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

4.7.1 Source magnitude and hazard threshold level. The magnitude of potential energy sources is compared with hazard threshold levels to identify those combinations that could result in a hazard.

4.7.1.1 Worst-case scenario for current level.  An upper boundary of 30 µA has been described as a worst-case scenario of a static triboelectric energy source.  Since in an ungrounded aircraft this current flows from Earth, through the aircraft tire resistances to the airframe, and thence to the snow, dust, etc., causing the effect, the potential between airframe and Earth is determined almost entirely by the tire resistance.  While laboratory measurements of tire resistance well over 100 M  have been made, the worst-case measured value of 40 M  obtained in the field, is used here to compute airframe potential utilizing equation (1):

V = I·R = (30 µA) · (40 M ) = 1,200V

Where:

V = potential in Volts from the airframe to ground

I = triboelectric current in amperes

R = resistance in ohms from the airframe to ground

Using equation (2) and a value of C = 0.005 µF, the energy level (U), could reach 3.6 mJ.

U = i C·V2

= i (0.005 µF) · (1,200V)2  = 3.6 mJ

The values of U and V exceed the hazard threshold levels for fuel vapor ignition and non-lethal shock to personnel.

4.7.1.2 Worst case static voltage level.  The value for the worst-case static voltage level for personnel is 27 kV.  Energy levels produced by this voltage may range as high as 0.18 J or

180 mJ.  Comparing this value with the hazard threshold values in Table IV shows that the thresholds for reflex action shock to personnel and ordnance EED ignition levels are exceeded. (For further information on P-Static refer to MIL-STD-464.)

4.7.1.3 Charge induced by storm activity.  The source level has been established as due to charge induced by storm activity.  Energy levels may then reach:

U = i C·V2

= i (0.005 µF) · (60 kV) 2  = 9 J

These levels exceed threshold values for all hazards.

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