Human exposure to electromagnetic fields generated by overhead lines with voltages above 1000 V. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Human exposure to electromagnetic fields generated by overhead lines with voltages above 1000 V. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electrical safety, fire safety

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Electromagnetic field of ultrahigh voltages negatively affects the human body. A medical examination of personnel working for a long time near high-voltage overhead lines showed that the electromagnetic field of industrial frequency causes increased fatigue in a person, a decrease in blood pressure, and a drop in heart rate; in the heart there are sharp pains, accompanied by palpitations and arrhythmia.

The harmful effect of the electric field on the human body, located near the overhead line, is determined by two factors: the biological effect of electromagnetic radiation and the occurrence of electrical discharges between a person and current-carrying parts; between a person and metal structures that have different potentials.

Current up to 50 μA does not have a noticeable effect on the human body. At a current exceeding 50 μA, it is necessary, when working near overhead lines, to use special shielding suits or stationary portable shielding devices.

The amount of current flowing through a person is directly proportional to the potential of the electric field at the location of the person. In turn, the potential of the electric field depends on the voltage of the electrical installation, on the distance between the person and the current-carrying part. Obviously, the higher the voltage of the electrical installation and the closer the person is to the current-carrying part, the higher the potential will be and the greater the current will pass through the person. The potential under the overhead line is determined at a height of 1,7 m from the ground (average human height). The highest values of currents fall in the middle of the intermediate span of the overhead line at a distance of 1-3 m from the projection of the extreme phase wire to the ground. Depending on the increase in the voltage of the overhead line, the current strength increases. At a voltage of 110 kV, 220 kV, 500 kV and 750 kV, the current strength will be 10 μA, 38 μA, 113 μA, 200 μA, respectively. When a person moves under the extreme phase from the middle of the span to the overhead line support, the current strength decreases to 3 μA at an overhead line voltage of 110 kV and to 60 μA at 750 kV. Under the middle phase wire and between the phase wires of the overhead line, the current is reduced by 25-50%. Potentials of a dangerous magnitude are also found on machines and various mechanisms if they are near the overhead line. Therefore, all rubber-running machines and mechanisms used in the zone of influence of an electric field must be equipped with a metal chain connected to the chassis or body. Before entering the zone of influence, the chain should be lowered to the ground.

Measurements of currents under overhead lines at a height of 1,7 m from the ground made it possible to conclude that at an overhead line voltage above 220 kV, the current passing through a person significantly exceeds the permissible limit, therefore, it is undesirable for a person to be under an active overhead line without appropriate protection. A person can stay near the overhead line for a long time without the use of protective equipment, if the distance from the place of work to the projection of the outermost wire on the ground is (not less than): with 110 kV overhead lines - 5 m, 220 kV overhead lines - 10 m, 330 kV overhead lines - 20 m , VL 500 kV - 30 m, VL 750 kV - 40 m, VL 1150 kV - 55 m.

The amount of current flowing through a person when he is in the zone of influence of an overhead line depends not only on the voltage of the latter, but also on weather conditions. In wet weather, the current passing through a person increases (with dry shoes). Usually, animals are anxious and try to leave the zone of influence of the overhead line.

The maximum permissible levels of electric field strength should not exceed the following values:

- on the territory of a residential area, vegetable gardens, orchards - 5 kV / m;

- inside residential buildings - 0,5 kV/m.

At voltages above 1 kV / m, measures should be taken to exclude the impact on a person of perceptible electrical discharges and currents flowing to the ground.

These measures include:

- organization of a sanitary protection zone - the territory along the overhead line route, where the tension does not exceed 1 kV / m. For overhead lines of various voltages, the distances from the projection onto the ground of the extreme phase wires to buildings or a person should be as follows: at a voltage of 330 kV - 20 m, at 500 kV - 30 m, at 750 kV - 40 m and at 1150 - at least 55 m;

- removal of a residential building from overhead lines or the use of shielding devices. The presence of residential buildings and the performance of work within the sanitary protection zone are unacceptable.

Indirectly, a person himself, without the use of measuring instruments, can determine the electric field strength, which does not exceed 5 kV / m. At this voltage, most people do not feel the effects of electrical discharges by touching grass or shrubs with the back of their hands.

If it is necessary to cross overhead lines of ultra-high voltage, one should not wear synthetic clothing and shoes, since a spark discharge may occur between a person and the ground or grass and the fabric will be burned. In this case, it is better to take off your shoes and walk barefoot under the overhead line.

In the sanitary protection zone of overhead lines with a voltage of 330 kV and above, it is forbidden to perform any manual work, put up haystacks, stacks of bread, stacks of peat, firewood, etc.

It is unacceptable to stay in the sanitary protection zone of the overhead line, even for a short time, for children and adolescents under 18 years of age.

It is forbidden to make fire within the sanitary protection zone of the overhead line. You can not approach the overhead line during a thunderstorm, while holding braids and pitchforks over your head.

Author: Korshevr N.G.

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