1.5.4 Hazards To Fuel (HERF)
HERF is a consequence when, under certain conditions, strong RF electromagnetic fields can induce voltages and currents in conductive objects such as pipelines, refueling hoses, fuel nozzles, aircraft structures, ships, and vehicles. Under certain circumstances, these induced voltages may produce electrical sparks or small electrical discharges when conductive objects come into contact or when a gap between them is sufficiently small. If such a discharge occurs in the presence of a flammable fuel-air mixture, it could provide an ignition source capable of initiating combustion.
The hazard is not caused by the fuel itself becoming electrically charged by the RF field. Rather, it results from the possibility that RF-induced electrical energy may produce a spark in an environment where flammable vapors are present. Whether ignition occurs depends on several factors, including the strength and frequency of the RF field, the geometry of nearby conductive objects, the concentration of fuel vapor, the availability of oxygen, and the minimum ignition energy of the particular fuel involved. All of these conditions must exist simultaneously for ignition to be possible.
HERF considerations are encountered most commonly during aircraft refueling, ship refueling, fuel transfer operations, tanker loading facilities, petroleum storage terminals, and other industrial processes involving volatile fuels or solvents. During these activities, fuel vapors may be present around vents, filling points, or transfer equipment, making careful control of potential ignition sources an essential part of safe operating practice. Although electrostatic discharge is generally a more common ignition source than RF-induced sparking, RF transmitters are nevertheless considered as part of the overall hazard assessment wherever explosive atmospheres may exist.
To minimize this risk, established operating procedures are followed whenever fuel handling takes place near RF transmitters. Depending on the circumstances, these procedures may require transmitting equipment to be switched off, operated at reduced power, or kept beyond specified separation distances while refueling is in progress. Equipment is also bonded and earthed where appropriate to minimize the accumulation of electrical charge, and operating procedures are carefully coordinated to ensure that potential ignition sources are controlled.
Although HERF incidents are uncommon, the potentially serious consequences of fuel ignition mean that conservative engineering and operational precautions continue to be applied throughout the aviation, maritime, defense, and petroleum industries. HERF provides another example of the broader principle that RF radiation safety extends beyond protecting people alone. It also involves understanding how electromagnetic fields interact with equipment, conductive structures, and the surrounding environment so that hazards can be identified and effectively controlled before they present a risk.