Can’t control the weather. Although there will be heavy rain and light rain, it can prevent the protective work of the electromagnetic flowmeter. In industrial applications, a variety of equipment is used. Even if someone walks on the power grid, lightning strikes, or carpets, electrostatic induction voltages of tens of thousands of volts will be generated. These are the deadly invisible killers of electromagnetic flowmeters. When using equipment, unexpected transient voltages and surges often occur, causing damage to electronic equipment. The reason for the damage is that the semiconductor devices (including diodes, transistors, thyristors, integrated circuits, etc.) in the equipment are burned or damaged. 75% of the failures of statistical instruments are caused by transients and surges. Therefore, in order to improve the reliability of the instrument and the safety of the human body, necessary protective measures must be taken against voltage transients and surges.1. The harm of static electricityElectrostatic discharge (ESD) and electrical transient response pulse group (EFT) have varying degrees of harm to the instrument system. Electrostatic discharge produces strong radio frequency radiation in the frequency range of 5~200MHz. The peak value of this radiant energy mostly self-oscillates between 35MHz and 45MHz. The resonant frequency of many information transmission cables is usually also within this frequency range, so a large amount of electrostatic discharge radiation energy is mixed into the cable. The electrical transient pulse group also produces quite strong radiation, which is coupled to the cable and the casing circuit. When the cable is exposed to an electrostatic discharge environment of 4~8kV, the measurable induced voltage of the terminal load of the information transmission cable can reach 600V, which far exceeds the threshold voltage value of 0.4V of a typical digital instrument and meter, and the duration of a typical induction pulse About 400 nanoseconds.2. Lightning protection port countermeasuresAccording to the engineering practice of the electromagnetic flowmeter application, when the electromagnetic flowmeter is struck by lightning, it can be roughly divided into direct lightning, induced lightning and conducted lightning. However, no matter how it arrives at the equipment, it can be summarized as lightning surges intruding from the following four places. Here, these places are called lightning protection ports, and instruments and meters are taken as examples for description.1. Shell portFor example, sensors, transmission lines, signal relays, field devices, DCS systems and other large or small devices such as instruments or systems may be completely exposed to the environment and be directly struck by lightning, resulting in equipment damage. The standard stipulates that when 4kv lightning electrostatic discharge occurs on the equipment shell, it will affect the normal operation of the equipment instrument and system. For example, the sensor terminal box placed outdoors may be exposed to the contact discharge of lightning; the DCS cabinet in the machine room may be exposed to the air discharge when flowing from the building column.2. Signal line port (including antenna feeder line, data line, control line, etc.)).In the control system, in order to realize the signal and information transmission of the integrated electromagnetic flowmeter, the general wiring rack of the signal exchange end of the process control system, the terminal of the data transmission network, the power supply port from the microwave equipment to the antenna, etc. are required to be connected to the outside. In this area, most of the surges that are received and sent from the outside from the signal port outside the building pass through long cables, so the 10/700s waveform is adopted. The standard stipulates that the surge voltage between lines is 0.5kV and the surge voltage between lines is 1kV. On the other hand, the signal transmission port between the instruments in the building receives a surge which is equivalent to a surge on the power line. It uses a combined wave of 1.2/50 (8/20) s, from line to line and from line to ground. The limit value of surge voltage remains unchanged. If the limit value is exceeded, the signal port and the equipment behind the port may be damaged. 3. Power portThe power ports are the most widely distributed and are the most likely to sense or transmit lightning waves. They can be located anywhere from the distribution box to the power socket. The standard stipulates that it is 1.2/50 (8/20 )s waveform, the surge voltage limit value between the underline and the line is 0.5kV, and the surge voltage limit value between the lines is 1kv. However, the surge voltage here means that the working voltage is 220V AC input. When the operating voltage is low, this cannot be used as a reference. A small surge on the power cord does not necessarily damage the device immediately, but at least it will affect the life of the device.4. Grounding portAlthough there is no specific index for the grounding port in the standard, in fact the grounding port of information technology equipment is very important. In the event of thunder and lightning, the grounding port may be affected by the counterattack of the ground potential, the rise of the ground potential, or the ground resistance may be too large due to poor grounding or grounding errors, which may not meet the requirements of the reference potential, resulting in damage to the equipment. The grounding port not only has requirements for grounding resistance/grounding wire pole (length, diameter, material), grounding method, grounding grid installation, etc., but also directly related to the electrical characteristics of the equipment, working frequency band, and working environment. In addition, it is also possible to counterattack the DC power port from the ground terminal and destroy the DC operating voltage equipment. As mentioned above, lightning protection for information technology equipment can be considered starting from four key portsThree, port protection guide1. Shell portThe cabinet port protection of the electromagnetic flowmeter must include not only the cabinet of the building, but also the cabinet of a certain equipment or the cabinet of a certain system, such as cabinets, computer rooms, etc. The scope of application of the first part (general principles) of IEC1312-1 “Lightning Electromagnetic Pulse Protection” is the design, installation, inspection and maintenance of effective lightning protection systems for electromagnetic flowmeter systems in or above buildings. There are three main protection methods, grounding, shielding and equipotential bonding.2. GroundingIEC1024-1 has described the method of lightning protection and grounding of buildings, mainly through the underground mesh grounding system of the building to meet the requirements. In the lightning protection of the instrument system, the communication cable must also be connected to the building grounding system (cannot form a circuit) through the power line between two adjacent buildings, in order to use multiple parallel paths to reduce the current in the cable.The grounding of the instrument integrated electromagnetic flowmeter system should pay more attention to the safety of the system and prevent interference from other systems. In the working state, the grounding of the instrumentation system generally cannot be directly connected to the lightning protection ground. If not connected, stray current may enter the instrumentation system and cause signal interference. The correct connection method should be to connect two different ground grids underground, connect with a low-voltage arrester with a discharger, and automatically connect under lightning strikes.
3. ShieldIn theory, shielding is very effective for lightning protection of the electromagnetic flowmeter housing. However, from an economic and reasonable point of view, different shielding methods must be selected according to the anti-interference of the equipment components and the requirements for shielding performance. Line shielding, that is, the use of shielded cables in the instrumentation system has been widely used. However, the situation with the scene depends on the situation.
Post time: 21-09-21