Part 1. Lightning protection status of mixing plant

The lightning protection measures of concrete mixing plant are mainly to prevent the induction of lightning surge along the external line. With the electronization and high integration of measurement mode of concrete mixing plant, real-time intelligence of computer control, remote network control and informatization of Internet access, a perfect computer and network communication system has been formed, This brings greater challenges to the lightning protection of the mixing plant. The application of new technologies such as electronic high integration, computer real-time control, network remote control and satellite communication in the control system of the mixing plant has further reduced the lightning withstand capacity of weighing instruments and signal amplifiers that were originally sensitive to surge, especially the communication port of the computer in the control room, the communication port of the console PLC, the communication port of weighing instrument are more vulnerable to lightning. Therefore, in the design of the mixing plant, it is required to design the lightning protection and grounding system in many aspects from the perspective of the overall lightning protection of the mixing plant.

Part 2. Lightning protection and grounding of the overall structure of mixing plant

The lightning protection of the overall structure of the mixing plant is inseparable from the lightning protection of surrounding buildings. According to the latest national mandatory standard gb50054-95, equipotential connection shall be adopted for the lightning protection and grounding of buildings and equipment, rather than independent grounding grid traditionally. Therefore, different implementation schemes of combined grounding shall be adopted for mixing plants of different building types. The so-called equipotential bonding is to weld the building itself and various conductive objects inside and outside it with conductors to ensure equipotential bonding. Because the peak value of lightning current is very large, and the places flowing through it immediately rise to a high potential (relative to the earth), there is side flicker for nearby electrical and electronic equipment and staff still at the earth potential, which is easy to cause equipment and personal accidents. Therefore, equipotential connection is the key measure for lightning protection. At the site of the mixing plant, the parts requiring equipotential connection include all buildings in the plant, the main body of the mixing plant, electrical appliances in the control room, power distribution system of the mixing plant, grounding wires of anti electromagnetic radiation facilities, etc.

Part 3. Lightning area classification and area division

The damage caused by lightning is transient overvoltage. In our daily life, due to opening and closing switches, line faults bring temporary overvoltage. Transient overvoltage will cause destructive faults such as sensitive semiconductor breakdown and circuit board line fusing. There are three ways for lightning to enter the power supply system: intrusion, induction and counterattack.

Intrusion: it is because lightning strikes the power supply, resulting in high overvoltage and damaging electrical equipment.

Induction: it is due to the leading down current generated by the lightning arrester and the high induced voltage generated by the metal parts of the equipment.

Counterattack: when an object near the equipment is struck by lightning, the current returns to the equipment shell through grounding.

For different areas, we can divide the lightning protection area into lpz0a area, lpz0b area, lpz1 area and lpzn + 1 are. All objects in lpz0a area may be directly struck by lightning and conduct all lightning currents; The magnetic field strength does not decay. All objects in lpz0b area cannot be directly struck by lightning greater than the selected rolling ball radius, and the electric field intensity does not decay. Lpz1 area cannot be directly struck by lightning. The current passing through each conductor is smaller than lpz0b area, and the electromagnetic field can be attenuated. If the subsequent lightning protection area of lpzn + 1 area needs to reduce the incoming current and electromagnetic field strength, the subsequent lightning protection area shall be added.

Part 4. Lightning protection solutions

1. The concrete mixing plant includes several major mechanical metal structures such as cement silo, mixing plant main body, aggregate batch and conveyor belt. The lightning protection downlead of each major mechanical structure can use the metal structure of the equipment, but corresponding electrical connection measures must be taken between the metal structures of each major mechanical equipment to eliminate the contact resistance of the paint layer between the two contact surfaces, To ensure reliable electrical connection between structures. In order to prevent direct lightning strike, one or more early discharge lightning rods can be installed on mechanical equipment as required. The length of early discharge lightning rod (lightning arrester) shall be 1-2m. In order to meet the requirements of protecting all outdoor equipment, the power, control, lighting, signal and communication lines used by mechanical equipment equipped with lightning rods shall be laid in steel pipes or meet the requirements 《Code for design of steel cable tray engineering》The cable tray of cecs31z 91, and the steel pipe and tray shall be reliably electrically connected with the metal structure of the mechanical equipment. The insulation coating shall be removed from the grounding hole, and the connection shall be connected with braided copper wire. At the same time, the structure bus and transformer neutral point outside the control room shall be protected with lightning arrester, a standard grounding grid shall be made outside the control room, and the grounding down lead of all equipment shall be connected with the grounding body. The welding and grounding system shall be grounded nearby. Only by reliably connecting the main steel structure of the mixing plant and installing connection measures at each section of the trunking, can ensure good contact at all connection parts and equipotential grounding of all parts of the mixing plant.

In order to prevent overvoltage caused by lightning stroke, the insulation level of various equipment shall meet the insulation requirements of voltage on the equipment. During equipment ordering and delivery test, it shall be strictly controlled to ensure the insulation and withstand voltage level of equipment according to the requirements of regulations, so as to prevent lightning damage breakdown.

2. The control room is the working environment of the computer used for the control of the mixing plant, i.e. the machine room. The floor of the control room is made of channel steel, wood and flame retardant leather. The floor surface shall be electrostatic conductive and it is strictly prohibited to expose metal parts. The working table and seat cushion materials in the control room shall be electrostatic conductive, and the electrostatic leakage resistance of the control room floor and working table shall meet the current national standards《Technical conditions for raised floor for computer room》. Electrostatic grounding must be carried out for electrostatic conductive floor, worktable and seat cushion cover. Electrostatic grounding can be connected with grounding device through current limiting resistance and its own connection. The resistance value of current limiting resistance should be 1MΩ, and the electrostatic potential of insulator in control room should not be greater than 1KV. Due to the existing installation method (directly placed on the chassis), the contact is unreliable. What is more serious is that there are two layers of paint films between the two contact surfaces, which leads to the failure of the grounding of the control room to meet the requirements. The solution is to carry out electrical connection measures at the connection between the control room and the support, such as removing the insulating coating at the grounding hole and using braided copper wire at the connection to ensure reliable electrical connection.

3. The control room is the first protective barrier for indoor computers, control instruments, television monitoring, air conditioning and other electronic equipment. Since the control instruments must be connected with outdoor sensors through signal cables, the cable shield wire must be grounded (multi-point repeated grounding),shall be connected with the grounding grid of the mixing plant to form equipotential, and lightning arrester shall be installed at the same time; as computer control communication cables, there are mainly RS232, RJ-45 and network port. Because the communication control line will be interfered by various high frequency and strong voltage, it is particularly sensitive to lightning induction invasion. Therefore, it is necessary to use shielded cables and the shielding layer shall be grounded. The communication line is dry, the line needs to be connected with the last stage surge protector specially matched with RS232 and RJ-45 interfaces. The overvoltage needs to be limited to less than 1.5kV, level B and category of last stage protection.

4. At the beginning of the design of the mixing plant, the lightning protection and grounding of the distribution system should be designed scientifically and systematically based on the overall structure and comprehensive consideration of lightning protection and grounding,at low voltage 220 / 380V in the power supply system, a three-phase five wire (TN-S) system shall be used to facilitate device grounding (PE) and neutral (N),the lines shall be separated, and the PE line shall be connected to the terminal floor of each layer and section of the device grounding. The surge protector (SPD) with level D 3 + 1 protection mode must be adopted. The key is the selection of its maximum current capacity. When the lightning protection zone is divided according to the type of the mixing plant and the surrounding environment, select surge protective devices (SPDs) with different protection levels, and select electrical equipment with reliable quality and lightning protection equipment with high reliability. Truly make a common grounding grid that meets the requirements according to the principle of equipotential. Only in this way the distribution line and mixing plant equipment can avoid the harm of lightning stroke.

Part   5 Summary

The lightning protection of mixing plant is a comprehensive project. Every detail needs to be done well, and the system also needs maintenance. After lightning weather, check whether the surge protector fails, whether the grounding wire is firmly connected, and whether the grounding resistance meets the design requirements. The protection of the system is indispensable. Lightning damage is pervasive.