Lightning protection technology for instruments and meters

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Electrostatic discharge (ESD) and electrostatic fast transient pulses (EFT) can cause varying degrees of damage to instrumentation systems. Electrostatic discharge generates intense radio frequency radiation in the frequency range of 5–200 MHz. The peak of this radiant energy often occurs between 35MHz and 45MHz, where self-oscillation takes place. The resonance frequencies of many information transmission cables also typically fall within this frequency range, resulting in a large amount of electrostatic discharge radiation energy being coupled into the cables. Electrical fast transient pulse groups also generate quite strong radiation emissions, which thus couple to the cable and chassis circuits. When the cable is exposed to a 4–8 kV electrostatic discharge environment, the induced voltage that can be measured across the terminal load of the data transmission cable can reach 600 V. This voltage is far above the threshold value of 0.4 V for typical digital instruments; the duration of such induced pulses is approximately 400 nanoseconds. Instruments and meters often encounter unexpected voltage transients and surges during use, which can lead to damage to electronic devices. The reason for this damage is that the semiconductor components within these instruments and meters (including diodes, transistors, thyristors, and integrated circuits) get burned out or damaged. According to statistics, 75% of failures in instruments and meters are caused by transients and surges. Voltage transients and surges are everywhere: power grids, lightning strikes, explosions – even walking on a carpet can generate tens of thousands of volts of static electricity. All of these are hidden, deadly threats to instruments and equipment. Therefore, to improve the reliability of instruments and equipment as well as human safety, protective measures must be taken against voltage transients and surges. 1. Lightning protection ports: Based on engineering practices in the field of instruments and meters, lightning strikes on such devices can be roughly categorized into direct lightning strikes, induced lightning strikes, and conducted lightning strikes. However, regardless of the form in which it reaches the equipment, lightning surges can be categorized as those invading from the following 4 locations; these locations are referred to here as lightning protection ports, and instruments are used as examples. 1.1 Enclosure ports: For example, we can consider any large or small instrument or system as a single enclosure; sensors, transmission lines, signal repeaters, field instruments, DCS systems, and so on – all of these can be completely exposed to the environment and subjected to direct lightning strikes, resulting in equipment damage. Standards stipulate that when the equipment enclosure is subjected to a 4 kV lightning electrostatic discharge, it will affect the proper operation of instruments or systems. For example, sensor terminal boxes placed outdoors may be subjected to lightning contact discharge ； The DCS cabinets located in the machine room may be exposed to air discharges resulting from current leakage from the building’s columns. 1.2 Signal line ports (including antennas, data cables, control cables, etc.) In control systems, in order to transmit signals or information, there must be points of connection to the outside world; for example, the main distribution frames at the signal exchange points in process control systems, the terminals of data transmission networks, and the feed ports that connect microwave equipment to antennas. All such interfaces, which receive signals from the outside or transmit signals outward, can be affected by lightning surges. Since the surges entering through the signal ports on the outside of the building often travel over long cables, a 10/700μs waveform is used; standards specify that the surge voltage between wires is 0.5 kV, and the surge voltage from a wire to ground is 1 kV. The ports through which signals are transmitted between the instruments inside the building are subject to surge impacts, similar to those on the power supply lines; a 1.2/50 (8/20) μs combined wave is used, with the surge voltage limits for line-to-line and line-to-ground connections remaining unchanged. Once the limit is exceeded, the signal port and the devices connected to it may be damaged. 1.3 Power ports: Power ports are the areas where lightning strikes are most likely to occur or be conducted, and they are distributed widely; these power ports can be found anywhere, from distribution boxes to power sockets. The standard specifies that the surge voltage limit between wires under a 1.2/50 (8/20) μs waveform is 0.5 kV, and the surge voltage limit from the wire to ground is 1 kV. However, the surge voltage mentioned here refers to a working voltage of 220V AC. If the working voltage is lower, this standard cannot be applied. A minor surge on the power cable may not immediately damage the equipment, but it will at least affect its lifespan. 1.4 Grounding Port: Although the standards do not specify any particular requirements for grounding ports, in practice these ports are very important for information technology devices. During lightning strikes, the grounding terminal may be affected by backflow of ground potential or increases in ground potential; or due to poor or improper grounding, the ground resistance may become too high to meet the requirements for a reference potential, thereby causing damage to the equipment. The grounding port has requirements not only regarding ground resistance/ground wire electrodes (length, diameter, material), grounding methods, and the setup of the ground grid, but it is also directly related to the electrical properties of the device, its operating frequency band, and the operating environment. At the same time, a reverse current can also flow from the ground terminal to the DC power port, damaging devices that operate on DC voltage. In summary, lightning protection for information technology equipment can be approached by focusing on four key ports: 2. Port protection for instruments and meters. 2.1 Enclosure ports – The protection of the enclosure ports of instruments and meters refers not only to the building’s exterior enclosure but also to the enclosures of individual devices or systems, such as cabinets or computer rooms. The scope of application of IEC 1312—1 \"Protection against lightning electromagnetic pulses\" Part 1 (General principles) is: the design, installation, inspection, and maintenance of effective lightning protection systems for instrument and control systems located within buildings or on their rooftops. There are mainly three protection methods: grounding, shielding, and equipotential bonding. 2.1.1 Grounding ； IEC1024—1 has outlined the methods for lightning protection and grounding of buildings, primarily achieving these requirements through a networked grounding system underground in the building. When protecting instrumentation systems against lightning, it is also required that the power lines and communication cables passing between adjacent buildings be connected to the building’s grounding system (so as to prevent the formation of loops), in order to use multiple parallel paths to reduce the current in the cables. The grounding of instrumentation systems should pay more attention to system safety and preventing interference from other systems. Generally, in operation, the grounding of instrument systems should not be connected directly to the lightning protection ground wire; otherwise, stray currents will enter the instrument system and cause signal interference. The correct connection method involves linking two separate ground grids underground using a discharge device or low-voltage lightning arrester, so that they can be automatically connected in the event of a lightning strike. 2.1.2 Shielding ； Theoretically, shielding is very effective for protecting the enclosures of instruments from lightning. However, from an economic perspective, different shielding methods should still be chosen based on the immunity of the equipment components and the requirements regarding shielding effectiveness. Cable shielding, that is, the use of shielded cables in instrumentation systems, has been widely applied. However, the shielding of equipment or systems depends on the specific circumstances. The IEC provides examples of measures to connect the rebar in buildings to metal frames. The full text, as well as the addresses of any images and tables mentioned in the article, can be found at: http://www.kzcd.cn/mforum/forum_more.asp?scriptID=1234. The articles on this site are intended solely for reference purposes!