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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equipment, and the threat to personnel safety, electrical grounding becomes essential.  Electrical airframe grounding, like safety belts in automobiles, is statistically indicated by, among other things, the vast numbers involved.

4.7.4 Effects of grounding. In each of the scenarios considered, the use of a proper ground connection ensures that the airframe is maintained at the same potential as the ground point for those sources considered, except in the case of external power systems.  In addition, since the airframe resistance to ground is greatly reduced, the duration of such effects as induced voltages is reduced to fractions of a millisecond.  As an additional advantage, the use of a proper grounding procedure ensures that any arcing or electrical discharge associated with the act of connecting grounds takes place at the ground point rather than near the airframe.

TABLE V. Potential hazard relationship to energy sources and scenarios.

TABLE NOTES:

1.    A = Static shock to personnel,

2.    B = Ordnance EED/stores actuation/uncommanded release,

3.    C = Electronic equipment damage.

5. DETAILED GUIDANCE

5.1 Testing and identification of ground points (ashore). This section provides guidance that should be used to measure resistance of the grounding system.  It includes theoretical background, measurement equipment, measurement methods and procedures.

5.2 Theoretical background. The following figures illustrate the Three-Terminal Fall of Potential Method. Using Ohm's Law, the Earth's resistance can be determined using known values of voltage and current.  With a 30 V source and a measurement current of 6 amperes, the resistance of the resistor is 5 ohms (see Figures 6 and 7).  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. (see Figures 8 and 9).

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