Electrical Hazards for Aircraft and the NEC
The National Electric Code, Article 90.2(B)(1)(Scope, Not Covered) specifically excludes electrical installations in aircraft. How then, can there be electrical hazards involving the NEC and aircraft applications? Whenever the aircraft is receiving electrical power from fixed premises wiring, such as when loading passengers or undergoing maintenance at an aviation facility such as an airport.
Such electrical hazards are created by the inherent aircraft electrical design philosophy colliding with specific requirements for electrical safety contained in the NEC.
During ground operations, aircraft may be receiving electrical power, defined by MIL-STD-704 as a three-phase, four-wire, grounded-WYE electrical service of 115/200 volts, 400-Hz. This Military Standard is used for all commercial aircraft using 400-Hz power. This power is derived from the building electrical service and is a permanent installation, connecting to the aircraft via a pendant cord. The NEC, Article 513.10(C)(3) requires that such cord “shall include an equipment grounding conductor.” The cord assemblies available today do not contain the required grounding conductor. See Figure 1.
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Figure 1: Typical Aircraft Cord Connector |
In this design, pins A, B, C, and N carry the respective circuit conductors, including the grounded conductor. The two smaller pins are reserved for the 28-VDC Safety circuit; pins E & F. The aircraft mating plug does not provide for an equipment grounding conductor and the grounded conductor is bonded to the aircraft skin. The aircraft structure then serves as a Neutral or Return conductor.
It is common practice in aircraft power distribution design to provide single phase load power via a single conductor, the aircraft structure then providing the missing conductor [1, 2].
Consider then a commercial aircraft at the boarding gate, powered by the premises power system, with fuel in the tanks, people boarding, and power currents flowing through the structure of the aircraft, including the fuel tanks.
By not providing a grounding conductor, it will be difficult to detect a low level single-phase power fault. This is especially true since the skin or fuselage is acting as the grounded conductor.
Author:
Mike McClelland
Principal Engineer
559-583-7491
mmcclelland43@comcast.net
Bibliography
1. Standard Handbook for Electrical Engineers, 12th Edition, Donald G. Fink and H. Wayne Beaty © McGraw-Hill 1987, Chapter 23, page 23-4.
2. Aircraft Electromagnetic Compatibility Final Report, Boeing Commercial Airplane Company, Seattle, WA, June 87, published by U.S. Department of Commerce, National Technical Information Service. DOT/FAA/CT-86/40. Page 77. Download this report from: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870014423_1987014423.pdf
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