Shulou(Shulou.com)11/24 Report--

High-voltage wires are dangerous. Don't touch them if you have nothing to do. Don't touch them if you have something to do. This is common sense. But what should you do if a high-voltage power line falls to the ground nearby?

Remember four words: never run. There is only one walking position that will save your life. Today we will learn the correct operation in case of grounding high-voltage wires nearby.

High-voltage electricity is closely related to our daily life and industrial production. It can be said that it is the "blood" of human society.

The power supply station and substation are transmitted by 60,000 - 500,000 volt transmission lines. At substations, the voltage of high-voltage electricity is reduced and then transported to homes. In general, the voltage of high-voltage power lines on the street is between 4 and 25 kV.

This H-shaped transmission line is a relatively old transmission system. It is more common in old residential areas. The transmission line at the top has high voltage, and the communication line at the bottom. Image source: hsseworld the danger of high-voltage power lines is self-evident. Once electrocuted, the heat generated by the current passing through the body can burn organs, and the current can also cause cardiac arrest. But what many people don't know is that there is a "boundary" near the high-voltage power line, and that "boundary" is actually very dangerous.

When a high-voltage wire touches the ground directly or indirectly, the potential at the point of contact (which you can simply understand as voltage) spreads out like waves, and the closer you get to the point of contact, the higher the potential and the more dangerous it is. This ripple-like diffusion and weakening of the potential along the contact point is called an earth potential rise, which is equivalent to expanding the range of hazards of high-voltage wires.

Ground potential rises when wires are in direct contact with the ground. Image source: Western power How large is the "boundary" formed by this high voltage region?

If a 60,000 volt high-voltage wire falls to the ground, then a circle with a radius of 10 meters centered on the contact point is as dangerous as the high-voltage wire. If you don't want to get electrocuted, keep a distance of at least 10 meters from the contact point. If it is a higher voltage high voltage wire, then this circle has a larger range.

Of course, if anything that conducts electricity (such as a tree branch) is placed in this area, it can become as dangerous as high-voltage power lines and can produce electric shocks to humans.

Ground potential near the substation rises. In the field of electrical engineering, the hazards of elevated ground potential have long been known, and many international organizations have protection standards for elevated ground potential, such as the Institute of Electrical and Electronics Engineers (IEEE). 80-2000。The U.S. Occupational Safety and Health Administration (OSHA) also classifies elevated ground potential as a "known hazard" and proposes corresponding management and disposal regulations.

So, what if a high-voltage power line does fall nearby?

Once again, do not run at this time, run is dead. According to the U.S. Occupational Safety and Health Administration, there are only two correct practices at this time: it is best to stay still if it is not necessary, and if you must leave, take the "geisha step," that is, the legs rub against each other in small steps, moving a few centimeters at a time, and keeping your feet off the ground.

Method of movement within the area of elevated ground potential: geisha step. Photo Credit: SCGA Why? Middle school physics tells us that electric current is caused by potential difference. Because if there is a potential difference between two points in the human body, then the person will be overcharged.

In the field of safe production of electricity, the potential difference between two steps is called step potential. In the region of elevated ground potential, it is obvious that the larger the step, the larger the step potential, the easier it is to be electrically split.

Step potential. Picture source: SCGA Imagine that one leg of a person steps on the contact point of 60,000 volts of high voltage and the earth, the other leg is 10 meters away, the potential difference between his two feet reaches 60,000 volts, Pikachu explosion is unavoidable, unless you are small wisdom, otherwise do not try.

In turn, reducing the potential of the stride reduces the likelihood of pickup, which is how the geisha step works: keeping the feet in contact and moving along the legs reduces the potential difference between the feet.

In addition to the step potential, another danger is called touch potential, which means that a person stands on the ground with a low potential and touches something with a high potential, such as an excavator connected to a high-voltage wire. Of course, current will flow from the high potential through your foot to the low potential ground.

Contact potential. Image source: SCGA The principle of contact potential sounds simple, but if you really encounter such a situation, what should you do, such as sitting in a car and high-voltage power line contact?

That's the second important suggestion.

At this point, the most critical self-help measure is not to try to leave the car. Because as long as you're in the car and your body isn't in contact with a low potential ground, no electricity will be generated and you're safe.

The U.S. Occupational Safety and Health Administration recommends that if you have to abandon your car, the best posture is to put your feet together, keep your hands close to your body, and bounce off the car like an inchworm. The idea is to leave the car without touching the ground at the same time.

The correct posture to escape from a powered car: inchworm. Photo source: SCGA Of course, after leaving the car, you have to use the geisha step you learned before.

At this point, you might think that high-voltage power lines are dangerous if they touch the ground, but this is actually happening because of artificial settings: most high-voltage power lines have a neutral line that is grounded, that is, connected to the ground.

In other words, if the neutral line is not grounded, then standing on the ground while touching the live wire of the high-voltage wire will not get an electric shock. But does such a system exist?

Yes, it is called floating ground, or floating ground. The neutral line of a floating-grounded transmission system is not grounded, and the main purpose of doing so is to prevent electromagnetic interference caused by the current loop generated by grounding.

Image credit: wikimedia since floating grounding seems safer than grounding, why don't we use this type of transmission system?

The reason is that the floating ground voltage is not stable, especially when the user switches the appliance. Overvoltage may occur in the ungrounded loop when the appliance is turned on and off, thus damaging the appliance. Therefore, floating grounding is mainly used in large factories, and there are places where it is impossible to ground the zero line, such as aircraft.

After seeing the safe position and floating grounding near the high-voltage wire, careful children may ask a question: since the neutral line of the general transmission line is grounded, can I directly regard the earth as the zero line?

Such systems, in which the earth acts directly as a neutral, are called single wire earth return systems. Sometimes, it is not the earth that acts as the zero line, but the water. Of course, such a system has very high requirements for the insulation of the fire line, otherwise it is really Pikachu runaway everywhere.

Single-line earth loop transmission line in Kruger National Park, Cabra Bassa District, Mozambique. Obviously, the main reason for treating the earth as a zero line is to save the cost of manufacturing and laying zero lines, so single-line earth circuits are mostly used in sparsely populated areas and isolated power consumption places.

In 1981, a single-wire ground-loop transmission line was built between Bethel and Napak, Alaska. Some developing countries also use such systems as Laos, Mozambique and South Africa as major transmission facilities. There was a tingling sensation when stepping on the ground in these countries, which might not be an illusion.

Step lightning, okay?

References:

https://docs.qq.com/doc/DVGxoeGlXYmtTRHhv

This article comes from Weixin Official Accounts: Bringing Science Home (ID: steamforkids), by Qi Jun

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.