Risk of Accidental Ignition
It is prohibited to use open flames and, in some places, mobile phones on the forecourt of a filling station because of the risk of igniting gasoline vapor. In the U.S. the fire marshal is responsible for regulations at the gas pump. Most localities ban smoking, open flames and running engines. Since the increased occurrence of static-related fires many stations have warnings about leaving the refueling point.
Cars can build up static charge by driving on dry road surfaces. However many tire compounds contain enough carbon black to provide an electrical ground which prevents charge build-up. Newer "high mileage" tires use more silica and can increase the buildup of static. A driver who does not discharge static by contacting a conductive part of the car will carry it to the insulated handle of the nozzle and the static potential will eventually be discharged when this purposely-grounded arrangement is put into contact with the metallic filler neck of the vehicle. Ordinarily, vapor concentrations in the area of this filling operation are below the lower explosive limit (LEL) of the product being dispensed, so the static discharge causes no problem. The problem with ungrounded gas cans results from a combination of vehicular static charge, the potential between the container and the vehicle, and the loose fit between the grounded nozzle and the gas can. This last condition causes a rich vapor concentration in the ullage (the unfilled volume) of the gas can, and a discharge from the can to the grounded hanging hardware (the nozzle, hose, swivels and break-a-ways) can thus occur at a most inopportune point. The Petroleum Equipment Institute has recorded incidents of static-related ignition at refueling sites since early 2000.
Although urban legends persist that a mobile phone can cause sparks, this has not been duplicated under any controlled condition. Nevertheless, mobile phone manufacturers and gas stations ask users to switch off their phones. One suggested origin of this myth is said to have been started by gas station companies because the cell phone signal would interfere with the fuel counter on some older model fuel pumps causing it to give a lower reading. In the MythBusters episode "Cell Phone Destruction", investigators concluded that explosions attributed to cell phones could be caused by static discharges from clothing instead and also observed that such incidents seem to involve women more often than men.
The U.S. National Fire Protection Association does most of the research and code writing to address the potential for explosions of gasoline vapor. The customer fueling area, up to 18 inches (46 cm) above the surface, normally does not have explosive concentrations of vapors, but may from time to time. Above this height, where most fuel filler necks are located, there is no expectation of an explosive concentration of gasoline vapor in normal operating conditions. Electrical equipment in the fueling area may be specially certified for use around gasoline vapors.
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