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Photocoupler 1: It is difficult to ensure that there is no potential difference between the grounding points at both ends. If there is a potential difference, there will be a weak current, so that the shielding layer actually becomes the grounding line; 2: The shielded wire with grounding at both ends works at the working site where the high-frequency interference is more serious. The line capacitance formed between the shielding layer and the internal signal line is coupled to the signal loop, which will seriously affect the signal misjudgment. 3: I have seen all kinds of governors and plc instructions that the signal line shield must be grounded at one end and the ground should be on the controller side. There are two main types of grounding: 1. Earth grounding, first of all to ensure the reliability of the total grounding body, the grounding resistance should be small, and at the same time try to stay away from the building lightning protection grounding and high grounding system. Ensure the equipotential of each ground point. Used for grounding the device case and grounding the double shielded outer layer. If it is determined that the grounding points are reliable and absolutely equipotential, multi-point grounding is certainly good. (This is my personal opinion, in my actual electrical operation, it has a certain effect on the occasion of large interference) 2. Technical grounding (signal ground), mainly used for the second layer of shield grounding, so that the signal distortion is reduced, because the signal ground is actually the zero potential of the working power of the electronic printing board, which is the zero point of the system signal power supply. The two-layer shield should be insulated and shielded from each other! If not insulated from each other, it should be treated as a single layer shield! The grounding of the outermost shield is caused by the induced potential difference, thereby generating a magnetic flux that reduces the strength of the source magnetic field, thereby substantially canceling the voltage induced when there is no outer shield; The innermost shield is grounded at one end. Because there is no potential difference, it is only used for general anti-static sensing. The following specifications are the best proof! "GB 50217-1994 Power Engineering Cable Design Specification" - 3.6.8 The grounding method of the control cable metal shield shall comply with the following provisions: (1) The analog signal loop control cable shielding layer of the computer monitoring system shall not constitute two or more points of grounding, and it is recommended to use a centralized one-point grounding. (2) In addition to the (1) item and other control cable shielding layers that require a little grounding, when the electromagnetic induction interference is large, two points of grounding should be used; the electrostatic induction interference is large, and one point can be grounded. For double shield or composite general shield, it is better to use one point for the inner and outer shields and two points for grounding. (3) The choice of two-point grounding should also consider that the shielding layer will not be melted under the action of transient current. "GB50057-2000 Building Lightning Protection Design Code" - Article 6.3.1 stipulates: ... When shielded cable is used, its shielding layer should be equipotentially connected at least at both ends. When the system requires only equipotential bonding at one end. Two layers of shielding shall be used, and the outer shielding shall be treated as described above. To prevent static interference, it must be grounded at a single point, whether it is a layer or a second layer. Because the electrostatic discharge rate of single point grounding is the fastest. However, the following two cases are excluded: 1. There is strong current interference outside, and single point grounding cannot meet the fastest discharge of static electricity. If the grounding wire has a large cross-sectional area and can ensure the fastest discharge of static electricity, it must also be grounded at a single point. Of course, that's true, there is no need to choose two layers of shielding. Otherwise, two layers of shielding must be used. The outer layer shielding is mainly to reduce the interference intensity. It is not to eliminate the interference. At this time, it must be grounded at multiple points. Although it cannot be finished, it must be weakened as soon as possible. To be weakened, multi-point grounding is the best choice. For example, the cable tray in an enterprise is actually an outer shield. It must be grounded at multiple points, and the first line of defense reduces the intensity of the interference source. Inner shield (in fact, we don't buy double-layer cables, usually the outer layer is the cable tray, the inner layer is the shield of the shielded cable). It must be grounded at a single point, because the external strength has been reduced, discharge as soon as possible, and eliminate interference. It is the purpose of the inner layer. 2. Safety requirements such as external electric shock and lightning protection. This situation requires two layers of protection. The outer layer is not used to eliminate interference. It is for safety reasons. To ensure personal and equipment safety, it must be grounded at multiple points. The inner layer is to prevent interference, so a single point grounding is like a drainage pipe in a city. The static generated is like the water in the drain. How the pipeline allows water to be removed as quickly as possible requires the following: 1. The pipe diameter of the pipe should be large, just like the grounding pole and the grounding wire should be thick. 2. The pipe network reduces the nodes as much as possible, allowing the water to drain directly into the ground. If there are many nodes in the drainage pipe, the water flow will naturally be slow, as the electrostatic discharge of the grounding electrode is slow, and the interference cannot be quickly eliminated. Therefore, the grounding of all equipment You must use a single point to access the ground. That is, single point grounding. 3. The water in the pipeline is not uniform. In some places, there is more water. In some places, there is less water. The entrance to the ground may be placed in a place with plenty of water. In this way, the water in the pipeline is placed fast. The same is true for static electricity. Grounding points should be placed as close as possible to equipment that is prone to static electricity, such as inverters and thyristors.

Power cables are an important part of the maintenance of power plants and substations. The wide distribution of cables in power plants and substations, the flammability of cables, the stringiness of cable fires, and the severity of the consequences of cable fires have caused the fire protection of cables to receive high attention from the power sector, fire department, and other agencies. There are multiple cable fire incidents and it is necessary to discuss the fire safety measures of the cable. 1. Use methods such as sealing, blocking, and separating to ensure that a single cable fire does not extend to multiple cables. The cable enters the cable trenches, cable tunnels, and cable trays; the cable outlets must be tightly sealed to prevent the ignition of a single cable or a small number of cables and ignite a large number of cables. Between the power cable and the control cable, there shall be fireproof partitions, etc. The control cables shall be fireproofed or flame retardant cables used to ensure that the main equipment can be safely stopped in any emergency. 2. The tightness and thickness of the fireproof plug must be guaranteed. Fire doors should be automatically closed after a fire. 3. The fire blocking layer must have sufficient mechanical strength. Because the cable catches fire, especially the electrical short circuit occurs, it will cause the rapid expansion of the air and produce a certain amount of impulse, destroy the fireproof plugging layer with low mechanical strength, and make the fireproof plugging stop working.

This paper includes insulation resistance measurement of cable main insulation, cable main insulation withstand voltage test, cable outer sheath insulation resistance measurement, cable outer sheath DC withstand voltage test, measurement of metal shield resistance and resistance ratio, cross-elective system test, inspection The phase and cable line parameters at both ends of the cable line are measured in eight parts. First, the insulation resistance measurement of the main insulation of the cable 1. Purpose of the test Initially determine whether the main insulation is damp and aging, and check whether the main insulation of the cable is defective after the withstand voltage test. A decrease in insulation resistance indicates that the insulation is wet or deteriorated, which may cause cable breakdown and burnout. It can only effectively detect the overall moisture and penetration defects and is not sensitive to local defects. 2, measurement method In each phase measurement, the non-test phase and the metal shield (metal sheath) and the armor layer are grounded together. With a megohmmeter, a large-capacity digital megohmmeter is recommended (eg, short-circuit current >3 mA). The 0.6/1kV cable measures 1000V. The cable measuring voltage above 0.6/1kV is 2500V. The cable above 6/6kV can also be used for 5000V. For the cable of 110kV and above, the electric megohmmeter with 5000V or 10000V is used. The electric megohm meter is equipped with self-discharging function. Insulated gloves are used each time the wiring is changed. After each phase of the test, it should be fully grounded and discharged. 3, test cycle Handover test, after new terminal or joint 4, pay attention to the problem The "L" end lead and the "E" end lead of the megger should have reliable insulation. The cable should be fully discharged before and after the measurement, and the time is about 2-3 minutes. If you shake the megohmmeter by hand, do not stop shaking the handle before disconnecting the high voltage lead. The other end of the cable test equipment shall be guarded and no one shall be allowed to approach and contact. If the leakage current of the cable joint surface is large, shielding measures can be used, and the shielded wire is connected to the "G" end of the megohmmeter. 5, the main insulation insulation resistance value requirements Handover: Before and after the withstand voltage test, there is no significant change in insulation resistance. Pre-test: greater than 1000MΩ Cable main insulation insulation resistance value reference standard Note: The values listed in the table are the insulation resistance values when converted to a length of 1km. Conversion formula R conversion = R measurement / L, L is the length of the cable under test. When the cable length is less than 1km, no conversion is required. Second, the cable main insulation withstand voltage test 1, pressure test type The cable withstand voltage test is divided into DC withstand voltage test and AC withstand voltage test. The DC withstand voltage test is applicable to paper insulated cables, and the rubber and plastic insulated power cables are suitable for AC withstand voltage test. Our conventional cable is AC polyethylene insulated cable (rubber-insulated power cable), so we will only introduce the AC withstand voltage test below. 2, pressure test wiring diagram 3, pressure standard For 110kV and above cables, the recommended frequency is 20hz ~ 300Hz resonant withstand voltage test. The AC withstand voltage standards at the time of handover are as follows: For 110kV and above cables, the recommended frequency is 20hz ~ 300Hz resonant withstand voltage test. The AC withstand voltage standards during the pre-test are as follows: Third, cable outer sheath insulation resistance measurement 1. Purpose of the test Detect if the cable is damaged or damp after laying or during operation. The reasons for the damage of the outer sheath are: excessive tension or excessive bending during laying; direct external force due to construction and transportation during laying or operation; end/intermediate joints subjected to internal stress, natural tension, and electrodynamic force; termites devour , chemical corrosion, etc. 2, measurement method For 110kV and above cables, the 500V electric megger is used, and the electric megohm meter is equipped with self-discharging function. Insulated gloves are used each time the wiring is changed. After each phase of the test, it should be fully grounded and discharged. The sheath overvoltage protector must be disconnected during the test. GB50150-2006, Q/CSG 1 0007-2004 requires that the insulation and resistance of the outer sheath insulation resistance value is not less than 0.5MΩ/km. 3, test cycle Handover test, 3 years (for the outer sheath with lead wire) 4, pay attention to the problem The "L" end lead and the "E" end lead of the megger should have reliable insulation. The cable metal sheath should be fully discharged before and after the measurement, and the time is about 2-3 minutes. If you shake the megohmmeter by hand, do not stop shaking the handle before disconnecting the high voltage lead. The other end of the cable test equipment shall be guarded and no one shall be allowed to approach and contact. Fourth, the cable outer sheath DC withstand voltage test 1. Purpose of the test Detect if the cable is damaged or damp after laying or during operation. 2, test voltage The overvoltage protector of the sheath must be disconnected during the test, and the test is to be conducted - DC 10kV, duration 1min, preventive test - DC 5kV, duration 1min 3, test cycle Handover test, 3 years 4, test judgment No breakdown occurs. 5, the detection site Non-metallic sheath and joint outer sheath (the thickness of the outer sheath is 2mm or more, and the surface is coated with a conductive layer, basically for 110kV and above voltage grade cables). For the cross-external system, the DC withstand voltage test is carried out on each section of the cross-elective system. During the test, the cross-connection of the cable metal sheath is broken, and the metal sheath of the test section is connected to the DC test voltage. The metal sheath of the non-test segment cable on one side is grounded, the outer sheath of the insulated joint, the insulating splint between the interconnecting box segments, and the lead coaxial cable are tested together with the outer sheath of the cable. 6, typical defects and defects analysis The defect of serial number 1 is a typical construction problem. After the fault point is located, the construction party indicates that the cable has been injured by the iron shovel, and the test passes after the treatment. This defect exposes the problems in construction management. No. 2 Installation errors of the same type of insulated joints Four points were found in the two cables, reflecting the low level of the installer. The outer sheath test detected the defects to avoid the occurrence of the operation failure similar to the serial number 5. The reason for the defect of serial number 3 is that the construction management is not strict, and the reason for the serial number 4 defect is that the installation quality of the accessory is poor. The serial number 5 is an example of a 110kV cable failure in a unit, and there are problems in both the attachment installation and the handover test. First of all, the manufacturer's process requirements are unreasonable. The outer layer of the copper braid of the cable preform requires only one layer of semi-insulated insulation tape, and the preform is severely eccentric in the copper shell, resulting in insufficient insulation margin. Secondly, in the DC 10kV/1min withstand voltage test of the outer sheath of the cable, the test voltage punctures only one layer of insulation tape, but the metal sheath of the non-test segment on the other side of the interconnection box is not grounded during the test, resulting in defects. Not found in time. After the live operation, the insulated joint is internally turned on, causing the cable sheath to cross the lian system to fail, and the sheath generates an induced current of about several tens of amps. The induced current flows through the joint of the copper braid and the copper shell of the joint, and the generated heat fuses the intermediate joint preform. The burned area destroys the insulation performance of the stress cone of the rubber preform, the field strength is severely distorted, and the joint is instantaneously broken down. The conductor discharges the copper shell, causing the line to trip. Fifth, measure the resistance and resistance ratio of the metal shield 1. Purpose of the test Measuring metal shield resistance and conductor resistance can monitor changes in corrosion, and measuring resistance ratio can eliminate the effect of temperature on DC resistance measurements. 2, test cycle Handover test 3. Test method The DC resistance of the metal shield and conductor at the same temperature was measured with a double-arm bridge. 4, test judgment There should be no major changes compared to the measurement data before commissioning. When the ratio of the current to the latter increases compared with that before the operation, it indicates that the DC resistance of the shield layer increases, and the copper shield layer may be corroded; when the ratio is reduced compared with before the operation, the conductor in the accessory is indicated. The contact resistance of the connection point is increased. Sixth, cross mutual lian system test 1. Cross-peel system diagram 2, cross-interconnect effect and composition Compared to the non-interconnected interconnection, the current flowing through the metal sheath is greatly reduced. The high induced voltage on the non-grounded metal sheath is the voltage induced on the metal sheath of the longest length of the cable. The cross-connection must be disconnected from the metal sheath, and the insulation between the fracture and the ground should be well insulated. Generally, the interconnection box is used for the cross-connection of the metal sheath of the cable. The grounding metal sheath is introduced into the direct grounding box through the coaxial cable; the non-grounding metal sheath is introduced into the cross-connected grounding box through the coaxial cable, and the sheathed overvoltage protector is used to limit the possible overvoltage. 3, cross-connect performance test DC withstand voltage test of cable outer sheath, insulated joint outer sheath and insulating splint During the test, the over-voltage protector must be disconnected, and the three-section cable metal sleeve on the other side should be grounded in the interconnection box, so that the insulating rings of the insulated joint can also be tested together. Nonlinear resistance type sheath overvoltage protector test. The following two items are handover test items. One of the preventive tests is selected: the volt-ampere characteristic or reference voltage should be in accordance with the manufacturer's specifications; the non-linear resistance sheet and its lead-to-ground insulation resistance are measured with a 1000V megohmmeter. The insulation resistance between the lead and the case should not be less than 10MΩ. Check the contact resistance and connection position of the interconnecting box knife (or connecting piece), and the connection position should be correct. When measuring at the normal working position, the contact resistance should not be greater than 20μΩ. Seven, check the phase of the two ends of the cable line 1. Purpose of the test After the new line is put into operation and the line connection mode is changed during operation, the phase and phase sequence of both ends are checked to prevent phase errors and cause accidents. 2, test cycle Handover test. 3. Test method Check that the ends of the cable line are in phase and consistent with the grid phase. For cable lines of 110kV and above, all need to be completed in the power outage state, and the method is basically the same as the overhead line. Eight, cable line parameter measurement 1. Purpose of the test Cable line DC resistance, positive sequence impedance, zero-sequence impedance measurement, capacitance measurement, as the actual basis of calculations (such as system short-circuit current, relay protection setting value, etc.) after the new line is put into operation and the line connection mode is changed. . 2, test cycle Handover test. 3. Test method The same as the overhead line parameters, because the positive sequence capacitance and the zero sequence capacitance of the cable are the same, it is usually expressed only by the capacitance between the conductor and the metal shield.

On September 12th, the annual event ICH Shenzhen 2018 Shenzhen International Connector, Cable Harness and Processing Equipment Exhibition, which attracted much attention from the industry, opened in Shenzhen Convention and Exhibition Center. Nearly 400 manufacturers showed the latest application in various industries. Products and the latest manufacturing technologies, including: industrial / automotive / electronic connectors, cable harnesses, testing equipment, connector manufacturing technology, wire harness automated processing equipment, wire harness ultrasonic welding, wiring equipment and materials, etc. Wait. On September 12th, the annual event ICH Shenzhen 2018 Shenzhen International Connector, Cable Harness and Processing Equipment Exhibition, which attract much attention from the industry, opened in Shenzhen Convention and Exhibition Center. Nearly 400 manufacturers showed the latest application in various industries. Products and The latest manufacturing technologies, including: industrial / automotive / electrical connectors, cable harnesses, testing equipment, connector manufacturing technology, wire harness automated processing equipment, wire harness ultrasonic welding, wiring equipment and materials, etc. As an influential professional exhibition in the connector wire processing industry in China, the opening of ICH Shenzhen 2018 ignited the industry trade exchange boom, and new products, new technologies and new solutions were exhibited, which aroused the interest of visiting buyers. The exhibition will last until September 14th, and more industry professionals are expected to visit the Shenzhen Convention and Exhibition Center to participate in the annual feast of connector and wire harness products, manufacturing and processing technology industry!