This article was posted 05/20/2009 and is most likely outdated.

Electrical Hazards for Aircraft and the NEC
 

 

Topic - Grounding
Subject - Electrical Hazards for Aircraft and the NEC

May 20, 2009
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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.

Image1
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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Comments

  • With all due respect Sir.

    As an individual with an airframe and powerplant license, multi-decade experience with avaition, railroad and commercial electricity, please make sure this isn't a tempest in a teapot, an engineer in search of a cause. Because if this was an actual issue, we would be seeing aircraft exploding and people dropping like flies trying to board aircraft all over the world.

    Every multiphase piece of equipment I have ever installed in an aircraft was completly self contained. That's why polyphase is used. No external neutral. This might be the issue, in that while it's called a "neutral", nothing in the airplane actually uses it, so in practice it becomes the bonding conductor.

    Wiring, especially wiring carrying AC, to the sub equipment is usually shielded, with the shield grounded to the frame. The case of the equipment is bonded to the airframe or frame of the railroad car. You don't want current in the frame and shields, because then you have a big antenna, interfering with the navigation and communication equipment. Additionally each piece of equipment has it's own sub-breakers or fuse.

    Another issue is that these peices of equipment, both the whole machine, and discreet appliances, recieves much more regular critical and intense inspection than building or house electrical system would ever recieve post approval. A "low level fault" would show up as something wrong, either with the system itself, or as interference with some other system.

    In all of my experience, the metal frames, both the airframe itself and the individual components NEVER carried current. All equipment was bonded together, yes, but what have you seen inside the airplane to make you think that there is actually current in the airframe? Or are you looking at the plug and assuming that the inside of the airplane is working just like all the ground bound and stationary systems that you have designed?

    The fuel tanks you mentioned as exploding? What reason was given for their explosion. Operator error? Did they explode while they were connected to the airplane? As far as I know it's always sop to ground the airframe to the fuel delivery device, either stationary or mobile. Was it actually the issue of current in the neutral that caused the explosion.

    In conclusion, what I think we have here is something called a "legacy". When the MIL spec was written, it was sop. The industry worked to the spec. Now there are many, many things out there worked to that spec. The NEC moved on, rightfully so, but to change all of the things out there that are configured to the old spec, it is too impractical and too expensive to change.

    Thank You

    Steve
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