1、 Selection conditions of 1-5-core cable for 1kV and below
(1) Single core cable
When the line is long and the working current is large, or when it is laid under water, in order to avoid or reduce the intermediate joints, or when the single core AAC Cable has better comprehensive technical economy than the multi-core cable, the single core cable can be selected.
Low voltage DC power supply circuit, when necessary, single core cable can be used.
(2) Core cable
When the neutral point of 1kV and below power supply is directly grounded, the number of cable cores of single-phase circuit, when the protection line and neutral line share the same conductor, the 2-core cable shall be used.
2-core cable should be used for DC power supply circuit.
(3) Core cable
When the neutral point of 1kV and below power supply is directly grounded, the number of cable cores of single-phase circuit is: when the protection line and neutral line are independent, 3-core cable should be used.
(4) Core cable
For 4-core cable (3 + 1 cable) of 1kV and below, the fourth core not only serves as protective grounding, but also transmits unbalanced current and short-circuit current of power system. Its size is determined by unbalanced current and short circuit current, but generally not less than 1 / 2 of phase line.
When the protective line and neutral line share the same conductor, 4-core cable should be used in the three-phase four wire low-voltage distribution system of 1kV and below, instead of 3-core cable combined with single core cable to form a loop, or even directly using the metal sheath or armor layer of 3-core cable as neutral line. Otherwise, when the three-phase current is unbalanced, it is equivalent to the operation state of single core cable, which is easy to cause power frequency interference.
(5) Core cable
For the three-phase four wire low-voltage distribution system of 1kV and below, some distribution lines of electrical devices with high safety requirements, and some communication centers and automation equipment that not only ensure electrical safety, but also have anti-interference grounding, the TN-C low-voltage distribution system with 5 cores should be adopted, so that the protection line (PE line) and neutral line (PN line) are independent, The cross-sectional area of cable core is generally 3 large, 2 small or 4 large, 1 small or 5 large.
2. Selection of medium and high voltage cable cores
For 3 ~ 35kV three-phase power supply circuit cable core number selection. When the loop or cable with large working current is laid underwater, 3 single core cables can be selected for each loop; In addition to the above conditions, 3-core cable should be selected. The 3-core cable can be of common package type or 3 single core cable stranding structure type.
For 110kV three-phase power supply circuit, except for laying in Lake, sea water and other places and when the cable section is small, three single core cables can be selected for each circuit.
For three-phase power supply circuits above 110KV, three single core cables shall be selected for each circuit.
For high voltage AC single-phase power supply circuits such as electrified railway, two core cables or two single core cables shall be selected for each circuit.
Single core cable should be selected for HVDC transmission system; Coaxial 2-core cable can also be used for underwater laying in Lake and sea.
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