Hazards in the Workplace – RF Shock and Burn
Most engineers and operators in the wireless industry think of radio frequency (RF) safety as an exposure issue and often forget RF shock and burn that can be a concern for tower climbers and construction workers. Occupational safety and health are of prime importance in the workplace especially towers and structures that may be energized by RF energy. The large radio frequency energy fields developed by high power RF equipment pose RF safety concerns, as well as OSHA, FCC, and other agency requirements for worker protection. The ANSI/IEEE standard for RF radiation provides safety guidelines for human exposure but currently does not address shock and burn. The Canadian Safety Code 6 Standard stipulates a hand grip contact current rating of 100 mA. In general, the scientific literature points out many shipboard structures can be excited sufficiently by their own radio and microwave transmissions to present RF burn hazards to personnel. The literature also describes cranes can be potent sources of RF burn hazard. Moreover, RF burn hazards are present on structures when irradiated at field strengths much lower than the maximum permissible for human exposure.
An RF burn hazard can exist on various transmitting antennas and simple precautions can be taken to avoid it. However, such a hazard can also arise on metallic objects excited by radiation from nearby transmitting antennas especially AM broadcast facilities. Metallic objects include communication towers, water tanks, cranes and bridges. For a case in point consider a tall steel crane located on a floating barge in a coastal river within a ½ mile of a 10 kW AM transmitting antenna. Lawrence Behr Associates, Inc. (LBA) was recently contacted by a construction firm to assess radio frequency (RF) safety hazards on cranes and electromagnetic interference (EMI) to on board electronic control system at a high rise bridge construction site.
Personnel on the barge reported spark discharge from crane cables while handling equipment connected to the cable hook. Also present was an induced current grasping hazard. After consultation with LBA engineers, client personnel checked and cleared the site of possible problems with onboard tools and equipment including welders, shore power, generators but the problem persisted. An engineer was mobilized to the construction site where he was able to precisely locate the relationship to AM stations, and coordinate office computations of impact on the contractor’s equipment. The LBA engineer discovered the induced currents in the crane cable also extended to the computer system communication/data lines used to electronically control the hoist cables.
Any ungrounded, conductive (usually metal) object that is in the field of a strong RF source like a nearby AM radio station can be illuminated by the RF field and re-radiate the energy. Because a crane can be an effective antenna for AM broadcast signals, the hoist cable, gantry and base create a resonant loop which possesses induced currents. LBA measured the maximum current in the crane cable which was significant for RF shock and burn. However, the radiated power from the cable was insignificant for RF exposure.
LBA was able to devise rigging and work protocols that eliminated all RF safety hazards from the project, which was able then to resume. Recommendations were also provided on mitigating the EMI issue with the computer control system. The next time you work on tall structures like cranes, bridges and towers, don’t overlook the potential for RF shock and burn. LBA offers RF Safety Training as well as an on-line search tool for potential construction site RF hazards: http://devlbagroup.com/associates/tools.php?tool=rfhazard