Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3 KHZ to 300 GHZ - Safety Code 6

3. Siting and Installation

3.1 Basic Principles

In order to institute protective measures in areas where RF devices are used, it is necessary to take into account the time workers may spend in these areas. When surveying around RF devices:

1. field levels shall be known in controlled areas where restricted occupancy is allowed. These areas shall be designated accordingly, and the maximum worker occupancy time shall be posted where applicable, and
2. exposure as well as induced and contact currents in uncontrolled areas shall not exceed the limits specified in Section 2.2.

3.2 General Recommendations

1. Warning signs, specified in Section 7.1, or suitable substitutes, indicating the presence of RF fields, shall be posted according to recommendations of Section 7.2 or 7.3.
2. The areas surrounding unmanned, high-power sources of RF radiation shall be fenced off to prevent unauthorized access to places where an overexposure could occur. If a metallic fence is used, the contact current limits specified in Tables 3 and 7 shall be met.
3. The siting of an RF device shall take into account the practical possibility of multiple source exposures from fields and leakage from other devices in the vicinity.
4. Metallic objects that are not necessary shall not be present near any radiating RF device, as they may cause high intensity fields in some locations.

3.3 RF and Microwave Heating Devices

RF and microwave heating devices are used to convert RF energy to heat. They may consist either of electrodes, coil(s) or antenna(s) which produce fields in the object to be heated as well as stray fields in the surrounding region. These devices have applications in the areas of scientific investigation, medical care and industrial processes. Examples of these devices are RF dielectric heaters and sealers for processing dielectric materials and short-wave and microwave diathermy equipment for therapeutic application. When such devices are used, the following recommendations apply:

1. Where radiated energy is directed into occupied areas, appropriate precautions shall be followed to ensure that exposure of people does not exceed the maximum levels specified in Section 2. Where the maximum level at the operator position exceeds those specified in Section 2, steps shall be taken, e.g., through the use of shielding, to bring unintended exposure levels to those specified in Section 2. Information about the shielding of RF dielectric heaters and sealers is given elsewhere ⁽¹⁰⁾.
2. Where the fields produced by an RF and microwave heating device exceed the limits specified in Section 2.2 in locations accessible to persons not classified as RF and microwave exposed workers, there shall be an indicator that provides a visible warning when the RF power is on. Precautions shall be taken to ensure that when the RF power is on, no such persons are present in the locations where the limits are exceeded. In evaluating operator exposures, it is advisable to measure induced body current in addition to electric and magnetic field measurements⁽¹⁰⁾.

3.4 Medical Devices and Electromagnetic Interference

It is advisable to use medical RF devices only in a room specifically designed, selected or modified to accommodate the patient and the device. Electromagnetic interference from devices such as shortwave diathermy and electrosurgical units may cause malfunctions, with potentially adverse consequences, to various medical devices, such as electrocardiographs, electromyographs, electroencephalographs, cardiac pacemakers, etc. Other sources of interference are radio, television and radar transmissions as well as mobile communications devices (e.g., cellular telephones, walkie-talkies and vehiclemounted transceivers)⁽¹¹⁾. Electromagnetic interference can generally be eliminated by means of shielding, filtering or grounding of the spurious signals, or relocating the devices being affected. Pacemaker design has improved to the extent that its susceptibility to electromagnetic interference has been largely eliminated. Other medical devices are, or may be, designed to minimize the effects of electromagnetic interference. Information concerning RF electromagnetic interference with medical devices has been reported⁽¹²⁾.

3.5 Microwave Ovens

1. Adjusting applied voltages, replacing the microwave power generating component, dismantling the components of the oven and refitting waveguides, shall be undertaken by persons specially trained for such assignments. The services of a qualified repair person should be sought when any malfunction is suspected.
2. Special care is required to ensure that no damage occurs to the part of the oven making contact with the door or door seals.
3. The power mains shall be disconnected before removing the oven outer shell.
4. Before reaching into the rear compartment of the oven, the high voltage capacitors must be discharged in a safe manner.
5. The door interlocks shall not be defeated⁽¹³⁾.

3.6 Radar and Communications Systems

1. Wherever possible, the antenna beam shall be directed away from occupied areas. However, if the beam does irradiate occupied areas, the applicable maximum exposure limits given in Section 2 shall be observed. If needed, antenna sweep restrictions or RF power reduction may be employed to prevent the exposure limits from being exceeded in occupied locations.
2. For any installation that is to be operated, all signs and barriers demarcating areas of restricted or denied occupancy and all interlock systems shall be permanently installed to prevent accidental access to the zone of denied occupancy.

3.7 Electroexplosive Devices

Care shall be taken to ensure that electroexplosive devices are not placed in RF fields of a level sufficient to cause serious risks. Firing circuitry along with the wires of electric blasting caps may, under certain circumstances, pick up sufficient energy from RF fields to cause caps to explode ⁽¹⁴⁾. The susceptibility of the blasting caps to RF fields depends on the frequency, polarization and the strength of the field, and various factors in the design of the detonator - including to what extent it is electrically screened from radio interference. The level of field intensity that may prove hazardous depends on its frequency: the lower the frequency, the more susceptible are the detonators.