Constant and low-frequency magnetic and electromagnetic...

Constant and low-frequency magnetic and electromagnetic fields

Magnetic fields can be constant from artificial magnetic materials and systems, pulsed, infra-low frequency (with frequency up to 50 Hz), variable.

The impact of EMF industrial frequency is associated with high-voltage transmission lines, sources of permanent magnetic fields used in industrial plants.

The sources of permanent magnetic fields are permanent magnets, electromagnets, electrolysis baths (electrolyzers), DC transmission lines, busbars and other electrotechnical devices using direct current. An important factor of the production environment in the manufacture, quality control, assembly of magnetic systems is a constant magnetic field.

Magnetoimpuls and electrohydraulic units are sources of a low-frequency pulsed magnetic field.

The constant and low-frequency magnetic field rapidly decreases with distance from the source.

The magnetic field is characterized by two quantities - induction and tension. Induction B is the force acting in a given field on a conductor of unit length with a single current, measured in Tesla (T). The intensity H is a quantity that characterizes the magnetic field regardless of the properties of the medium. The vector of tension coincides with the induction vector. Unit of measurement of tension - ampere per meter (A/m).

Electric power lines with a voltage of up to 1150 kV, open distribution devices, switching devices, protection and automation devices, measuring devices belong to the electromagnetic fields (EMF) of the industrial frequency.

Overhead power lines (50 Hz). The influence of EMF of industrial frequency is connected with high-voltage lines (BL) of electric transmissions, sources of permanent magnetic fields used in industrial enterprises.

The intensity of EMF from overhead transmission lines (50 Hz) depends largely on line voltage (110, 220, 330 kV and above). Average values ​​at workplaces of electricians: Е = 5 ... 15 kV/m, Н = 1 ... 5 A/m; on the routes bypassing maintenance personnel: Е = 5..30 kV/m, Н = 2 ... 10 A/m. In residential buildings located near high-voltage lines, the electric field strength, as a rule, does not exceed 200 ... 300 V/m, and the magnetic field 0.2 ... 2 A/m (B = 0.25 ... 2 , 5 mT).

The magnetic field near the power lines (power lines) with a voltage of 765 kV is 5 mT directly under the power line and 1 mT at a distance of 50 m from the power line. The picture of the distribution of the electromagnetic field as a function of the distance to the power line is shown in Fig. 5.6.

The EMF of the industrial frequency is mainly absorbed by the soil, so at a small distance (50 ... 100 m) from the power lines, the electric field strength falls from tens of thousands of volts per meter to the standard values. A significant danger is represented by magnetic fields arising in the zones near power transmission lines (LEP) of industrial frequency currents, and in zones adjacent to electrified railways. High-intensity magnetic fields are also found in buildings located in close proximity to these zones.

Fig. 5.6. Electric and magnetic field under a power line with a voltage of 765 kV (60 Hz) at a current of 426 A, depending on the distance to the power line (line height 15 m)

Railway electric transport. The strongest magnetic fields in large areas in a densely populated urban environment and at workplaces are generated by public rail electric transport. The theoretically calculated picture of the magnetic field generated by typical currents from the railway is shown in Fig. 5.7. The experimental measurements carried out at a distance of 100 m from the track yielded a magnetic field value of 1 μT.

The level of transport magnetic fields can exceed the corresponding level from the power line by 10 ... 100 times; it is comparable, and often exceeds the Earth's magnetic field (35 ... 65 μT).

Electric networks of apartment houses and household low-frequency devices. In everyday life, EMF and radiation sources are television sets, displays, microwave ovens and other devices. Electrostatic fields in conditions of low humidity (less than 70%) create clothing and household items (fabrics, carpets, capes, curtains, etc.). Microwave ovens in industrial design are not dangerous, but the failure of their protective screens can significantly increase the leakage of electromagnetic radiation. Screens of TVs and displays as sources of electromagnetic radiation in everyday life do not pose a great danger even with prolonged exposure to humans if the distance from the screen exceeds 30 cm.

Fig. 5.7. The configuration of the magnetic field from the electrified railway

Quite strong magnetic fields can be detected at a frequency of 50 Hz near home appliances. So, the refrigerator creates a field of 1 mcT, a coffee maker - 10 mT, and a microwave oven - 100 mT. Similar magnetic fields of a much larger extent (from 3 ... 5 to 10 μT) can be observed in working zones of steelmaking production using electric furnaces.

The intensity of electric fields near the extended wires included in the 220 V network is 0.7 ... 2 kV/m, near household appliances with metal housings (vacuum cleaners, refrigerators) - 1 ... 4 kV/m

In Table. 5.6 the values ​​of the magnetic induction about some household appliances are given.

In the vast majority of cases, a network with one zero (zero working) conductor is used in apartment houses, networks with zero working and protective conductors are quite rare. In such a situation, the risk of electric shock is increased when the phase wire is closed to the metal case or chassis of the device; metal enclosures, chassis and instrument housings are not grounded and are the source of electric fields (when the device is switched off with a plug in the socket) or electric and magnetic fields of the industrial frequency (with the device switched on).

Table 5.6. The values ​​of the magnetic induction B near household appliances, mcT

Devices

Distances from the devices, cm

3

30

100

Hair Dryers

6 ... 2000

0.01 ... 7

Less than 0.01 ... 0.3

Electric Shavers

15 ... 1500

0.08 ... 9

Less than 0.01 ... 0.3

Drills

400 ... 800

2 ... 3.5

0.08 ... 0.2

Vacuum cleaners

200 ... 800

2 ... 20

Less than 0.13

Mixers

60 ... 700

0.6 ... 10

-

Wiring

0.1-100

0.01 ... 4

0.02 ... 0.25

Portable heaters

10 ... 180

0.15 ... 5

Less than 0.01

TVs

2.5 ... 50

0.04 ... 1.2

Less than 0.01 ... 0.15

Washing machines

0.8 ... 50

0.15 ... 2

Less than 0.01 ... 0.15

Electric irons

8..30

0.12 ... 0.3

0.01 ... 0.025

Fans

2 ... 30

0.03 ... 0.4

0.01 ... 0.35

Refrigerators

0.5 ... 1.7

0.01 .... 0.25

Less than 0.01

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