1. Why does a single-core cable jacket ground in one end of the cable must be installed with a return cable laid parallel to the cable?
Answer: In the cable line grounded at one end of the metal sheath, in order to ensure that the induced voltage in the sheath does not exceed the allowable standard, a conductor laid in parallel along the cable line must be installed, and both ends of the conductor are grounded. This conductor is called back Streamline. When a single-phase ground fault occurs, the ground short-circuit current can flow back to the center of the system through the return line, and the magnetic flux generated by the ground current through the return line cancels the magnetic flux generated by the ground current of a part of the cable conductor and can be reduced. The induced voltage of the sheath in the event of a short circuit.
2. Can cable engineering be divided into several projects?
(1) Site transportation: Including the loading, unloading and transportation of engineering materials from the warehouse to the construction site and return of empty vehicles.
(2) Local projects: including road excavation, tunnels, and trench construction.
(3) Laying project: including laying, intermediate head making, flipping cover plate, buried pipe, school tide, traction head making and so on.
(4) Both ends of the project: including the production and installation of supports, suspension bridges and their foundations, the production of terminal heads, the installation of oil pressure and signal devices, and the testing of electrical properties.
(5) Plug and stop project: including the production of oil-filled cable plugs and heads, installation of fuel tanks, automatic drainage and signal devices, etc.
(6) Grounding: Including insulation joints, transposition boxes, protectors, and grounding box installation.
3. What inspections should be performed on the acceptance of cable lines?
Answer: (1) The cable specifications should conform to the regulations, the arrangement should be neat, no damage, complete signs, correctness, and clarity;
(2) The fixed bending radius of the cable, the relevant distance and the wiring of the metal sheath of the single-core power cable shall meet the requirements;
(3) The cable terminal and the middle head are not leaking oil, and the installation is firm. The oil pressure of the oil-filled cable and the setting value of the meter should meet the requirements;
(4) Good grounding;
(5) The cable terminal is in correct color, and the metal components such as brackets are complete;
(6) There should be no debris in the cable trench, in the tunnel and on the bridge, and the cover is complete.
4. What are the requirements for the positive sequence impedance measurement of cable lines?
A: The AC resistance of the cable conductor and the phasor of the same three inductive reactance of the cable are called the positive sequence impedance.
The positive sequence impedance of the cable line can be measured directly on the cable tray. Generally, lower voltages are used during measurement. Therefore, step-down transformers are required to perform step-down voltages. The voltage drop device uses star-type wiring, and the capacity is generally 10 kVA or more. For wider voltage regulation range, the AC power supply should be relatively stable during measurement to ensure that the current reaches the specified requirements during measurement. The actual voltmeter reading must be the voltage of the cable end, and the test current is preferably close to the long-term allowable ampacity of the cable. For each meter value, the three tables are read at the same time after closing the current.
5. According to the “Safety Regulations for Electric Power Production”, what are the conditions that electricians must have?
Answer: The following conditions must be met:
(1) A medically certified illness that does not interfere with work;
(2) Having the necessary electrical knowledge, according to their duties and the nature of their work, mastering relevant regulations, professional techniques and safe operating techniques, and passing the examination;
(3) Proficiency in emergency methods for electric shock.
6. What should be noticed during the transportation and handling of cables?
Answer: (1) In the process of transportation and loading, cables and cable trays should not be damaged. It is forbidden to push the cable tray directly from the vehicle. Cables should not be transported flat and stored flat.
(2) Before transporting or rolling cable trays, it must be ensured that the cable trays are securely fastened, and the oil conduits between the oil-filled cables and the pressurized oil tanks should be fixed and not damaged. The pressure oil tanks should be firm and the pressure indications should meet the requirements.
7. What are the measures for cable fire protection?
Answer: (1) Use flame-retardant cables;
(2) Use fireproof cable brackets;
(3) using fire retardant paint;
(4) Fireproof partition walls and fire dampers are installed at cable tunnels, mezzanine exits, etc.;
(5) Overhead cables should avoid oil pipes and explosion-proof doors. Otherwise, fire protection measures should be taken for local pipes or heat insulation.
8. What aspects should be considered in selecting the section of the power cable?
A: The following aspects should be considered:
(1) Operating current allowed to pass through the cable for a long period of time;
(2) Thermal stability in the event of a short circuit;
(3) The voltage drop on the line must not exceed the allowable operating range.
9. What are the advantages of power cables and overhead lines?
A: (1) Operation is reliable. Because it is installed in a hidden place such as underground, it is damaged by external forces, there is less chance of failure, and the power supply is safe. It will not cause harm to the human body.
(2) Maintenance workload is small, and frequent inspections are not required;
(3) No erection of towers;
(4) Helps increase power factor.
Contact: Sheng Cable
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