Most modern measuring instruments with microelectronic devices as the core, can withstand the voltage is very low, and most of the distribution of the instrument is scattered, the installation in many places is open air, so the instrument is greatly increased by the possibility of lightning damage.
At present, the main way to prevent lightning instrumentation electromagnetic shielding, equipotential grounding, the rational use of surge protectors and other kinds of, then, specifically what to do?
1. Electromagnetic shielding
Electromagnetic shielding is a basic measure for reducing electromagnetic interference and lightning protection. Electromagnetic shielding reduces the penetration of alternating electromagnetic fields into the instrument, and at the same time reduces the electromotive force induced on the external connection lines of the instrument.
All signal cables are shielded twisted pair cables. The grounding of shielded cables is, in principle, specified to be grounded at one end and suspended at the other.
About one end of the grounding requirements are unique to the instrumentation and other professionals, electrical engineers do not understand this requirement, they believe that the electrical equipment or household appliances after the shell grounding, the leakage to the shell on the charge to the earth, so that there is no high potential, personal safety is guaranteed. For electronic instrumentation, the problem to be solved is not only personal safety issues, it is more important to shield the external electromagnetic field interference, that is, the interference will be rejected. In the instrument signal transmission, emphasizing the use of metal shielded wire, is to solve the problem of shielding.
Instrumentation in the relevant regulations emphasize the transmission of signals in the form of twisted wires is also very reasonable. Electrical engineers in the selection of two-core wire like to use parallel lines, because this cable is not only flat and beautiful and easy to use, but for the instrumentation profession is a taboo, because the external electromagnetic field on the two wire interference is not absolutely symmetrical, so the two parallel lines on the inductive potential may be different, this difference is finally superimposed on the instrument signal input, which caused the measurement error.
Shielded cables are protected through steel pipes or enclosed steel raceways, avoiding as much as possible to be exposed outside the steel protection pipes.
Emphasize that the steel tube protection is also extremely important. This layer of steel pipe not only to protect the cable from mechanical damage, more importantly, the cable outside the possible existence of electromagnetic interference and its lightning electromagnetic pulse to be shielded, so that it is not harmed.
This protective tube, the alignment channel must be well connected in the middle, for example, at the connection of the two steel tubes, the two sections of the tube are electrically connected with an electrical collet from an electrical specialist and a copper conductor with a cross-sectional area of 1.5 mm2. This is not superfluous at all. Because of the connection between the steel pipe and the steel pipe, on the surface of the connection between metal and metal, has been through, but do not know that such a connection in the electrical performance is not secure. This protective pipe and alignment channel must be grounded at both ends.
Use of the building's own shielding
Building walls, roof steel, metal doors and windows, etc., for equipotential connection, and connected to the grounding device to prevent direct lightning, so that the shape of a Faraday cage, thus forming the building of instrumentation and electronic equipment and wiring of the outer layer of protection.
Some control room itself is not a reinforced concrete structure, but in the walls and roof with steel wire mesh for shielding, can also receive a good shielding effect.
2. Isoelectric grounding
Specific provisions have been made for lightning protection of buildings, and a lightning protection grounding network covering a large area has been built according to this standard, and the grounding of transmitters, sensors and secondary meters installed on equipment and pipelines is connected to this public grounding network to realize equipotential grounding.
Although the aircraft is not grounded, it is protected from the effects of lightning because its internal electronic equipment is equipotentially connected to the metal casing of the aircraft, forming an equipotential body.
3. Reasonable selection of surge protectors
A surge protector (SPD) is a device that resists conducted line overvoltage and overcurrent. They include discharge gaps, varistors, zener or avalanche diodes, filters, etc. The shielding and grounding measures described earlier are effective. However, shielding cannot be done seamlessly, for example, power has to be brought in from the outside, and in some cases and on some specific occasions it has not been possible to eliminate overhead lines completely. This is similar to the case of residential neighborhoods and roads where overhead wires are still plentiful.
This must be done with the help of an SPD for lightning protection of the instrument and the system.
There are many parts that may be harmed by the surge, such as: power supply, AI port, DI port, AO port, DO port, communication port, etc., like a simple steam flowmeter, there is a power supply, flow signal input port, pressure signal input port, temperature signal input port, 4 ~ 20mA signal output port, communication signal interface and so on 6 ports, if each port is equipped with a stereotyped SPD, it is necessary to increase a lot of equipment and corresponding investment.
The intensity of the surge that may be introduced at each port varies, as do the methods used to deal with it, so it is important to take appropriate measures according to the specific circumstances, which should be both effective and economically rational.
In the selection of SPD, it should be reasonably selected according to the actual need and the characteristics of SPD, especially pay attention to the SPD’s operating voltage, load current and system circuit matching, its maximum continuous operating voltage should be slightly greater than the maximum normal operating voltage of the circuit, the load current should be greater than the maximum normal operating current of the circuit. Ensure that the SPD works normally in the circuit. For intrinsically safe circuits, it is necessary to pay attention to whether the SPD has the corresponding intrinsically safe certification, and its line resistance value should be small enough to ensure that the field instrumentation voltage can work normally. For circuits loaded with communication signals, it is necessary to pay attention to the bandwidth of the SPD to ensure that normal signals are passed.
The importance of lightning protection of instrumentation can not be ignored, as a key device, bears an important measurement and monitoring tasks, once damaged by lightning, will bring serious impact on production and life. Therefore, it is important to strengthen the instrumentation lightning protection work, choose the appropriate lightning protection measures to protect the safe and stable operation of instrumentation equipment.