Static Ignition of Gasoline
You may have seen placards at the gas pump that tell you to touch your vehicle’s body during fueling. One of the test cases that may have prompted this unusual placarding at the pump was investigated by EDT Corp. Once the circumstances are explained, you might understand how some of the doubts formed as investigators had to be convinced through hard test results to believe that just getting in and out of a car could ignite vapors.
You may have thought, “What?, Getting out of the car can set me on fire?! I am not pumping gas again.” Yes, but full service costs a bundle. Read on. Once you understand a little more, going to self service won’t be a problem.
One such incident occurred on a cold, dry day while a young, not so affluent, party-goer in his polyester leisure suit filled his gas tank. When our carefree fella got out to pump the gas, he left the engine running. The filler hose was old and worn to the point that its internal electrical conductor had broken. The dude’s sneakers used PVC sole material with effective electrical insulating properties.
As mentioned, it was cold that day (around 35 degrees F), and our hero decided to get back in the car to stay warm.
Before the auto-shut-off feature turned off the flow of fuel to the vehicle, the driver got out of the vehicle without touching any of the conductive surfaces such as the metal trim or the car body. He went to the filler nozzle to top off the tank. As his hand touched the nozzle body, flame erupted from the filler neck, burning him.
Afterward, investigators found that the hose resistivity was about 500,000 ohms, (way higher than the expected 1 to 10 ohms.) Because of the high hose resistance, initial investigators assumed that static charge was formed by the flow of nonconductive fuel into the vehicle. However, mathematical and physical analysis showed that the static created by fueling bled off to safe levels where the vehicle’s electrical potential never exceeded five volts.
Tests with a similar vehicle model found no accumulation of static charge from engine operation.Other tests measured the capacitance of the vehicle as it sits on the ground. Capacitance is the ability to store charge and is measured in farads or fractions thereof. Any two conductive surfaces separated by an insulative gap can form a capacitor, like the underside of a car and the ground, separated by rubber and air.
These results raised the obvious question:
Where did the energy for a static discharge come from if not from the engine or the fueling process? The limited conductivity of the filler, although damaged, was enough to bleed away the charge from those processes.
To answer the question, EDT personnel measured electrical properties such as resistance and capacitance of the vehicle, the victim, the victim’s clothing and the pump hoses. Capacitance of the vehicle was first estimated by mathematical methods, then physically measured by “wallpapering” the asphalt with foil and driving the vehicle onto the conductive surface for the measurement. Computer recorded measurements of the capacitance agreed within 10% of the calculated values. Capacitance magnitude determines how fast the voltage can build and how much “oomph” it will have when the capacitive system discharges. In this case, the capacitive system consisted of the victim and the car body.
In order to pose a credible ignition hazard, static discharges must exceed one milli-joule of energy at more than 1500 volts of potential although some gases such as hydrogen can ignite at lower energy levels. Heavier vapors require more energy for ignition. If the energy content of the spark is too low, air in the spark zone does not heat enough to exceed the flash point of the hydrocarbon fumes. Likewise, a lower voltage will not bridge an air gap that is wide enough to cause ignition because of heat loss to the metal surfaces.
Unique conditions led to the ignition:
- the victim’s synthetic (polyester) clothing
- low humidity (less than 50%)
- synthetic car seat covers
- insulating footwear
- and an exit from the vehicle without touching it
Based on the measurements and the unique circumstances, we found that the victim created a static charge when he slid his polyester-clad body across the car seat’s nylon fabric. His insulating sneakers kept the charge on his body as he walked to the hose nozzle without touching a metal surface. His body charge sparked to the hose nozzle when he grabbed the still-flowing fuel nozzle. As the spark traveled from his fingers to the filler nozzle, it ignited the flammable gasoline vapors at the filler neck opening.
Subsequent calculations found that if the driver had hesitated for more than 15 seconds, his body charge would have dissipated. If he had touched the car’s metal skin, the static charge would have dispersed within 2 milliseconds. These observations explain the warnings placed at most gas stations.
So if you walk away from the nozzle while filling, upon your return, touch the metal car body or the metal pump frame before grabbing the nozzle, just in case you are wearing synthetic fibers.
An associated warning is: Always place your portable gas can on the ground before filling it to discharge static created by the fuel flow.
Books about static electricity:
Electrostatic Ignitions of Fire & Explosions by Thomas H. Pratt, Burgoyne, Inc., Marietta, Georgia.
Electrostatics by A.D. Moore, Joseph Crowley, Laplacian Press
Static Electricity, NFPA 77-1993, National Fire Protection Association, Quincy, MA
About the Author
Richard T. Edwards, P.E. is a consulting engineer with our Birmingham Office. has provided consultation in the analysis and selection of materials, analysis of fractures, metallurgical analysis, materials processing, and evaluation of industrial processes toward resolution of industrial failures and accidents. You may contact Richard for your forensic engineering needs at firstname.lastname@example.org or (205)838-1040.