Отчет мсэ-r bt. 2140-1 (05/2009)

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3.8 Precautions to control the direct health effects of RF radiation

Recommendation ITU-R BS.1698 contains the precautions to be taken into account. Two groups of people are considered in terms of the precautions that can reasonably be taken. The first group is employees at, or regular official visitors to, transmitting stations. Whilst this group may be at a more frequent risk, the extent to which control measures can be applied is much greater than that for the second group, being members of the general public.

3.8.1 Employee (occupational) precautionary measures Physical measures

Some form of protective barrier must be provided to restrict access to any area where either the basic biological limits are exceeded or contact with exposed RF conductors is possible. Access to such areas must only be possible with the use of a key or some form of tool. Mechanical or electrical interlocking should be provided to enclosures where access for maintenance is needed. Screening of equipment should be sufficiently effective to reduce the level of RF radiation.

Other physical measures such as warning lights or signs should also be used in addition to, but not instead of, protective barriers.

The risk of shock or burns from RF voltages induced on conducting objects, such as fences and support structures, should be minimized by efficient and properly maintained RF earthing arrangements. Particular attention should be paid to the earthing of any temporary cables or wire ropes, such as winch bonds, etc.

Where such objects need to be handled in a RF field, additional protection from shocks or burns should be provided by the wearing of heavy-duty gloves and through effective labelling. Operational procedures

RF radiation risk assessments must be carried out by suitably trained and experienced staff at regular intervals and also when any significant changes are made to a transmitting station. The initial objective must include the identification of the following:

− The areas where people may be exposed to “derived” or “investigation” levels.

The different groups of people, e.g. employees, site sharers, general public etc., who may be exposed.

− The consequences of fault conditions, such as leakage from RF flanges, antenna misalignment or operational errors.

An initial check on the RF radiation levels can be done by calculation or mathematical modelling, but some sample measurements should also be carried out for verification purposes. In most cases, however, measurements will be needed to determine RF radiation levels more accurately. The actual quantities to be measured (E field, H field, power fluxdensity, induced current) should be determined based on the specific circumstances. These include station frequencies, field region (near/far field) being measured and whether it is proposed to check compliance with basic restrictions (SAR) or only “derived/investigation” levels. These circumstances will also largely determine whether the three individual field components should be measured separately or whether an isotropic instrument should be used. RF radiation surveys should then be carried out by staff trained in the use of such instruments, following prescribed measurement procedures, and recording results in a specified format.

A nominated competent person should be made responsible for the identification and provision of suitable types within any organization or company. Such measuring instruments must always be used in accordance with manufacturers’ instructions and be subject to regular functional testing and calibration. Labels showing expiry dates must be fixed to instruments following such tests or calibration. Records of calibration should be kept, including whether adjustments and/or repairs were needed on each occasion. This information should then be used to determine the interval between calibrations.

Systems of work should be implemented that not only ensure that RF radiation limits are not exceeded, but also minimize exposure in terms of time and number of employees. Maintenance work, in areas subject to access restrictions due to high RF radiation levels, should be planned around scheduled transmission breaks or radiation pattern changes where possible. However, there should always be a balance between exposure to RF radiation and other risks, such as working on masts at night, even when floodlit. Where necessary, transmitters should be switched to reduced power or turned off to allow safe access for maintenance or repair work.

Prohibited areas on transmitting stations must be clearly defined and marked, and “permit to work” systems should be implemented. Appropriate arrangements should be put in place for any systems, antennas, combiners or areas shared by other organizations. All staff who regularly work in areas with high levels of RF radiation should be issued with some form of personal alarm or RF hazard meter.

Records must be kept of exposure above specified RF radiation levels. Companies or organizations responsible for operating transmitting stations should monitor the health of staff who regularly work in areas with high levels of RF radiation and take part in epidemiological surveys, where appropriate.

Details of general policies and procedures relating to RF radiation safety should be included in written safety instructions and given to all appropriate staff. In addition, local instructions for each transmitting station should be issued to ensure compliance with such policies and procedures.

Safety training should also include the nature and effects of RF radiation, the medical aspects and safety standards.

3.8.2 Precautionary measures in relation to the general public Physical measures

Similar considerations apply to the general public, as those detailed in § for employees.

Particular attention should be given to areas where RF radiation limits could be exceeded under fault conditions. Protective barriers should be provided in the form of perimeter fencing, suitably earthed where needed. Additional hazard warning signs will probably be necessary. Operational procedures

Risk assessments, carried out under § above, must take into account the possibility of members of the public having medical implants. A procedure for providing health hazard information to such potential visitors should be adopted with appropriate restricted access procedures. Basic RF safety instructions should be provided for regular site visitors.

The need to carry out RF radiation surveys beyond site boundaries must be considered, in particular where induced voltages in external metallic structures (cranes, bridges, buildings etc.) may cause minor burns or shock. In carrying out such surveys the possibility of the field strength increasing with distance, usually due to rising terrain, should be taken into account. Where necessary, a procedure for monitoring planning applications or other development proposals should be implemented.

An example which illustrates the text above is given in § 3.10 and Figs. 29 and 30 of this Report.

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