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

Equation:                                                                                                                                 (3)

t = R·C·ln  ( E  l

Where:

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 40 M  an aircraft capacitance to ground of 0.005

µF, a safe voltage limit of less than 30 V, and 60 kV as the initial voltage:

Equation:                                                                                                                                (4)

( 60 kV l

t = (40 M ) · (0.005 µf) · ln 1

\

1

30 V  )

= 1.52 seconds

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

4.4.1.3 Friction. Static electricity is generated when certain materials rub together. During such close moving contact between two materials, one of the materials is depleted of electrons, causing a surplus in the second. The magnitude of the static electrical charge thus produced depends on the materials involved and on the amount of humidity present during such friction. With synthetic materials (such as nylon) undergoing friction in a cold dry climate, the effect is greater.  Voltages as high as 27 kV have been recorded in some instances.  Because of hazards presented by such high charges, personnel subjected to such conditions should bring themselves to the same voltage potential as the aircraft and weapon system.  Since the risk of generating static electricity is increased in a dry atmosphere, such risk is decreased in a damp, highly humid atmosphere which inhibits the accumulation of static electrical charges.  Moisture in the air or on the skin provides a conductive path for the positive and negative charges to be rejoined, neutralizing the static charges.  To further protect against the effects of friction, it is essential that the aircraft be grounded.  Static electricity is generated when certain materials rub together.

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