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Electric fields of industrial frequency currents

Electric Fields of Industrial Frequency Currents
It has been established that negative influence on the body of workers is exerted by electromagnetic fields of industrial frequency currents (that are characterized by frequency of fluctuations from 3 up to 300 Hz). Adverse effects of currents of industrial frequency manifest themselves only at intensity of a magnetic field of the order 160-200 A/m. Frequently, the magnetic field strength does not exceed 20-25 A/m, therefore the estimation of danger of influence of the electromagnetic field is to be made on the basis of the value of the electric field strength. 

To measure the strength of electric and magnetic fields “IEMP-2” ("-2") devices are used. Density of radiation flow is measured by various types of radar-testers and thermistor measuring devices of low-power , for example, "45-", "VIM", etc. 

Protection against electric fields

In conformity with the standard “GOST 12.1.002-84 SSBT. Electric fields of industrial frequency. Admissible levels of strength and the requirements for monitoring procedure at workplaces” norms of admissible levels of electric fields strength depend on time of staying of people in a dangerous zone. Presence of the personnel at a workplace within 8 hours is allowed at strength of the electric field (E), not exceeding 5 -20 kV/m. When readings of the electric field strength is within the range of 5 -20 kV/m: the duration of admissible stay in the working zone in hours makes: 

 =50/-2.
(3.1)  


Work in conditions of an irradiation form the electric field with the strength of 20-25 kV/m is permitted for not more than 10 minutes. 

In the working zone characterized by various levels of strength of the electric field, staying of the personnel is limited to time (in hours): 




  (3.2)  


where and – respectively, denominate actual and permissible duration of staying of the personnel (in hours) in controlled zones with strengths 1, 2, ..., n. 

The basic types of means of collective protection against exposure to the electric field of industrial frequency currents are shielding devices. Shielding can be general and individual. In case of general shielding high-frequency installation is closed in a metal casing - a cap. Control of the installation is carried out through windows in walls of a casing. With a view of safety the casing is connected with the grounding of the installation. The second type of general shielding – isolation  of a high-frequency installation in a separate premises with remote control. 

Structurally shielding devices can be executed in the form of screens, canopies or partitions made of metal ropes, rods, grids. Portable screens can be produced in the form of demountable screens, tents, boards, etc. Screens are made of sheet metal with thickness not less than 0,5 mm. 

Along with stationary and portable shielding devices individual shielding  kits are used. They are intended for protection against influence of an electric field which intensity does not exceed 60 kV/m. The of individual shielding  kits include: overalls, special footwear, means for head protection as well as for face and hands 

Components of shielding kits are equipped with terminal leads connection that provide a uniform electrical network and carry out high-quality grounding (more often through footwear). 

Check of a technical condition of shielding  kits is carried out periodically. Results of checkups are registered in a special logbook. 

Field surveying works can be carried out in the vicinity of transmission aerial power lines. Electromagnetic fields of aerial power lines  of high and ultrahigh voltage are characterized by magnetic and electric strength components totaling , accordingly, up to 25 A/m and 15 A/m (sometimes at the altitude of 1,5-2,0 m from the ground). Therefore, with a view of reduction of negative influence to health, when carrying out of field works close to aerial power lines with 400 kW and above, it is necessary either to limit duration of stay in a dangerous zone or to use individual means of protection. 

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