How exactly are airplanes electrically grounded?

How exactly are airplanes electrically grounded? - E&S Grounding Ask the Experts

Michael Asked:
How are airplanes electrically grounded? Is a lightning protection system (LPS) effective if it is not grounded? If you install a lightning rod that is not grounded, is it more or less dangerous?

Our Answer:

Great question. To start with, you need to provide a path to earth for the lightning protection system, otherwise, where are all those electrons going to go? The energy will jump to earth no matter how insulated your structure is, the question is whether or not the arc-flash will cause a fire to start in the process.

If you have an ungrounded structure (think of an airplane flying in the sky), you could have two possible scenarios:

1. The airplane is properly bonded from tip to tip and has an extremely low difference in potential from any given point in the plane.
2. The airplane is not properly bonded and has large resistances from front to back and side to side.

In scenario #1, the airplane would get hit with lets say, 100,000 amps. But the resistance across the plane is very low (because it is properly bonded), so lets say it has a resistance of 0.0001 ohms. If we do a little Ohms Law math, we find that the voltage of 100,000 A times 0.0001 ohms = 10 volts. So the plane suffers a 10 volt rise of potential across the entire plane, and the lightning jumps out of the plane moving on towards the earth. 10 volts is very manageable to the planes electronic systems, and no thermal damage occurs.

In scenario #2, the airplane would get hit with lets say, 100,000 amps. But the resistance across the plane is very high (because it is not properly bonded), so lets say it has a resistance of 1.0 ohms. If we do a little Ohms Law math, we find that the voltage of 100,000 A times 1.0 ohms = 100,000 volts. So the plane suffers a 100,000-volt rise of potential across the entire plane, and the lightning jumps out of the plane moving on towards the earth. 100,000 volts is not manageable to the plane, its electronics are fried, and fires start due to the extreme thermal heating…. it’s a bad day for the airplane.

Now, consider your building or structure and the addition of an ungrounded lightning protection system (LPS); it could, in fact, increase the bonding of the structure, thereby reducing the resistance differential across the structure. This is a good thing.

Note: Contrary to popular belief, adding a lightning protection system (LPS) to a structure does not increase (or decrease) the likelihood of a strike.

The concern for the ungrounded lightning protection system on a structure, is what happens when the lightning energy leaves the structure heading to earth? It can only leave the structure as an arc-flash event. This is bad. It is also against every single code and regulatory system we know of. A structure’s lightning protection system must be bonded to ground, in order to meet National Electrical Code (NEC) requirements.

Please see NEC Article 250.106 and 250.104(D)(1). (NFPA 780 also mandates electrodes for every down conductor).

The Engineering Experts at E&S Grounding Solutions

grounding, Grounding & Earthing Principles, how to ground an airplane, airplane

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