In November 2001, RSI (Radiofrequency Safety International) was contacted by a trucking company from the beautiful island of Oahu in Hawaii. The trucking company had been contracted by the Honolulu Water Board to dredge a canal near an industrial portion of Honolulu. The trucking company had a crane with a 110-foot boom at the site that they were using to lower steel pilings into the canal to hold back water during the dredging project.
The crane was located on a dirt pier at the western side of the canal, and the boom of the crane would extend to nearly an 80-degree angle above the canal. On the first day of construction when the crane was to be used, an odd phenomenon suddenly occurred. As the crane operator lowered the hook attached to the line of the crane and the ground crew attempted to attach a sling to the hook, the ground crew experienced an intensely uncomfortable electric shock. Due to the presence of power lines in the immediate area, they assumed that the shock might somehow be due to the static or flux emanating from the power lines.
A quick call to the local electric utility, however, put that theory to rest. As it happens, the local utility, Hawaiian Electric Co., had hired RSI to train its workers on radiofrequency exposure safety. Some of the workers had gleaned from the class knowledge about induction from communications towers. Though thoroughly familiar with the phenomenon of induced current, or induction related to 560 kilovolt, 60 hertz power lines, the phenomenon of induction of energy from nearby broadcast or communications antennas was new to them.
Though not commonplace, this situation has happened enough times to warrant an OSHA standard 29 CFR §1926.550 (a) (15) (vii), Cranes and Derricks:
"Prior to work near transmitter towers where an electrical charge can be induced in the equipment or materials being handled, the transmitter shall be de-energized or tests shall be made to determine if electrical charge is induced on the crane. The following precautions shall be taken when necessary to dissipate induced voltages:
1926.550(a)(15)(vii)(a) "The equipment shall be provided with an electrical ground directly to the upper rotating structure supporting the boom;" and
1926.550(a)(15)(vii)(b) "Ground jumper cables shall be attached to materials being handled by boom equipment when electrical charge is induced while working near energized transmitters. Crews shall be provided with nonconductive poles having large alligator clips or other similar protection to attach the ground cable to the load."
Induction is the process by which electric or magnetic forces are created in a circuit by being in proximity to an electric or magnetic field or a varying current without physical contact. Inductance or capacitance are both familiar words in electric theory, but would be a far reach for laypersons that are not electrical engineers.
The reason the trucking company personnel were experiencing this phenomenon was the close proximity of a combination AM/FM broadcast tower. The tower, which is a 400-foot-tall, self-support tower, held five AM broadcast channels and one FM broadcast channel. The total combined power of the stations was in excess of 117,000 watts of power. In addition to the high power levels, all the frequencies emanating from the tower operated between 30 and 100 megahertz. This frequency range has a strong potential to induce electrical current in nearby conductive, or metal objects. If you have ever been close to an AM broadcast tower, you will have noticed the tall wooden or brick fence with several "High Voltage" danger signs.
RF energy (also known as electromagnetic energy), as the name implies, has two distinct components: the electric field and the magnetic field. In 1996, the Federal Communications Commission (FCC) issued a rule that limited the amount of RF energy to which humans can be exposed. However, this rule deals strictly with power density, or the amount of RF energy that is emitted from an antenna. The FCC rule imposed no limits on induced current or contact current. OSHA, however, adopted the Institute of Electrical and Electronics Engineers C95.1 standard for human exposure to radiofrequency radiation which allows 100 milliamps (mA) of induced current in working environments.
A thorough sample of the amount of inductance in the energized crane line indicated 200 mA to 1 amp of induced current in the crane line, crane hook, attached sling and piling. 200 mA of current is enough to cause heart palpitations in individuals. Under certain environmental conditions such as dampness or humidity, the amperage present in the crane lines has the potential to kill.
During the current measurement, it was also discovered thatthe inductance only happened when the crane line was extended its full length to the ground. When the crane boom was lowered to about parallel with the ground, no inductance was detected. This was due to the fact that only when the crane line matched the wavelength or wavelengths of radiofrequency signals emanating from the radio tower did the crane line have the ability to become a "receive antenna."
To determine the inductance capacity or resonance potential of a metal object, or for that matter a human being, a simple math formula can be used. Wavelength can be derived by taking the speed of light in centimeters (3 x 104) divided by the frequency in megahertz. For example, we can take a known frequency in megahertz from the radio tower (let's use 1610 on the AM dial), convert it to megahertz (1.060 MHz) and run the formula:
wavelength = 30,000/1.1610 = 18633.54 cm = 7453.416 in or 621.11 ft.
Since objects can resonate at full, half and quarter wavelengths and so on, we should divide 621 by 2 to get 310.5 feet for a half wave, and then divide 310.5 by 2 to get 155.25 for a quarter wavelength. Induction occurs more frequently at quarter wavelengths. That means with the boom line fully extended, the crane line was resonating to approximately one-quarter wavelength. Depending on the length of the line extending down from the tip of the boom, the induction potential varied. However, since there were more than six different frequencies on the tower, it is likely that the induced current was influenced by all the frequencies on the tower.
Any conductive (metal) object in close proximity of high power RF fields can exhibit inductance of RF energy, thus posing a strong shock or burn potential. If you have a crane, or for that matter any object that could be vertically or horizontally oriented, you may have to do testing on it (making sure you wear high voltage lineperson's gloves) in order to verify that there is not a buildup of inductance that could pose a shock or burn hazard.
This happenstance may not be something everyone has heard about, but as noted before, it has been documented. See the following excerpt from a September 5, 1990 OSHA Hazard Information Bulletin on "Radiofrequency Radiation-caused Burns":
"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."
Since the American National Standards Institute (ANSI) indicates a grasping current of 100 mA at this date (passed in committee), OSHA recognizes induced current as a physical hazard, and is able to issue citations for exposure to the hazard without controlling methods (training, PPE, warning devices, etc.).
In essence, you never really know when you may be impacted by induced current shocks or burns when you are near a radio broadcast tower, especially one that broadcasts an AM signal where the entire tower is the antenna. It is best to be prepared to monitor the situation you are getting into with a simple amp probe for detecting induced current. Even holding a copper wire (ungrounded) while wearing a gauge 00 insulated lineperson's glove may be enough to detect induced current if you see that tell-tale spark at the end of the wire while touching it next to the object. After you determine the hazard, it is up to you (or a competent consultant) to determine how to mitigate the hazard.
About the author: Tim Denton authored this article while serving as manager of the technical department at Radiofrequency Safety International (RSI). RSI is an environmental health and safety service company specializing in helping organizations comply with federal standards for human exposure to radiofrequency energy. To contact RSI, phone (888) 830-5648 or visit their Web site at www.rsicorp.com.