Thirty cable common problem knowledge popularization（5）
23. Where are the inner shielding layer and outer shielding layer of power cables located? What materials are used? What does it do?
Answer: In order to have better contact between the insulation layer and the cable conductor and eliminate the increase in electric field intensity on the conductor surface caused by the roughness of the conductor surface, the conductor surface is generally covered with an inner shielding layer of metallized paper or semiconductor paper tape. In order to ensure better contact between the insulation layer and the metal sheath, an outer shielding layer is generally wrapped around the insulation layer. The outer screen layer is made of the same material as the inner screen layer, and sometimes it is also bound with copper tape or braided copper wire tape.
24. What should we pay attention to during the transportation, loading and unloading of cables?
Answer: (1) During transportation, loading and unloading, cables and cable drums should not be damaged. It is strictly forbidden to push cable drums directly off the vehicle. Cables should generally not be transported or stored flat.
(2) Before transporting or rolling the cable drum, it must be ensured that the cable drum is firm and the cable is wound tightly. The oil pipe between the oil-filled cable and the pressure oil tank should be fixed and not damaged. The pressure oil tank should be secure and the pressure indication should meet the requirements.
25. What aspects should be considered when selecting the cross-section of power cables?
Answer: The following aspects should be considered:
(1) The long-term operating current allowed by the cable;
(2) Thermal stability in the event of short circuit;
(3) The voltage drop on the line cannot exceed the allowable working range.
26. What are the advantages of power cables compared with overhead lines?
Answer: (1) Reliable operation. Because it is installed in hidden places such as underground, it is less damaged by external forces and has fewer chances of failure. The power supply is safe and will not cause harm to people;
(2) The maintenance workload is small and frequent inspections are not required;
(3) No need to erect poles and towers;
(4) Helps improve power factor.
27. What measures are taken to prevent cable fire?
Answer: (1) Use flame-retardant cables;
(2) Use fireproof cable brackets;
(3) Use fire retardant coatings;
(4) Fire partition walls and fire baffles should be installed at cable tunnels, mezzanine exits, etc.;
(5) Overhead cables should avoid oil pipelines and explosion-proof doors. Otherwise, local pipe penetration or heat insulation and fire prevention measures should be taken.
28. What is a cable failure? How many common types are there?
Answer: Cable failure refers to the failure of insulation breakdown of the cable during preventive testing or the failure of the cable line to cause a power outage due to insulation breakdown, wire burnout, etc. during operation. Common faults include ground faults, short circuit faults, disconnection faults, flashover faults and mixed faults.
29. According to the "Regulations on Safety in Electrical Industry", what are the conditions that electrical workers must meet?
Answer: The following conditions must be met:
(1) Healthy as determined by a doctor, with no illness that would hinder work;
(2) Have necessary electrical knowledge, master relevant procedures, professional techniques and safe operation techniques according to their duties and nature of work, and pass the examination;
(3) Be proficient in first aid methods for electric shock.
30. Where are the inner shielding layer and outer shielding layer of power cables located? What materials are used? What does it do?
Answer: In order to have better contact between the insulation layer and the cable conductor and eliminate the increase in electric field intensity on the conductor surface caused by the roughness of the conductor surface, the conductor surface is generally covered with an inner shielding layer of metallized paper or semiconductor paper tape. In order to ensure better contact between the insulation layer and the metal sheath, an outer shielding layer is generally wrapped around the insulation layer. The outer screen layer is made of the same material as the inner screen layer, and sometimes it is also bound with copper tape or braided copper wire tape.