Whenever fuel is poured, pumped or moved from one container to another, a static charge is generated. The same principle is in effect when you walk across a carpet in the winter and get a shock from a doorknob. The charge level and the distance that can be jumped or arced depends on several factors—pump rate, temperature, humidity and containers.
Static electricity is the reason why a ground wire is always connected to commercial airliners and transport trucks whenever fuel is being transferred. When you transfer fuel into your car or light aircraft, the hose has a built-in ground wire that acts as an electrical path to dissipate any static charge. As an added precaution, there is usually an excessively rich air/fuel ratio in the fill pipe which will not burn. There are two primary areas where a pilot should exercise caution when transferring fuel.
First is draining an aircraft tank. For example, if you’re draining a wing tank, you should always connect a jumper cable from the plane to the fuel container. This will dissipate the charge and eliminate the chance of a spark jumping from plane to container, causing a fire. Remember, when you’re draining fuel, there can be enough air circulation so that the air/fuel ratio is in the burnable range.
The second area of concern is the filling process. Many FBOs use a ground wire when filling an aircraft. But in some cases, fuel is transferred from a drum or can into an aircraft. Here, a jumper wire is a good safety precaution to ensure that the charge is dissipated. If you use a metal funnel with metal cans, make sure that the can, funnel and plane are always touching during transfer.
With metal containers, the electrical charge is dissipated to the conductive container where it can be discharged by a ground wire or contact. In plastic containers, there is no good electrically-conductive path to dissipate the charge. Although some people put metal strips into the plastic container, I would recommend the use of metal containers with a good jumper wire. It's the safer way to go.