Crane and Lift Operators – Beware RF Shock and Burn!
Recent information from OSHA has brought to our attention a potentially serious hazard involving burns on longshoremen caused by radiofrequency radiation. Two hazards were noted at a West Coast, USA location. The burns were apparently caused by spark discharges from crane cables. Also present was an induced-current grasping hazard.
LBA engineers have responded to a number of situations involving cranes where RF energy has caused worker shock and burns, and has caused damage to crane lifting cables. One instance is discussed in the blog https://lbagroup.com/blog/hazards-in-the-workplace-rf-shock-and-burn/. Note that RF damage to cables is specifically cited in OSHA crane operator standards!
The incident here was researched by OSHA, and their description and conclusions are of interest to all in the lifting and rigging workplace environment.
The longshoremen were working on a pier that is located in close proximity to several AM radio station transmitting towers. The radiofrequency radiation emanating from the transmitters induces electric currents in the longshoring operation cranes’ cables due to the cables acting as antenna receptors to the radiation.
The OSHA Health Response Team measured currents as high as 200 milliamps (mA). The American National Standards Institute C95 committee is considering a limit for this type of grasping current hazard of 100 mA. Measurements also indicated that electric field strengths in the general vicinity of the ship were on the order of 10 volts per meter. However, this is well within the ANSI C95.1 – 1982, Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 300 kHz to 100 GHz, limit of 632 volts per meter for AM radiofrequencies. Because of this induced current and an open circuit voltage from cable end to ground measured at approximately 300 volts by the Health Response Team, spark discharges occur just before and after grasping the cable. These discharges have resulted in burns.
Controls involve either isolating the crane hook from the crane cable and block assembly by an insulator or grounding the crane cable with a ground chain or wire. Either control has drawbacks. Insulators or insulated blocks are expensive and can affect the lift capability of a crane. Moreover, an isolated hook cannot prevent inadvertent employee contact with the crane cable itself. A grounding chain would have to be located manually with each individual lift thereby inviting inadvertent contact. Failing these controls, personal protective equipment in the manner of rubber-insert leather gloves, long sleeve shirts, safety helmets and safety glasses should be employed.
The LBA safety team is available to assist those planning crane or lift operations in the vicinity of AM broadcast stations. Contact Bryan Dixon at 252-757-0279 or firstname.lastname@example.org. Convenient, economical on-line RF Awareness certificated training is available at http://devlbagroup.com/associates/rftraining.php.