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GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION REQUIREMENTS CHAPTER 4: EXTERNAL GROUNDING (EARTHING)
• Grounding electrode encasement material shall be packaged for the purpose of grounding electrode
encasement.
• Grounding electrode encasement material shall be environmentally safe and approved by the
environmental authority having jurisdiction.
• Grounding electrode encasement material shall be used in accordance with the manufacturers'
instructions.
• Grounding electrode encasement material shall not have a corrosive effect on the grounding
electrode system components.
• The use of charcoal or petroleum based coke breeze is not recommended as it may result in rapid
corrosion of copper electrodes and copper conductors (BS 7430:1998, clause 8.5; BS 6651:1999,
clause 18.4.2; and FAA STD 019d-2002, section 3.8.3.5). Charcoal and petroleum based coke
typically contains high levels of sulfur, which in the presence of moisture will accelerate corrosion.
Coke breeze derived from coal in coke ovens is generally considered acceptable; all the corrosives
and volatiles have been cooked off at extremely high temperatures (FAA STD 019d-2002, section
3.8.3.5).
Per MIL-HDBK-419A, the suggested grounding electrode encasement (backfill) material is a mixture
of 75 percent gypsum, 20 percent bentonite clay, and 5 percent sodium sulfate. The gypsum, which is
calcium sulfate, absorbs and retains moisture and adds reactivity and conductivity to the mixture. Since
it contracts very little when moisture is lost, it will not pull away from the ground rod or surrounding
earth. The bentonite ensures good contact between the ground rod and earth by its expansion, while the
sodium sulfate prevents polarization of the ground rod by removing the gases formed by current
entering the earth through the ground rod. This mixture is readily available from cathodic protection
distributors as standard galvanic anode backfill. The backfill mixture should be covered with 305 mm
(12 in.) of excavated soil. See MIL-HDBK-419A Volume I, section 2.9 for additional information.
4.4.2 GROUNDING (EARTHING) CONDUCTORS
Grounding (earthing) conductors are the conductors used to connect equipment or the grounded circuit
of a wiring system to a grounding electrode or grounding electrode system. These conductors may
connect grounding electrodes together, form buried ground rings, and connect objects to the grounding
electrode system. See BS 7430:1998, clause 3.17 and NFPA 70-2005, Article 100 for additional
information.
4.4.2.1 GENERAL SPECIFICATIONS
General specifications for grounding (earthing) conductors are listed below.
• Unless otherwise stated, all below-ground, or partially below-ground, external grounding electrode
system conductors shall be 35 mm2 csa (#2 AWG) or coarser, bare, solid, tinned or un-tinned,
copper conductors (ANSI T1.313-2003 and ANSI T1.334-2002, section 5.3). For areas highly
prone to lightning, and/or military installations, larger conductors, such as 50 mm2 csa (#1/0 AWG)
or coarser, should be considered (MIL-HDBK-419A); stranded conductors may be used in this
application. Tinned conductors are recommended for stranded conductors.
• Solid, bare, tinned, copper conductors should be used to help minimize galvanic corrosion between
tower legs and other parts of the grounding electrode system (ANSI T1.313-2003, section 10.7).
See “Dissimilar Metals and Corrosion Control” on page 4-34.
4-28 68P81089E50-B 9/1/05