Why don't high voltage lines have insulating skins?

High voltage line transmission voltage up to 500,000 volts, if you want to give such a line package insulation, ordinary insulation thickness is far from enough. Our home electricity wires, the voltage is only 220V, as long as a thin layer of insulation wrapped up to prevent contact with the firewire can prevent electrocution.

High voltage electrocution can be categorized into two types:

One is step voltage electrocution. It refers to the high-voltage transmission line if the ground, there is a current flow into the earth, the ground near the potential will be gradually reduced to the distance, then the two feet of the human position there is a voltage difference, the current flows through the human body resulting in electrocution, this is the step voltage electrocution.

Another is the high-voltage arc electrocution. It is close to the high-voltage charged body to a certain distance, high-voltage charged body and the human body between the discharge phenomenon will occur, the current flows through the human body caused by high-voltage arc electrocution.

The latest regulations define voltage levels above 1kV as high voltage. Now in the city's overhead lines (street poles on the kind) are basically 10kV lines, most have been replaced with insulated lines, that is, the subject said "wrapped with insulation", the thickness of the insulation layer is not high, so the impact on the cost is not very large, due to the 10kV lines and the public contact with the opportunity to more, so insulated lines can improve the safety of operation. So insulated lines can improve the safety of operation, which is very worthwhile.

However, if the 110kV and above lines are made into insulated lines, the thickness of its insulation layer is very large, which brings the result that the cost is substantially increased, and it brings some technical difficulties, firstly, the heat dissipation is not good, in order to reduce the heat dissipation, it is only possible to reduce the amount of its current-carrying capacity, which in fact also improves the cost, and secondly, the strength of such a heavy line will be greatly reduced, and the span of the line will be lowered, the Need to build more towers, three, installation and construction is not greatly improved, in some places where the road is not accessible, may not be able to carry out construction.

Therefore, if the 110kV and above will be changed to insulated lines, the cost will be at least 3 to 5 times higher, each year's investment in electricity is limited, if the use of insulated lines will be less construction of 3 to 5 times the line, which is very unfavorable to the country's economic construction, the so-called insulation insulation, to put it plainly is to play an insulating, safety role, this insulator for high-voltage lines, is a setup, because in the In front of high pressure, the air can break through the conductive, insulator is useless, under normal circumstances, 10 kV of high pressure can break through more than 1 cm of air conductive, in accordance with national technical specifications, high-voltage power lines must be set up so that the wire away from the ground or buildings, the distance is at least a few meters or even tens of meters away, therefore, in the transmission line and the ground or buildings, as well as the possibility of people appearing in the middle of the space between the distance filled with air, the distance is not enough to make it easy for people to get to the ground or buildings. Therefore, in the transmission line and the ground or building, as well as where people may appear, there is a space distance filled with air in between.

The air itself is a good insulator, its breakdown voltage is generally more than 3 kV / mm, and the effect of ordinary resin insulating plastics is comparable, that is to say, for example, on a 110 kV high-voltage transmission lines, theoretically as long as the distance from the person more than 40 millimeters, the person is safe, but the actual spacing distance is generally 5 meters, that is, more than 5000 millimeters, at least in the more than 2 meters, the general situation, no need to wrap any other insulator. Under normal circumstances, do not have to wrap any other insulator, as long as you maintain a certain spatial distance, the air can play a good insulating effect, personal safety is fully guaranteed, if the high-voltage line wrapped in plastic insulators such as resin, one greatly increased the economic cost of manufacturing wires, and the second is to increase the weight of the wires and the difficulty of setting up, because the insulating plastic insulation with the air is almost the same, and finally the high-voltage line of the safety but did not get any improvement, so it is more than worth it.

The only danger of an exposed high voltage line is that it will cause a great risk of electrocution if it breaks and falls on the ground or a building, so there are high requirements in terms of the strength of the wires, the erection techniques and the daily inspections, which are sufficient to guard against the dangers of breaking and leaking, and only in the event of certain irresistible natural disasters, such as earthquakes, fires and floods, it is necessary to disconnect the wires in time to prevent the danger of a possible leakage of electricity, and in case you find that high voltage power lines break and fall to the ground, the safe practice is to maintain a distance of at least 20 meters from it to prevent electrocution accidents, while outdoor overhead lines, as long as they are erected to a certain height and there are no buildings under and near the overhead line, regardless of whether it is high-voltage or low-voltage, are made of bare conductors, which reduces the cost of construction, and with sufficient safety distance, the probability of electrocution accidents is extremely small, and in the case of outdoor overhead lines in the rural areas There is no need for wires with insulation.

High-voltage and low-voltage are generally bare wires, in the old urban areas of the city near the low-voltage overhead lines, the buildings are more dense, generally with the use of insulated wires; due to the high-voltage lines on the insulation requirements of the insulation layer is very high, the construction process is complex, high cost, generally still use bare wires, using the increase in the height of the overhead line to prevent electrocution, in order to look good and safe, the new urban areas now can not be seen in the overhead line, high-voltage and low-voltage Lines are buried in the ground, in fact, there are also high-voltage lines wrapped in insulation, that is, high-voltage cables, unless absolutely necessary, is not used, it is 10 times the cost of ordinary overhead lines, the same cross-section of the conductor, the cable's current-carrying capacity is also less than the overhead lines, which is still in the case of the cable using the copper (overhead lines with aluminum)

Due to more than two decades of power system systems, regulations and technology continue to improve, electrocution accidents have been greatly reduced, the general public contact with more than 10kV voltage level is not much opportunity, will rarely occur more than 10kV voltage level electrocution accident, so this high voltage line is completely unnecessary to use insulated wires.