Common Problems in the Maintenance of Lightning Protection Grounding for Television Monitoring Systems, Lightning Protection Knowledge, and Methods of Lightning Protection Grounding for Television Monitoring Systems - Part One
1. The Formation of Lightning
People commonly refer to clouds that produce lightning as thunderclouds, but actually several types of clouds are associated with lightning, such as stratocumulus, nimbostratus, cumulus, and cumulonimbus clouds. Among these, the cumulonimbus cloud is the most important, and what is generally referred to as a thundercloud in professional literature specifically means a cumulonimbus cloud.
The process of cloud formation occurs when water vapor in the air reaches saturation or supersaturation due to various reasons, leading to condensation. Achieving saturation of water vapor in the air is a necessary condition for cloud formation, and the main methods include:
(1) Constant water vapor content while the air cools down;
(2) Constant temperature while increasing water vapor content;
(3) Increasing water vapor content while also lowering the temperature.
However, for cloud formation, the cooling process is the most critical. In the cooling process, the cooling effect caused by ascending motion is the most common. Cumulonimbus clouds are formed during intense vertical convection. Since the ground absorbs more solar radiation heat than the air layer, the ground temperature rises significantly during the day, especially in summer, making this warming more noticeable. Therefore, the temperature of the near-ground atmosphere increases due to heat conduction and thermal radiation. As gas temperatures rise, it expands, its density decreases, and pressure also drops accordingly. Based on mechanical principles, it will then rise. The upper air layers have relatively higher densities and will descend. During the ascent of hot air currents, expansion and pressure reduction occur, along with heat exchange with high-altitude cold air, causing the water vapor in the rising air mass to condense into fog droplets, forming clouds. In strong convection processes, the fog droplets in the clouds further cool down, turning into supercooled droplets, ice crystals, or snowflakes, which increase with height. At the freezing level (-10°C), due to the large amount of supercooled water freezing and releasing latent heat, the cloud top suddenly develops upward, reaching the top of the troposphere and then spreading horizontally, forming anvil clouds, which are a significant characteristic of cumulonimbus clouds.
During the formation of cumulonimbus clouds, positive and negative charges accumulate in different parts of the cloud under the combined effects of atmospheric electric fields, thermoelectric effects, and fracture electrification effects. When charges accumulate to a certain extent, discharges occur between clouds or between clouds and the ground, which people commonly refer to as "lightning."
Lightning poses a tremendous destructive force, bringing catastrophic disasters to humanity and society. Especially in recent years, lightning disasters have occurred frequently, causing increasingly serious harm to the national economy. We should enhance our awareness of lightning protection, actively cooperate with meteorological departments, and carry out preventive work to minimize the damage caused by lightning.
2. Destruction Caused by Lightning
The destruction caused by lightning mainly occurs when the potential difference between cloud layers or between clouds and the earth, or between clouds and the air, reaches a certain level (25-30 kV/cm), resulting in violent discharge phenomena.
There are typically three forms of lightning strikes: direct lightning, induced lightning, and ball lightning.
Direct lightning is the rapid discharge phenomenon between charged clouds and a specific point on the ground. Devices like lightning rods can quickly introduce the high voltage and strong current generated by "direct lightning" into the ground, eliminating the impact of lightning strikes and thus protecting facilities.
Induced lightning occurs after a direct lightning strike when the charge in the clouds rapidly dissipates, causing localized high voltages in certain areas on the ground due to high scattering resistance. Or, during the discharge process of direct lightning, powerful pulse currents induce high voltages in surrounding wires or metal objects through electromagnetic induction, resulting in secondary lightning strikes known as flashover. Although within the protective range of lightning rods, objects may be spared from direct lightning strikes, "induced lightning" can generate high-voltage induction and current surges on power lines, communication lines, network cables, satellite antennas, and cable TV lines, entering distribution rooms, server rooms, offices, and residences through wires and inevitably damaging power supplies, communications, and electronic equipment. Therefore, preventing these modern-day lightning hazards has become both urgent and necessary.
Ball lightning refers to the phenomenon of spherical lightning.
3. Sources of Voltage Surges
Voltage surges can originate from outside electrical installations or from within, i.e., from electrical devices inside the installation.
External Voltage Surges: These surges are caused by lightning or the switching of public power grids. Both types of harmful power disturbances can disrupt the operation of computers and microprocessor information processing systems, causing downtime or permanent equipment damage.
When there is charge storage in the cloud layer, opposite-polarity charges of equal quantity are generated on the lower surface of the cloud layer, triggering a lightning discharge. Subsequently, the situation resembles the discharge of a large battery pack or a large capacitor, with the charge potential between the cloud layer and the ground reaching several million volts. During a lightning strike, current flows at several thousand amps through the discharge path, passing through all equipment and returning to the cloud layer via the ground, completing the electrical circuit. Unfortunately, this lightning path often passes through important or valuable equipment. The key concept of surge protection is to provide an effective short-circuit path for lightning-induced currents to the ground. This way, the lightning surge will bypass the equipment. The figure shows the reduction in lightning current at the device location. Large lightning current values are often cited, but their actual occurrence is rare.
Internal Voltage Surges: Internal voltage surges are frequent occurrences, such as those from air conditioners, air compressors, arc welders, electric pumps, elevators, switch-mode power supplies, and other inductive loads. For example, a 20hp induction motor (line voltage 230V, 4-pole, Y-connected) has approximately 39J of stored energy per phase at maximum torque. When its nominal root mean square current is interrupted, it generates transient overvoltage. This happens frequently, and other loads powered from the same distribution box are therefore susceptible to damage or malfunction.
Do not assume that overvoltage protectors on the power supply line of an electrical installation can protect electrical equipment from internal voltage surges. They cannot; they can only guard against external voltage surges entering the electrical installation along the power line because high-capacity incoming protection devices are too far from where internal voltage surges occur. Dion Neri wrote, Wang Yu Hou translated, Huang Miao Qing proofread from EC&M Electrical Construction & Management, October 1998, Volume 1, Issue 1. The following anecdote is excerpted from another website, and the authenticity of the story has not been verified by the moderator. "Not only presidents can be hit by transient voltage surges."
Voltage surges are abnormal large current pulses at the microsecond level. They can cause transient overvoltage damage to electronic equipment. Each year, the integration of semiconductor devices improves, the spacing between components decreases, and the thickness of semiconductors becomes thinner. This makes electronic equipment more susceptible to transient overvoltage damage. If the transient overvoltage caused by a voltage surge exceeds the tolerance of an electronic device, the device will either be completely destroyed or its lifespan will be greatly shortened.
Lightning is the most obvious cause of voltage surges. Lightning hitting transmission lines can lead to enormous economic losses. Every surge caused by utility companies switching loads shortens the lifespan of various computers, communication equipment, instruments, and PLCs. Additionally, large motor equipment, elevators, generators, air conditioning, refrigeration equipment, etc., can also cause voltage surges. UPS units can also be destroyed by voltage surges.
The lightning rod on the top of buildings can divert most of the discharge into the ground during a direct lightning strike, avoiding fires and explosions in buildings. Uninterruptible Power Supplies (UPS) handle severe voltage drops. Both are very useful, but neither can protect computers from voltage surges, and the UPS itself contains many microprocessors that can also be destroyed by voltage surges. About 25 years ago, IBM discovered that a more common source of voltage surges was the power grid switches of utility companies and large power equipment (such as air conditioners and elevators). Such voltage surges enter studios every day through distribution panels, damaging electronic equipment or shortening their lifespan. Therefore, almost all buildings with computers or other sensitive electrical equipment in the United States are equipped with surge protectors.
4. Electrical Equipment Susceptible to Damage from Voltage Surges
Electrical equipment containing microprocessors is extremely prone to damage from voltage surges, including computers and computer peripherals, programmable controllers, PLCs, fax machines, telephones, answering machines, etc.; program-controlled switches, broadcasting transmitters, microwave relay equipment; household appliances such as TVs, audio equipment, microwaves, video recorders, washing machines, dryers, and refrigerators. According to survey data from the United States, 63% of electrical products with issues during the warranty period are due to voltage surges.
5. Hazards of Voltage Surges to Computers and Other Sensitive Electrical Equipment
With the development of computer technology to multi-layer, ultra-scale integrated chips, circuits are becoming denser, trending towards higher integration, smaller component gaps, and finer wires. A few years ago, a one-square-centimeter computer chip had 2,000 transistors, while today's Pentium machines exceed 10,000,000. This increases the probability of computers being damaged by voltage surges. Due to the design and structure of computers, they must operate within a specific voltage range. When voltage surges exceed the level that computers can withstand, data corruption, chip damage, premature aging of components occur, and symptoms include unexpected data errors, failures in receiving/sending data, lost documents, malfunctions, frequent repairs, unexplained faults, and hardware problems, etc.
Lightning voltage surges far exceed the levels that computers and other electrical equipment can withstand, causing immediate destruction of computers and other electrical appliances or permanent loss of data in most cases. Even the start or stop of a small 20-horsepower induction motor can generate voltage surges of 3,000-5,000 volts, damaging or interfering with computers sharing the same distribution panel each time a surge occurs, and these surges happen very frequently.
CBEMA - The Computer and Business Equipment Manufacturers Association has established international standards that serve as the basis for designing and manufacturing computers by companies like IBM. Industry standards in China specify that the overvoltage that computers using 220/380V power systems can withstand should not exceed 2,000 volts.
The U.S. began widely applying computers about 20 years earlier than China and learned painful lessons before paying attention to voltage surge protection for computers. Surge protectors are installed in banks, the top 500 companies, defense facilities, financial insurance systems, service networks, telecommunications networks, petrochemical plants, etc., in the U.S. China will become the world's largest computer consumer market. Five years ago, Chinese banks were not yet computerized. With the popularization of computers, people have also recognized the importance of protecting such electronic devices. China issued and implemented industry standards for lightning protection security devices for computer information systems in 1998.
6. Basic Principles of Lightning Protection
To ensure good protection of equipment, an objective assessment should first be made of the environment it is in and the degree to which it is affected by lightning, as it relates to the amplitude, probability, network structure, equipment voltage resistance, protection level, and grounding. Lightning protection should be considered as a system engineering project, emphasizing comprehensive protection (including buildings, transmission lines, equipment, and grounding), comprehensive management, and ensuring scientific, reliable, practical, and economical approaches. Given the characteristics of induced lightning with instantaneous larger energy, according to IEC international standards on hierarchical energy absorption theory and the principle of classification of protection zones, multi-level protection is needed.