During refueling fuel is transferred from the fuel truck or underground tanks (sometimes above ground) to the aircraft and this process can cause a static charge to build up between the aircraft and fueling device. This charge build-up is the same principle as walking on a carpet in wintertime and getting a shock from the doorknob when opening a door.
Electrical testing has shown that when people 'feel' this shock the voltage is at least 3500 V (3,5 kV). Amperage is very low so no real power is transferred and no harm is done, but it can be uncomfortable though. Delicate electronics or aircraft avionics can be 'killed' by voltages much lower than this and you would not even know that it happened until strange unexplained failures occur.
According to Wikipedia the definition of static electricity is as follows:
"Electrostatics (also known as Static Electricity) is the branch of physics that deals with the forces exerted by a static (i.e. unchanging) electric field upon charged objects. Electrostatics involves the buildup of charge in objects due to contact between (generally) non conductive surfaces. These charges are generally built up through the flow of electrons from one object to another. These charges then remain in the object until a force is exerted that causes the charges to balance e.g. the familiar phenomenon of a static 'shock' is caused by the neutralization of charge built up in the body from contact with non conductive surfaces."
It is important to understand how static electricity is created. As in operations like refueling this can create dangerous situations as fuel vapors and sparks do not match. Aaircraft flying through the air can also build up static charges. To discharge these electrons, special devices are attached to ailerons and tail feathers to return these electrons to the surrounding air and these are called 'static wicks'.
Without these so called 'static wicks' an aircraft could experience radio and navigation difficulties due to static discharges on the outside of the fuselage resulting in St. Elmo's fire. This is ionization of air molecules producing a faint glow.
When an airplane is refueled, fuel is pumped from one container to another and throughout this process the fuel charge level is determined by several factors: pump rate, temperature and humidity. This charge is build up occurs between the two containers. When the charge is high enough to jump between the containers, a spark will occur. If there is a combustible mixture near the spark (above/near the fuel cap) a fire or explosion could happen. And this is the main reason why a ground wire is connected from the fuel station to the airplane whenever fuel is being transferred.
There are two areas where a pilot should exercise caution when transferring fuel. The first is when draining a fuel tank. During this draining fuel is transferred and a charge can build up. Connecting a ground wire from the aircraft to the fuel container makes sure a voltage differential cannot develop and eliminate the possibility of any spark.
Just remember, when you are draining fuel, there can be enough air in circulation so that the fuel/air ratio is in the combustible range. And the odd spark can then do the trick.
The second problem area is the filling process, especially in remote areas. Many fuel stations use a ground wire when refilling an aircraft and this is good. But when the aircraft is refueled from a barrel or drum this ground wire is nowhere to be seen. In this situation a jumper cable between the barrel and aircraft can be a good safety precaution to assure that any static charge is dissipated before it can build up.
If you use a metal jerrycan with a metal or otherwise conductive funnel (Mr. Funnel), make sure the aircraft, funnel and container are touching each other during the refueling process. Any static build up is not possible then.
Metal containers dissipate the electric charge easily. Plastic containers usually not, unless they are made of conductive materials. We recommend the use of metal containers with a good jumper wire. It is so much safer.
Static Discharge Reels
At an airport fuel station you will normally find a static discharge reel. These reels are designed to provide safe and reliable grounding of containers, equipment and vehicles. They provide grounding or bonding to prevent static electricity build up in these airport sensitive areas.
You will find reels in many different variants. And using them is easy, grab the large connector and slowly walk to the aircraft while unwinding the reel with the cable. Connect the wire to a conductive place (exhaust stack will do fine) or to a dedicated fuselage ground connector but not on a painted tiedown ring.
Finally: Do not forget to remove the ground cable when refueling is finished, you wouldn't be the first...