Background

In 1995, OFTA engaged the CityU Consultants Ltd. to take up a study on health protection of workers and members of public against radiofrequency electromagnetic radiation at radio sites. The findings of the study were presented to the RSAC meeting on 6 June 1996. A Working Group was set up to study the recommendations and propose follow-up actions.

2. In the consultancy report, the consultant has recommended a Code of Practice to be followed by radio site designers, radio site operators and occupational personnel. The Working Group has discussed in detail the proposed Code of Practice. Recommendations of the Working Group are given in Appendix 1. The Working Group has also prepared a revised draft of the Code of Practice which is attached at Appendix 2.

Advice Sought

3. Members are invited to give comments on the revised Code of Practice.

Office of the Telecommunications Authority
December 1996

								Appendix 1

Clause 4.3.1

1. New World PCS commented that the data base should not contain confidential information of operators. Members were of the view that the data base should only contain data which were necessary for radio hazards calculation. The Working Group recommends that the data base should contain location of transmitters, operating frequency band, power output etc. Detailed information such as name of operators and exact frequency channels should not be contained.

Clause 4.4.2

2. HKT CSL commented that it was impractical to inform everyone who was allowed to access to the areas where potential of health hazards might exist as operators did not have any control nor advanced knowledge on who might gain access to those areas. Members were of the view that the warning signs alone could already serve the purpose. The Working Group considers that there are difficulties in adopting the practice.

Clause 4.4.4

3. Pacific Link and HKT CSL commented that warning signs alone could serve the purpose and lighting was not required. HKRPA explained that lighting requirement was intended to apply to high power radio sites. HKRPA further explained that radio sites with power density exceeding 10 times of the IRPA occupational exposure limit are considered to be high power radio sites. Members doubted the use of lights in this situation as rooftops were dangerous places and should not be accessed by the general public or working personnel during night time. The Working Group recommends that the requirement of lights should only be applied to high power radio sites where power density in the working area exceeds 10 times of the IRPA occupational exposure limit.

Clause 4.4.5

4. HKT CSL suggested that the warning sign should include Chinese and English texts in addition to a warning symbol. HKRPA considered that putting up the warning symbol might cause unnecessary public panic. Smartone proposed an alternative symbol which is not used internationally. Members felt that the use of warning signs would be beneficial to the radio operators and the general public and it was preferred to use a sign which was used internationally. The Working Group recommends that the warning symbol proposed by the consultant which follows the British Standard BS5378 should be used. Brief text "RADIOFREQUENCY ENERGY CONTROLLED AREA" and "" should be added below the warning symbol. The recommended warning sign is as follows:

Clauses 4.4.7 and 4.4.8

5. Smartone and HKRPA commented that physically restricting personnel from gaining access to transmitting equipment would not be practicable. Smartone commented that it was difficult and not practicable to set up protection screen on rooftops. Such requirement would make it more difficult for the operator to acquire radio sites in the future. HKRPA was concerned that constructing protection screen, fence etc. might require approval from the Fire Services Department and the Housing Authority and it would delay the progress of installation of transmitters. Pacific Link questioned about the use of interlocked mechanism. The Working Group recommends that the construction of the protection feature and interlocked mechanism should only be required wherever it is practicable.

Clauses 4.4.12 and 4.4.13

6. Members shared the view that it would not be necessary to make NIR measurements at all sites. This would create heavy workload to all parties concerned. It would be sufficient to scrutinize carefully those sites which have potential radiation hazards. The Working Group recommends to put the second and last sentences of clause 4.4.12 to the end of clause 4.3.3 and delete clause 4.4.13.

Clause 4.4.14

7. Pacific Link questioned whether personnel should be allowed to work in areas which exceed the safety level. HKT CSL asked the requirements to obtain the work permit and by which authority it would be issued. HKRPA suggested the permit should be issued by OFTA and on a "per operator" basis. However, most members generally doubted the effectiveness of the use of permit. The permit requirement would only delay the progress of installation works and create workload to every party concerned. OFTA will consult other appropriate government departments on this issue. Meanwhile, the Working Group recommends not to include work permit requirement at this stage but radio operators should be responsible for training their occupational personnel so that they will be fully aware of the possible radiation hazards when working at radio sites.

Clause 5.1

8. Pacific Link questioned the practicality of physically restricting personnel from gaining access to transmitting equipment. Similar to clause 4.4.7, the Working Group recommends that barriers should only be constructed wherever it is practicable.

							Appendix 2

Code of Practice

for the Protection of Workers and Members

of Public Against Non-Ionising Radiation

Hazards from Radio Transmitting Equipment

1. Introduction

1.1 This Code of Practices has been prepared for those who design or operate radio systems and for those who work at radio sites. The Code gives guidance for the protection of workers and the general public from exposure to radiofrequency electromagnetic fields so as to provide a safe and healthy working or living environment under all normal conditions. Radio system designers, radio site operators and occupational personnel should observe the Code in the design, construction, installation, maintenance and operation of radio systems.

1.2 This Code of Practice cannot be so detailed as to deal adequately with every set of circumstances that may arise, and it may well be necessary for certain system designers and site operators to obtain more specialist advice.

1.3 Request for clarifications and proposals for updating this Code of Practice should be made to the Office of the Telecommunications Authority.

2. General Principles

2.1 The radio site operator should be responsible for ensuring that the radio systems operated by it complies with this Code of Practice, taking into consideration the combined effects of its own present/future transmissions and other present/future transmissions from the same site or sites in the vicinity.

2.2 The operator of a new radio system or a modified radio system should be responsible in ensuring that paragraph 2.1 is complied with, and should take whatever measure that is necessary to achieve such compliance prior to operation.

2.3 A radio system should be so designed, constructed, installed, maintained and operated to meet the health protection standards specified in this Code of Practice.

2.4 Compliance with the health protection standards specified in this Code of Practice should be achieved, as far as possible, through restrictions in the technical parameters of the transmitting equipment, and appropriate sitting of the antennas and ancillary equipment.

2.5 In case safety cannot be sufficiently guaranteed by the measures stated in paragraph 2.4, appropriate operational protection measures should be applied in order to comply with the health protection standards.

3. Health Protection Standard

3.1 The health protection standards specified in this Code of Practice are those laid down in the "Guidelines on Limits of Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 100KHz to 300GHz" published by the International Radiation Protection Association (IRPA) in 1988 (the 1988 IRPA Guidelines). The radio system operator should ensure that the electric and magnetic fields as well as the power density radiated from the antennas at the site operated by it do not cause exposure to occupational personnel and members of the general public in excess of the limits specified in Annex 1 which are extracted from the 1988 IRPA Guidelines. References should be made to the full Guidelines for the protection limits for pulsed fields, localised exposure, protection against RF shocks and burns and exclusion for low power devices.

3.2 In the case of multiple simultaneous exposure, the combined effect of such exposure should be assessed in accordance with the 1988 IRPA Guidelines.

4. Basic Guidelines

4.1 Safety Management

Operators are required to properly train their occupational personnel and keep them fully aware of the possible hazards in the working areas. A written procedure for safety management should be documented and may include the following points,

4.1.1 Safety policy

4.1.2 Training

4.1.3 Identification of potential hazards

4.1.4 Measuring equipment calibration

4.1.5 Non-ionising radiation (NIR) incident investigation

4.1.6 Record keeping

4.2 Equipment Performance Standards

4.2.1 The equipment should deliver the minimum radiofrequency output power necessary to meet operational requirements.

4.2.2 The antennas should be so designed and sited as to avoid, as far as possible, unnecessary radiation of radiofrequency energy.

4.2.3 The secondary radiation (scattered or leakage radiation) should be reduced to a negligible value or even prevented in the proximity of the equipment.

4.2.4 The equipment should be type-approved by OFTA or conform to specifications accepted by OFTA.

4.2.5 The manufacturer's guidance on the conditions for use and necessary precautions to be taken should be observed for all equipment not having the intrinsic safety.

4.3 Design of radio sites

4.3.1 For new radio sites, radio system designers will be required to submit to OFTA the relevant details necessary to set up a data base to assess the potential of NIR hazards at a radio site. For existing radio sites, operators may be required to submit to OFTA the relevant details necessary to set up a data base to assess the potential of NIR hazards at a radio site. The data base can only be accessed by operators on a need-to-know basis. Confidential information of the operators will not be contained in the data base. The required details of the data base will be,

I Site plan.

II Present and future allowable number of services and transmitters at the radio site.

III Antenna location, height and direction.

IV Worst case effective radiated power.

V Antenna model and radiation pattern.

VI Position of antenna and antenna mast for maintenance.

Current paging antenna masts are lowered for the maintenance of antennas, therefore it is important that its position for maintenance does not fall directly in front of another transmitting antenna.

4.3.2 Radio system designers should select the appropriate site and mounting for the antennas of transmitting equipment to reduce NIR levels to a negligible value at easily accessible areas for the general public, and minimised at easily accessible areas for workers. In any case, the power density at public areas should not exceed the IRPA exposure limit for the general public.

4.3.3 NIR levels should as far as possible be calculated first by the radio system designer, taking into account the combined effects of its own present/future transmissions and other present/future transmissions from the same sites or sites in the vicinity. This can be done by consultation with the site manager or the database of OFTA. Verification should be performed with a NIR site survey and, non-hazardous/hazardous areas established and documented. The measurement should be carried out based on the recommendations stipulated in the IEEE standard, C95.3-1991[2]. If the standard cannot be followed, then the method stipulated in the IEEE standard, 291-1991 [3] should be followed.

4.3.4 The conditions given in paragraph 4.3.2 may not be practical in many radio sites, under such conditions the NIR levels for the general public and workers should not exceed the exposure limits given in Annex 1.

4.4 Operational protection measures

4.4.1 Avoid any unnecessary exposure to NIR.

4.4.2 The radio site operator should properly inform everyone, who is allowed access to the areas in the vicinity of antennas, where potential of health hazards might exist, of all necessary precautions.

4.4.3 4.4.2 Areas in which the electromagnetic fields exceed the exposure limits given in Annex 1 should, as far as practicable, be made inaccessible to the general public.

4.4.4 4.4.3 If the measures stated in paragraph 4.4.3 4.4.2 are impractical, the radio site operator should delineate and identify these areas by warning symbols. For high power radio sites where the power density in the working area exceeds 10 times of the IRPA occupational exposure limit, lights are required at the radio sites to alert the occupational personnel for the possible hazard.

4.4.5 4.4.4 The warning symbol for identifying potential areas of NIR hazards should follow the British Standard BS5378[1]. The sign is based around black wavefronts radiating from a stylised point source antenna on a yellow background and is enclosed in a black triangle as shown in Figure 1. The actual dimensions used depends on the prominence required, a typical size for the triangle would be approximately 150mm or 200mm across the base. The warning sign should be made of reliable material. Fading should be avoided in outdoor environment. The warning sign should not carry sharp edges that may cause any potential danger to the general public. It should be securely mounted on appropriate positions such as ladders leading to the top of water tank where the antennas are mounted.

Figure 1 Non-ionising radiation warning symbol.

4.4.5 If the installation of new radio system or a modified radio system results in the power density at the site exceeds the IRPA exposure limit for general public, the operator of the new system or modified radio system will be responsible to put up the warning signs and carry out routine maintenance to ensure that the signs are in their proper positions and in good conditions. For existing sites where the power density at the site already exceeds the IRPA exposure limit for the general public, the major user of the radio site should be responsible to put up the warning signs and carry out routine maintenance to ensure that the signs are in their proper position and in good conditions.

4.4.6 Waveguides that carry power levels high enough to cause radiated power to reach dangerous levels if cut, should be identified. Prominent warning notices should be displayed in rooms where such waveguides are present to stress the danger of looking into open ended waveguides. All flanged joints in high power waveguides should be checked periodically for RF leakage.

4.4.7 Personnel must be physically restricted from gaining access to transmitting equipment or antenna structures when dangerous fields may exist. This should be enforced by the radio site operator with the use of protective features such as wire screens, automatic safety devices, etc.

4.4.7 Wherever practicable, the radio site operator should use protective features such as wire screens, automatic safety devices, etc. to physically restrict personnel from gaining access to transmitting equipment or antenna structures when dangerous fields may exist.

4.4.8 Wherever practicable, access to elevated platforms, maintenance towers, etc. in hazardous areas if the power density exceeds 10 times of the IRPA occupational exposure limit should be controlled by interlocked mechanism.

4.4.9 For steerable antennas, hazardous NIR may be avoided by using sector blanking or automatic restriction of power whenever the antenna beam is below a pre-determined angle.

4.4.10 The radio site operator should develop and implement safety instructions for the protection against excessive exposure to NIR at the various working places.

4.4.11 As a last resort, occupational personnel should use individual protection equipment such as protective clothing, glasses etc. when working under the exposure in excess of the limits specified in the health protection standards. The protective equipment should be regularly inspected to ensure its effectiveness. A personnel wearing a damaged piece of protective clothing which allows the coupling of electromagnetic radiation, may be subjected to an even greater hazard than without wearing the protective clothing.

4.4.12 The radio site operator should measure the level of NIR at areas in the vicinity of the site accessible to occupational personnel and the general public though the use of calibrated equipment. The measurement method should be carried out according to the recommendations stipulated in the IEEE standard, C95.3-1991[2]. If this standard cannot be followed, then the method stipulated in the IEEE standard, 291-1991 [3] should be followed.

4.4.13 The radio site operator should carry out the measurements referred to in paragraph 4.4.12 after commissioning of the radio system, completion of material changes to the radio system and at regular intervals of not more than one year. The results of the measurements should be documented.

4.4.14 Personnel likely to work in areas which may exceed the safety levels of NIR for the general population should either have a permit to work, or have a supervisor present that has a permit to work, to take responsibility. The issue of a permit to work should require that the worker be healthy and does not rely on devices such as heart pace-makers, which could be affected by NIR. A permit to work should require that the worker has an understanding of NIR and first aid, which may require suitable training. Where appropriate, medical surveillance adapted to the nature and the importance of the risk to which workers are exposed should be conducted.

5. Generic Guidelines

5.1 General

NIR levels can be reduced at any radio site by placing the antennas higher or locating them further away, but in some cases this may not be practical. Under these circumstances, access to areas where NIR hazards exist should be restricted by physical means and warning signs placed at all access points. Wherever practicable, protective features such as locked doors, locked ladders or barriers can be used to restrict personnel from gaining access to transmitting equipment or antenna structures when dangerous fields may exist. Restriction by physical means could be in the form of locked doors, locked ladders or barriers. If access is required when NIR hazard exists, the exposure time should be limited according to 1988 IRPA Guidelines. Other measures could include lowering the transmitter power if a complete shutdown cannot be avoided. Metal supporting masts or other metallic structure must be grounded if dangerous RF-voltages due to induction are anticipated. Radio sites should be kept clear of flammable atmospheres according to the British Standard BS6656:1991 [4].

5.2 Paging and Private Mobile.

Paging/private mobile transmitters are normally mounted on roof-tops using antennas that are mainly omni-directional. The antennas should be mounted high enough to eliminate NIR hazard at easily accessible areas in an uncontrolled environment. The height needed to avoid NIR hazard in an uncontrolled environment can be calculated using the model found in Annex 2, section 1. An absolute minimum height of 2.5m from the bottom of the antenna is recommended to avoid easy accessibility which may cause RF shocks or burns. Because the antennas used are mainly omni-directional, the position of the antennas should ideally be located at the centre of the roof top, this may not be critical and therefore should not be binding. If directional antennas are used, it would be preferable to mount the antenna near the edge of the building with the main beam pointing away from the roof top. Consideration should also be given to NIR radiated into adjacent buildings or adjacent roof-tops, again possible hazards can be calculated using equation (1) which can be found in Annex 2, section 1. If a minimum height of 2.5m is not practical then a NIR survey will need to be performed using spatial as well as time averaging to assess the hazard. Due to the possibility of future installation of additional transmitters, OFTA should be consulted on the expected maximum number of transmitters that are likely to be installed.

5.3 Public Mobile Cellular Radio

As far as practicable, the transmitting antennas should be mounted either on towers or poles of suitable height at the edge of the building and facing away from the building. To ensure that there is no possibility of NIR hazard, the transmitting antennas should be mounted at least 2.5m high in easily accessible areas. If the transmitting antennas have to be mounted on walls or water tanks where the general public can walk underneath, it should be mounted at a suitable height. The suitable height to avoid NIR hazards can be calculated using equations (1) and (5) which can be found in Annex 2, section 1 and 2. If a minimum height of 2.5m is not practical then a NIR survey will need to be performed using spatial as well as time averaging to assess the hazard.

5.4 AM Broadcasting

AM broadcasting radio sites are generally well restricted, as an additional precaution contractors for AM broadcasting transmitters should take into consideration the exposure limits in the 1988 IRPA Guidelines. Consideration should also be given for access to the towers during maintenance, even with the main transmitter switched off. The back up transmitters are usually located close to the main transmitter and can radiate significantly into the main tower. The same is true for the main transmitter radiating into the tower of the back up transmitter. The radio site should be kept clear of debris, especially metallic poles which is a common source of RF burns. Due to the high power levels involved the NIR hazardous area should be determined and restricted for either the occupational population and/or the general population. Calculation can be performed in the near field using equations (6-9) for AM transmitters using monopole antennas and can be found in Annex 2, section 3. A guide to expected field strengths can be found in reference [5].

5.5 FM/TV Broadcasting

FM/TV broadcasting radio sites are generally well restricted with antennas mounted quite high on towers, as an additional precaution, contractors for FM broadcasting transmitters should take into consideration the exposure limits in the 1988 IRPA Guidelines. Consideration should be given for NIR hazard when using the back up transmitter antennas, which are normally positioned lower than the main transmitter antennas thus producing a larger power density at ground level. Consideration should also be given for personnel which may need to climb up the tower for maintenance with the transmitters active. NIR radiation levels can be reduced in the tower by the use of shielding materials and it is recommended that reference [6] be consulted on any implementation. Far field NIR levels can be calculated using equation (1) and near field calculation can be calculated using equation (5) for antenna arrays which can be found in Annex 2, section 1. NIR occurring in masts have been found to be the highest at the top [7] and by the antenna spacing, with a coupling factor in the order of 10dB. This coupling factor can be used as a guide to the possibility of NIR hazard for personnel intending to climb up the tower to gain access for maintenance.

5.6 MMDS

MMDS operates at microwave frequencies, using either arrays or reflector antennas. The NIR levels can be calculated according to equation (1) in the far field and equation (10) for the near field which can be found in Annex 2, section 1 and 4. Further information can be found in reference [2] for near field power density in the axial direction. It is recommended that antennas are located on masts in order to restrict access by the general public. If the antennas are mounted on roof-tops where the use of towers are impractical, NIR radiation hazards can be minimised by positioning the antenna away from the centre of the roof-top with the main beam pointing away from the roof-top and any access points. Access to the antenna should be restricted physically for the general population and for the occupational population when power is present. Reference should be made to section 5.7 on additional safety practices.

5.7 Satellite Earth Station

Satellite earth stations can transmit power in the order of kW and can be a cause for concern regarding NIR hazards. It is recommended that reference [8, 9] be consulted on the possible levels of NIR levels near the reflector antennas. The safety precautions can be summarised as,

I Access to the reflection surface must be avoided.

II Access to the tubular beam must be avoided.

III Exposure to NIR hazards must be controlled.

IV Restrictions are necessary as regards masking of the site by obstacles.

V Restrictions are necessary as regards the height of the station buildings.

VI Consideration must be given to potential NIR hazards at collimation and other towers.

5.8 Point to Point Fixed Links

The guidelines for point to point have actually been covered in section 5.6 for microwave frequencies and section 5.2 for the lower frequencies.

5.9 RADAR

Although the average power for radar is usually small in comparison with the peak power, there is a danger of considering only the average power whereas letting the peak power increase without limit. The peak power density can be calculated according to equation (1) which can be found in Annex 2, and the average power density can be calculated according to Annex 3. The permissible level can then be determined according to the IRPA standard taking note of the conditions for pulsed fields. A major concern regarding radar is the possible large increase in NIR due to the failure of the rotating scanning mechanism, this danger can be minimised by the use of an interlock mechanism.

References

[1] "Safety Signs and Colours", British Standard BS5378, Part 1 1980, Part 3 1982.

[2] "IEEE Recommended Practice for the Measurement of Potentially Hazardous Electromagnetic Fields-RF and Microwave." IEEE Std C95.3-1991, Published by the The Institute of Electrical and Electronics Engineers, Inc., 345 East 47th Street, New York, NY10017-2394, USA.

[3] "IEEE Standard Methods for Measuring Electromagnetic Field Strength of Sinusoidal Continuous Waves, 30Hz to 30GHz." IEEE Std 291-1991, Published by the The Institute of Electrical and Electronics Engineers, Inc., 345 East 47th Street, New York, NY10017-2394, USA.

[4] "Guide to Prevention of inadvertent ignition of flammable atmospheres by radio-frequency radiation", British Standard, BS 6656 : 1991.

[5] "Radiation Levels and Protection Near Broadcasting Antennas", by G. E. Hatfield, International Broadcasting Convention 1988, pp125-128.

[6] "Antenna Shielding for the Protection of Engineering Personnel", by G. E. Hatfield and A. J. McCarthy, International Broadcasting Convention 1992, pp98-102.

[7] "Health and Safety with RF Broadcast Stations", by P. Shelswell, R. D. C. Thoday and S. Wakeling, International Broadcasting Convention 1988, pp120-124.

[8] "Safety Aspects of Radio-Frequency Radiation from Space Research Earth Stations", Recommendations and Reports of the CCIR, 1986, Volume II, Report 543-1, pp54-57.

[9] "Safety Aspects of Radio-Frequency Radiation from Fixed Earth-Station Antenna Systems", Recommendations and Reports of the CCIR, 1986, Volume IV, Part 1, Annex IV, pp176-177.

1. Annex 1- Annex 4
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