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

4.7.2 Time duration consideration. Some of the phenomena cited above are transient in nature.  Knowledge of their duration is necessary to assess them as realistic hazards.  Induced voltages, friction voltages, and voltage buildups following fueling are considered transients.

4.7.2.1 Time calculation.  Using the ungrounded aircraft resistance of 40 M , an aircraft capacitance to ground of 0.005 µF, and a safe voltage limit of less than 30 V, the following time durations were computed using equation (3):

§ E  ·

Where:

t = R · C · ln

1      i  1

\ Es  )

t = time to reach Es  after removal of source (seconds) R = aircraft resistance to ground ( )

C = aircraft capacitance to ground (F) Ei = initial (source) voltage (V)

Es  = safe voltage level (V)

ln = natural log

Using the ungrounded aircraft resistance of 40M , an aircraft capacitance to ground of 0.005 microfarad (µf), a safe voltage limit of less than 30V, and 60kV as the initial voltage,

t = (40 M )(0.005 µf) ln ቀ଺଴௞௏ቁ

ଷ଴ ௏

t = 1.52 sec

Heart discoordination (fibrillation) threshold levels have time durations as low as 0.2 second (s), a fact that makes the 1.52 second discharge time very dangerous and unacceptable.

4.7.2.2 Safety of transients.  Any transient is objectionable from a safety standpoint.  As stated earlier, heart action discoordination (fibrillation) threshold levels have time durations as low as 0.2 second.  Thus, the duration of even the shortest of the three transients considered is unacceptably long.

4.7.3 Hazards. Ungrounded aircraft should be considered to be in jeopardy from the indicated energy source, since at least one (and generally more than one) hazard threshold level is exceeded during each scenario.  These results are summarized in Table V.  The possibility of these hazardous events occurring is ensured by the physical data available.  However, each is dependent on a number of factors that may occur simultaneously.  In any single one-time event, such as an aircraft ordnance evolutions, consideration could be given to the fact that hazardous combinations appear so seldom that they may be neglected.  However, when consideration is given to the number of naval aircraft involved, the rapid tempo of operations, the fact that these are military operations (not always conducted under ideal conditions), the high cost of

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