21 JANUARY 2003
ELECTRICAL GROUNDING AND BONDING
9.1. General Information. This chapter provides general information related to the two hazardous
conditions that must be considered in handling and dispensing petroleum products: static electricity and
stray electrical current. See NFPA 77, Recommended Practice on Static Electricity, for additional
guidance on static electricity hazards.
9.2. Static Charge Generation in Refueling Systems. Low-conductivity liquids, such
as jet fuel,
become electrostatically charged while flowing through fuel systems. This can produce enough
electrical energy to cause ignition, fire, or explosion of the fuel-air mixtures above the liquid fuel
9.2.1. The mechanism of electrostatic charge generation is very complex, with many variables that
can increase or decrease the amount of electrical energy in fuel itself.
9.2.2. Certain equipment and conditions in fuel systems produce high static charges, necessitating
designs to retard this hazard. F/Ss are prolific static generators because of the filter media and must
be grounded directly to a grounding rod.
9.2.3. Fuel systems are grounded to earth potential, and each piece of equipment is electrically
interconnected by bonding through mechanical connections. Where no continuity exists, jumper
wires are installed across insulated sections. Where flange sections are broken, bonding is attained by
installing jumper wires. Isolation flanges provided for cathodic protection purposes require special
devices to provide continuity. Aboveground piping is tied to ground rods and underground sections
are grounded by being in contact with the earth. During refueling, static electricity is generated
through piping and especially the F/S. Although some of it is dissipated through contact with piping,
a residual charge remains that places the destination tank (usually the aircraft's tank) at a different
potential than the system or HSV. It is essential that the tank be bonded to the system to allow the
static charge to relax.
9.2.4. Many other factors contribute to electrostatic charge generation in aircraft fuels. More detailed
information may be found in technical libraries and T.O. 00-25-172.
18.104.22.168. During filling operations, aircraft refuelers and commercial transports have developed
measured electrostatic charges exceeding 50,000 volts. One reason for these high-voltage build-
ups is the insulating effect of the rubber tires from ground potential if the vehicle is not properly
bonded to the servicing system.
22.214.171.124. The overhead method of filling refuelers and transport trucks has been replaced with
bottom-loading methods. Top loading allowed fuel to free-fall, creating a large static charge
inside the vehicle's tank in an atmosphere conducive for an explosion. Bottom loading is much
safer since there is no free-fall of liquid to create static electricity.
126.96.36.199. Fuel flow through equipment and transfer pipes will generate sufficient static electricity to
create a potential hazard. Tests have shown that a typical flow of fuel through an F/S will produce
sufficient static electricity to create a spark.