1. Selection Criteria for 1~5 Core Cables up to 1kV
Single-Core Cable
Single-core cables are typically considered when the circuit is long, carries a large working current, or is laid underwater. They help reduce or eliminate intermediate joints and may offer better technical and economic performance compared to multi-core cables in certain applications. In low-voltage DC power supply systems, single-core cables can also be used where necessary.
Two-Core Cable
In 1kV and below power systems where the neutral point is directly grounded, a two-core cable should be used for single-phase circuits if the protective conductor and the neutral conductor are combined into one. For DC power supply circuits, a two-core cable is also recommended.
Three-Core Cable
When the neutral point of the 1kV and below power supply is directly grounded, a three-core cable should be used for single-phase circuits if the protective and neutral conductors are separate.
Four-Core Cable
A four-core cable (3+1) is commonly used for 1kV and below systems, with the fourth core serving as a protective ground. It carries unbalanced and short-circuit currents, and its size is determined by these factors but should generally not be less than half the cross-sectional area of the phase conductors.
In three-phase four-wire low-voltage distribution systems, if the protective and neutral conductors are combined, a four-core cable must be used. Combining a three-core cable with a single-core one or using the metal sheath of a three-core cable as a neutral line is not recommended. Otherwise, an imbalance in the three-phase current could cause power frequency interference, similar to that of a single-core cable.
Five-Core Cable
For three-phase four-wire low-voltage systems up to 1kV, especially in areas with high safety requirements such as communication centers or automation equipment, a five-core TN-C system is often used. This separates the protective earth (PE) and neutral (N) conductors, offering better electrical safety and interference protection. The core sizes are usually 3 large and 2 small, or 4 large and 1 small, or all five large.
Second, Selection of Core Numbers for Medium and High Voltage Cables
For 3~35kV three-phase power supply systems, single-core cables are preferred for long circuits or those with high operating currents, especially when laid underwater. Otherwise, three-core cables are generally used, either in a common sheath or as stranded single-core cables.
For 110kV three-phase power supply circuits, three-core cables can be used unless the cable is laid in lakes or seawater and has a small cross-section, in which case three single-core cables are more suitable.
For three-phase power supply circuits above 110kV, single-core cables are required for each circuit.
In high-voltage AC single-phase systems, such as electrified railways, two-core cables or two single-core cables per return path are typically used.
For high-voltage direct current transmission systems, single-core cables are standard. In underwater installations, coaxial two-core cables can also be used.
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