MIL-HDBK-274 Electrical Grounding For Aircraft SafetyThis handbook has been prepared to provide U.S. Navy and Marine Corps aircraft maintenance and operations personnel with the background and technical details of the different types of aircraft grounds and how they are applied. In addition, this handbook provides background information for aircraft grounding, static electricity theory and how it affects aircraft, and techniques used for measurement of grounding points. This handbook is intended for use by all U.S. Navy and Marine Corps aircraft maintenance and operations personnel for the purposes of ensuring that aircraft are properly and safely grounded. This handbook is for guidance only and cannot be cited as a requirement.

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MIL-HDBK-274A

5.4 Ground point measurement procedures. The three-terminal fall of potential method should be used when measuring ground points on the apron. This includes certified power grounds, certified static grounds, mooring eyes, and FLEDS lightning protection grounds. The two-terminal method should be used when measuring hangar ground points, the facility's AC neutral line, and in situations where the three-terminal method is impractical. A resistance value of less than 25 ohms is recommended for certified power grounds, FLEDS lightning protection grounds, hangar ground points, and the facility's AC neutral line. Certified static grounds and mooring eyes are recommended to be less than 10,000 ohms.

5.4.1 Three-terminal fall of potential method. The recommended test setup for performing the three-terminal fall of potential method is shown on Figure 10. This test is performed as follows:

a. Connect test lead I from the tester to the ground point being evaluated.

b. Drive a metal reference rod (rod C) into the ground at a distance of 100 feet (30.5m)

from the ground point being evaluated. Connect test lead C from the tester to this rod.

c. Drive a metal rod (rod P) into the ground approximately 62 feet (18.9m) from the ground point being evaluated and in line with the reference rod (rod C) installed in step b above. Connect test lead P from the tester to this rod (rod P).

d. Adjust the resistance dials on the tester until the indicator dial is centered (zeroed) on the scale. When the dial is centered, the resistance value (in ohms) of the ground point being evaluated can be read directly on the tester's digital readout.

5.4.2 Two-terminal method (direct method). The test setup for performing the two- terminal method is shown on Figure 11. This test should be performed as follows:

a. Connect a jumper across the P and C terminals of the tester.

b. Connect test lead I from the tester to the ground point being evaluated.

c. Locate a large, unpainted, metal water pipe at least 100 feet (30.5m) from the ground point being evaluated and connect test lead C from the tester to this water pipe.