21 JANUARY 2003
Aircraft Fuel Servicing, which requires all electrical equipment and wiring to comply with NFPA 70,
Article 515, Using Class 1 Liquids Requirements, for all applications.
9.7. Stray Currents.
9.7.1. General Information. Stray currents flow through different paths than the intended circuits, or
are any extraneous current in the earth. Sources of stray currents include electric railways, electric
power systems, electric welders, cathodic protection systems, and aircraft aeronautics electrical
equipment malfunctions. Since Air Force fixed refueling systems are in intimate contact with the
earth, stray currents sometimes take paths through the conducting parts of the system.
9.7.2. Hazards Due to Stray Currents. Stray currents cause arcs that will ignite combustible fuel-air
9.7.3. Eliminating Hazards Due to Stray Currents. The grounding and bonding method used for
reducing static hazards is important in eliminating stray current hazards. This does not eliminate
stray currents, but does ensure a continuous path is provided to conduct them into the earth without
9.7.4. Tank Car Loading and Unloading Facilities. Railroad spurs, used for loading and unloading
tank cars, should be insulated from the main line rails, so isolating them from stray currents that may
flow in the main line rails.
9.7.5. Marine Terminals. Stray currents from cathodic protection systems at marine terminals
require special attention. These systems, used for protecting piping and steel piers, cause currents to
flow in the water. The steel hull of the vessel acts as a conductor of these currents. The ship-to-shore
fuel-handling hose constitutes a conductor as it contains reinforcing wire, and will complete a low-
resistance circuit from the vessel to the shore-side piping. Arcs may then occur between the vessel
and the hose when the hose is connected, disconnected, or brought into contact with the vessel's deck.
To prevent arcing, a grounding cable is connected between the shore-side piping and the vessel before
operations begin. A switch wired in series with the cable is closed after the cable connection is made,
and before the fuel handling hose is taken aboard the vessel. Any stray current flows from the vessel
to shore by the cable, and arcing is avoided at the fuel handling hose and its connections (Figure 9.2).
9.7.6. Piping. Stray current may flow through piping systems because its electrical resistance is low
compared to the surrounding earth. When removing any section of piping, valve, meter, or other
component that will interrupt the continuity of the system, first install a bonding jumper wire. This
jumper, installed around the component to be removed, will prevent an arc when the component is
9.8. Electrical Inspection, Testing, and Identification Procedures.
9.8.1. Approved Static Grounds. Approved static grounds are provided for conducting electrostatic
charges and stray electrical currents to earth or ground potential. These electrostatic voltages may be
as high as 50,000 volts with low currents, and consequently the static ground resistance may be as
high as 10,000 ohms. The purpose of an approved static ground interconnected with aircraft and
support equipment is to place all components of the system to equal electrical potential to prevent
9.8.2. Identifying and Marking Static Grounds. All static grounds referenced in this manual will
have a resistance of less than 10,000 ohms. All existing static grounds will have a one-time test with