Common Sense Question And Answer of Wire And Cable

1. Why is it necessary to install a parallel cable in the grounding mode of one end of single-core cable sheath?

Answer: In the cable line grounded at one end of the metal sheath, in order to ensure that the induction voltage in the sheath does not exceed the allowable standard, a conductor laid parallel along the cable line must be installed, and both ends of the conductor are grounded. This conductor is called the reflux line. When a single-phase ground fault occurs, the grounding short-circuit current can flow back to the center point of the system through the reflux line. Because the magnetic flux generated by the grounding current of the reflux line cancels the magnetic flux generated by the grounding current of the cable wire, the induced voltage of the sheath in the short-circuit fault can be reduced.

2. How many projects can it be divided into for the cable engineering?

Answer: (1) site transportation: including engineering materials from the warehouse to the construction point, transportation and empty car return.

(2) Local works: including pavement excavation, tunnel, trench construction, etc.

(3) Laying engineering: including laying, middle head production, lifting cover plate, buried pipe, school tide, traction head production, etc.

(4) Two ends works: including production and installation of support and suspension bridge and its foundation, production of terminal head, installation of oil pressure and signal device, electrical performance test of names, etc.

(5) Stop engineering: including plug head production of oil-filled cable, fuel supply tank, automatic drainage and installation of signal device, etc.

(6) Grounding engineering: including insulation joint, transposition box, protector, grounding box installation, etc.

3. What inspections should be made for the acceptance of cable lines?

Answer: (1) the cable specifications should meet the provisions, the arrangement should be neat, no damage, complete, correct and clear signs;

(2) The fixed bending radius of the cable, the relevant distance and the wiring of the metal cover of the single-core power cable shall meet the requirements;

(3) The cable terminal and the middle head do not leak oil, the installation is firm, and the oil filling cable oil pressure and meter fixing value should meet the requirements;

(4) Good grounding;

(5) The cable terminal is correctly colored, and the metal parts such as bracket are complete;

(6) There should be no sundries in the cable trench and tunnel, with complete cover plate.

4. What are the requirements in the process of cable line normal sequence impedance measurement?

Answer: the AC resistance of the cable conductor and the phasor of the same cable resistance are called positive sequence impedance.

Cable line positive sequence impedance generally can be measured directly on the cable tray, measuring generally use low voltage, therefore, need to use step-down transformer, step-down with star wiring, capacity is generally above 10 kVA, has a wide range of voltage regulation, measuring ac power supply should be relatively stable, to ensure that the current to meet the required voltage, the actual voltage meter is cable voltage, the test current best near the cable for a long time, read the value of the meter, close the current and read the value of the three tables.

5, according to the "Electricity production safety regulations", what is the condition that the electrical staff must have?

A: The following conditions must be met:

(1) It is proved healthy by the doctor and has no symptoms that interfere with the work;

(2) Have the necessary electrical knowledge, master the relevant regulations, professional technology and safe operation technology according to their position and the nature of the work, and pass the examination;

(3) Proficient in the first aid methods of electric shock.

6. What should cables pay attention to in the process of transportation and loading and unloading?

Answer: (1) in the process of transportation loading and unloading, should not make the cable and cable tray are damaged, it is strictly prohibited to push the cable tray directly from the car, the cable should not be flat transportation, flat storage.

(2) Before transportation or rolling cable tray, the cable tray must be kept firm, the oil pipe between the cable winding, the oil-filled cable and the pressure tank should be fixed without damage, the pressure tank should be firm, and the pressure indication should meet the requirements.

7. What are the measures for cable fire prevention?

Answer: (1) using flame retardant cable;

(2) Use the fireproof cable bracket;

(3) Use of fire-retardant coating;

(4) Cable tunnel, sandwich exit and other places are equipped with fire partition wall and fire baffle;

(5) Overhead cables should avoid oil pipelines, explosion-proof doors, otherwise it should be taken for local pipe penetration or heat insulation and fire prevention measures.

8. What aspects should be considered to choose the section of the power cable?

A: The following aspects should be considered:

(1) Operating current allowed to pass by the cable for a long time;

(2) the thermal stability once short circuit;

(3) The voltage drop on the line shall not exceed the allowable working range.

9. What are the advantages of power cable and overhead line comparison?

Answer: (1) reliable operation, due to the installation in the underground and other hidden places, by external force damage is small, the chance of failure is less, power supply safety, will not cause harm to people;

(2) Maintenance workload is small, without frequent inspection;

(3) There is no need to erect poles and towers;

(4) Help to improve the power factor.