First, the difference between low-voltage cable and high-voltage cable
High-voltage cables are conductor-inner semiconductor layer-insulating layer-outer semiconductor layer-metal armor-sheath layer from inside to outside;
Low-voltage cables are conductor-insulation layer-steel belt-sheath from inside to outside (many low-voltage cables have no steel belt).
The main difference between high voltage cable and low voltage cable is that high voltage cable has more semiconductor layers and shielding layers than low voltage cable. Therefore, the insulation layer of high-voltage cable is much thicker than that of low-voltage cable, and the structure is complex and the process requirements are high.
(1) Semiconductor layer
The main function of the inner semiconductor layer is to improve the electric field effect. Because there is a gap between the conductor and the insulation layer of the high voltage cable, it is easy to produce partial discharge to destroy the insulation layer. In order to improve this situation, a shielding layer of semiconductor material is added between the metal conductor and the insulating layer to play a transitional role. The outer semiconductor layer has the same function as the inner semiconductor layer, and its function is to avoid partial discharge between the insulating layer and the metal sheath.
(2) Shielding layer
The metal shielding layer of high-voltage cable has three main functions:
1, shielding electric field
Since it is a shielding layer, as the name implies, it plays a shielding role. The voltage in the high-voltage cable is very high, and the electric field it produces is very strong. The shielding layer can effectively prevent the interference of the high-voltage cable to the outside world.
2, run through the capacitor current
The inner layer of the high-voltage line is a conductor, with an insulating layer in the middle and a metal sheath outside. Two conductors (inner copper wire+metal sheath) are separated by an insulating medium (insulating layer), which can be regarded as a capacitor. The basic principle of capacitance is two metal plates separated by an insulating medium. Alternating current will charge and discharge the capacitor, so the shielding layer can provide a circuit for charging and discharging the capacitor.
In addition, the metal shielding layer and metal armor of high-voltage cable need to be grounded at one end.
3, can be used as a short-circuit current channel.
When the insulation of high-voltage cable is damaged, if there is no shielding layer, the cable will leak to the earth, which has potential safety hazards. If there is a shielding layer, the leakage current will flow to the earth through the shielding layer.
Second, how to distinguish between low-voltage cables and high-voltage cables
1. Look at the cable structure. There are many layers of high-voltage cables, and there are armor, shielding layer, insulation layer and conductor inside the stripped outer layer. Low-voltage cables generally open the outer layer, and there is an insulating layer or conductor inside.
2. Check the section. Inside the cable, there is a conductive core (copper core or aluminum core), followed by an insulating layer, a semi-conductive layer, a shielding layer, a filling layer, a steel protective layer and a rubber protective layer (waterproof).
3. Check the insulation layer thickness: the insulation layer of high-voltage cable is relatively thick, and the insulation layer of low-voltage cable is relatively thin. Low-voltage cable insulation is generally within 3 mm, and high-voltage cable insulation is generally above 5 mm.
(1) Low voltage (below 1kv) is 1~3mm thick, 10kv cable is 5~8mm, and 35kv cable is about 10mm; Low-voltage or weak-current cables generally have an insulating layer and a protective layer.
(2) The high-voltage cable has an insulation layer after the sheath is removed, which is a white plastic main insulation layer wrapped outside the cable core. The low-voltage cable does not have this main insulation layer, but a rubber protective layer.
4. Look at the nameplate on the outer layer of the cable. Generally, the cable parameters are printed on the outer layer of the cable, including cable model, cross-sectional area, rated voltage, length and other parameters.
5. Check the voltage registration. Generally, there will be voltage levels such as YJV-1KV-4*150 or YJV-10KV-4*150 on the drawings.
Cables are customarily divided into: 1) weak current cables: 450/750V and below; 2) Low voltage cable: 0.6/1kv; 3) Medium voltage cable: 3-35kv; 4) High voltage cable: 35-110kv; 5) EHV cable: 110-750kv.
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