within the radio frequency range. With the advancement of technology, a large number of computers, digital technologies, and logic circuits are being used in people’s daily lives and work, and the definition of interference has been extended to refer to \"electromagnetic interference\". 　　Electromagnetic interference can be divided into conductive electromagnetic interference and radiative electromagnetic interference; the interference energy in the former type is transmitted from one circuit to another through wires or cables. Ways to reduce conductive electromagnetic interference include proper circuit design, the use of filters, and proper circuit grounding ； The interference energy of radiative electromagnetic interference is transmitted through the electromagnetic field in the air. Typically, when designing the enclosures and casings of electronic devices, as well as the wiring and grounding connections for these devices, radiation-induced electromagnetic interference is reduced by selecting appropriate shielding materials, employing suitable construction techniques, arranging the wiring and grounding connections properly, and using scientific grounding methods. It can be seen that proper grounding is an important way to effectively prevent electromagnetic interference. 　　Most low-frequency interference arises from mutual coupling through the lines, namely common impedance coupling. When the currents of two circuits flow through a common impedance, the voltage generated across this common impedance by the current in one circuit affects the other circuit; this is known as common impedance coupling. 　　Figure 1 (omitted) shows a common grounding network to which different wires are connected at various points. Due to impedance coupling, voltages such as Vg1, Vg2, etc., will exist between these wires. Points A, B, and C cannot be at the same potential, which creates interference sources; when amplified, this interference can directly affect communication and control signals. 　　Figure 2 (omitted) shows the same 3 devices, but with their grounds connected to point B; this prevents the common impedance coupling effect caused by the current Ic. At low frequencies, the potentials at points B and C are essentially the same as that at point B. Such a connection method is called the “single-point grounding method”. The single-point grounding method resolves the issue of equal potential among the grounding wires of various systems, thereby initially addressing the interference problems between these systems; in particular, the interference caused by 50Hz power-frequency signals is essentially eliminated. Therefore, the single-point grounding method is widely used in engineering. 　　Grounding at one point eliminates interference caused by common impedance coupling and low-frequency ground loops. Single-point grounding works well at frequencies of 1 MHz and below, and can also be applied up to 10 MHz when the size of the entire system is small (with a maximum size of less than λ/20, where λ is the wavelength of the interference signals of interest). 5 Ring grounding and equipotential bonding A ring grounding grid consists of grounding elements arranged in a closed loop around the building. Such a grounding grid can make the electric field distribution within the interface relatively uniform, reducing the harm caused by step voltage to people. It can also minimize the risk of high-voltage backfeed to equipment inside a building in the event of a lightning strike, due to the large potential gradient on the ground. 　　Equipotential bonding involves electrically connecting all metallic elements within a building, such as rebar in concrete, water pipes, gas pipes, other metal pipelines, the metal components of machinery foundations, large buried metal structures, the metal shielding layers of cables, the neutral wire in electrical systems, and the grounding wires of lightning protection systems. This is done through welding or reliable conductive connections, so that the entire building becomes a single equipotential body. When lightning strikes, the interior and vicinity of this building are generally at the same potential, preventing high voltages from affecting the internal equipment and causing electric shock to people. Furthermore, all metal wires that are in contact with the outside world, such as power lines, telephone lines, television signal cables, and computer signal transmission lines, must be equipped with appropriate overvoltage protection devices (lightning arresters). These devices must be electrically connected directly to the building’s lightning protection grounding system, so as to achieve equipotentiality (in practice, a quasi-equipotential state; since the potential difference between the various wires under normal conditions, as well as the residual voltage during a lightning strike, is negligible compared to the lightning voltage, such connections are also referred to as equipotential connections). 　　By using equipotential bonding, not only is the lightning protection capacity of the building and its internal equipment **improved**, but the requirements regarding the building’s grounding resistance can also be relaxed. This can reduce construction costs and the difficulty of construction, which is particularly important in areas with high soil resistivity such as arid regions, deserts, and mountainous areas. 　　Regarding the grounding resistance value for common grounding, many documents recommend that it should be below 1Ω. This is unreasonable and economically wasteful, not to mention whether this 1Ω value refers to the power-frequency or impulse grounding resistance. Assuming that the power frequency grounding resistance of 1Ω is equal to the impulse grounding resistance of 1Ω, for the 220/380V electrical equipment in buildings, the insulation impulse voltage is specified by the International Electrotechnical Commission to be 6 kV. When the building’s lightning protection system is struck by lightning directly, the amplitude of the lightning current is 35 kV when the number of occurrences is taken as 50%; the product of this value and a grounding resistance of 1 Ω is also 35 kV, which is 5.8 times the aforementioned impulse voltage of 6 kV. Under the condition of a common ground, the most important measure to prevent insulation breakdown in electrical equipment is to install overvoltage protectors between the insulated conductors and the common grounding system. Under special conditions, when certain types of metal objects cannot be directly connected to a common grounding system, overvoltage protectors must also be installed between them to prevent arcing and the generation of sparks in the atmosphere. An overvoltage protector is a device used to limit the surge overvoltage that occurs between two objects, such as a discharge gap arrester or a semiconductor device. 6 Application of foundation grounding electrodes There are various opinions regarding the use of foundation grounding electrodes: some people believe that since the rebar within the foundation is enclosed by concrete, it is impossible for it to make contact with the ground, so how can it function as a grounding electrode? In fact, dry concrete is an excellent insulator. Concrete containing water, however, is another matter. During the production of reinforced concrete foundations, portland cement and water react with each other; once it dries, the concrete contains many tiny, branched capillaries. When the concrete base remains in contact with moist soil, capillaries draw water into the concrete, thereby reducing its resistivity. The measured actual resistivity of concrete is shown in Table 1. 　　　　　　　　　Table 1: Measured data on the resistivity of concrete Conditions of the concrete Resistivity (Ω*m) In water 40–50 Buried in moist soil 100–200 Buried in dry soil 500–1300 As can be seen from the data in the table above, reinforced concrete foundations are suitable as grounding elements. For larger buildings, the grounding resistance after using foundation grounding electrodes generally meets the requirements. For smaller reinforced concrete buildings, using the buried reinforced concrete of their column and beam structures as a grounding grid means that even if the grounding resistance is not low enough, artificial grounding elements can be added as a supplement. This approach can at least help reduce the number of such artificial grounding elements, thereby saving costs; it is therefore a beneficial measure with no negative consequences. However, some reinforced concrete materials cannot be used as grounding devices; such as waterproof cement, aluminate cement, bauxite cement, and isobutyl silicate cement. Reinforced concrete foundations made from synthetic material cements cannot be used as grounding devices. 　　It is important to note that before concrete is poured, an electrical connection must be established between all the rebar bars. Primarily, it serves as the connection between the pile rebar acting as a grounding electrode and the foundation slab; therefore, the joints where the beam and column rebar are used as down conductors and equipotential ring shielding networks must be welded firmly to ensure a reliable electrical path. There is a view that buildings can meet the requirements for electrical connection simply by binding the rebar in their structure, and it is expected that the binding points will weld together under the impact of lightning current, similar to spot welding. In fact, this approach is unreliable. Based on analyses of cases related to the inspection, acceptance, and investigation of lightning protection systems, there are three issues with the above claim: First, in the humid and rainy regions of the south, rusting of steel rebar, along with vibrations during concrete pouring, result in poor contact at the joints where the rebar is tied together. This prevents the various components of the lightning protection grounding system from forming a proper electrical pathway, thereby hindering the discharge of lightning current ； Secondly, regarding the bonding of piles, beams, and column rebars selected as grounding devices, the transition resistance values at various joints vary, which affects the balanced distribution of lightning current ； Thirdly, the likelihood of welding occurring at the binding points due to lightning strikes is uneven; and each instance of \"spot welding\" caused by lightning currents results in localized damage to the building, whether it be the explosion of walls or columns, or the intense electromagnetic induction generated around the spots where welding occurs. The damage to human beings or equipment is significant, especially in high-rise buildings and what are now known as \"smart buildings.\" According to years of investigations into lightning disasters conducted by the Guangdong Provincial Meteorological Bureau and the Guangdong Provincial Public Security Department ； In Guangdong Province, many high-rise buildings suffer from insufficient lightning protection due to neglect of this issue; despite appearing to be under the \"protection\" of well-installed lightning rods, nets, and strips, partial damage to the buildings still occurs, and this phenomenon is even more common in rural areas. Therefore, in Guangdong Province, the inspection of the construction quality of building lightning protection facilities includes the inspection of concealed works, particularly the welding of piles, beams, columns, and rebar used as lightning protection components, as well as the quality of such welding. It has turned out that such measures are highly necessary. 　　In summary, the following principles should be followed when designing a ground grid for a building: (1) Try to use the rebar in the building’s foundation together with natural metallic grounding elements to form a unified grounding grid ； 　　(2) For the pile foundations and cap stones selected as the ground grid in buildings, as well as the column rebars used as down conductors, welding should be employed at their joints instead of binding ； 　　(3) Preferably use natural grounding electrodes as the basis, supplemented by artificial grounding electrodes; the shape should ideally be a closed ring ； 　　(4) The same grounding grid should be used, with grounding achieved through single-point grounding ；