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Questions and answers about wires and cables

November 24, 2020

Information source: China wire and cable network information center

 

1. Why is it necessary to install a return line parallel to the cable when the sheath of single core cable is grounded at one end?

 

Answer: in the cable line with one end of the metal sheath grounded, in order to ensure that the induced voltage in the protective sleeve does not exceed the allowable standard, a conductor parallel to the cable line must be installed, and both ends of the conductor are grounded. This kind of conductor is called return 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 return line. Because the magnetic flux generated by the grounding current through the return line offsets a part of the magnetic flux generated by the grounding current of the cable conductor, the induced voltage of the sheath during the short-circuit fault can be reduced.

 

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

A:

(1) Site transportation: including loading and unloading, transportation and empty vehicle return of engineering materials from warehouse to construction site.

(2) Local engineering: including pavement excavation, tunnel and trench construction.

(3) Laying engineering: including laying, making of middle head, lifting cover plate, burying pipe, checking tide, making traction head, etc.

(4) Two end project: including the fabrication and installation of the support, suspension bridge frame and its foundation, terminal head production, installation of oil pressure and signal device, and electrical performance test of various types.

(5) Plug up project: including the production of plug head of oil filled cable, installation of oil supply tank, automatic drainage and signal device, etc.

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

 

3. What inspection should be carried out for acceptance of cable line?

 

Answer: (1) the cable specifications should meet the requirements, the arrangement should be neat, without damage, and the signs should be complete, correct and clear;

 

(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 middle head shall be free from oil leakage and installed firmly. The oil pressure of oil filled cable and the setting value of meter shall meet the requirements;

(4) Good grounding;

(5) The phase color of cable terminal is correct, and the paint of metal parts such as bracket is complete;

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

 

4. What are the requirements of positive sequence impedance measurement of cable line?

Answer: the AC resistance of the cable conductor and the phasor sum of the three same inductive reactances of the cable is called positive sequence impedance.

 

The positive sequence impedance of the cable line can be measured directly on the cable tray. Generally, a lower voltage is used in the measurement. Therefore, a step-down transformer is required to reduce the voltage. The step-down transformer adopts star connection with a capacity of more than 10KVA and has a wide range of voltage regulation. During the measurement, the AC power supply should be relatively stable to ensure that the current meets the specified requirements during the measurement The reading value must be the voltage at the cable end, and the test current should be close to the long-term allowable current carrying capacity of the cable. When reading the values of each meter, read the values of three meters after closing the current.

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