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Name: Moe S.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: July 2004


Question:
If an electrical system needs to be "grounded" to the earth to work, how does the electrical system in an airplane work when the plane is in flight?



Replies:
In an airplane (as with a car or portable radio) the ground is sometimes termed an artificial ground. It is not technically tied to "the ground." Even in house wiring, there is a ground wire and a neutral wire. They have the same potential and are often connected together in the breaker box, but in theory serve different functions. The neutral (white) provided the return path of the current while the ground (usually green) provides a safety link to real ground.

What is needed for current to flow is a complete path, and there are different means of achieving that goal.

Larry Krengel


Moe,

Electrical "grounding" does not mean that all electrical systems have to be connected to the ground. For example, a battery operated flashlight is not connected to the ground, nor is the electrical system in your car.

Electrical grounding is a protective mechanism that helps prevent electrical shock. For example, a toaster has electrical connections to the heating elements inside the toaster. If a fault occurred that allowed a connection between those heating elements and a metal (electrically conductive) shell that is part of the toaster, then someone who touched the outside (shell) of the faulty toaster could be exposed to a high voltage. The circuit's fuse or circuit breaker only protects from a high current, so a fuse/circuit breaker would not protect the person in this situation.

By providing an electrical ground, current would be allowed to flow from the toaster's shell through the ground circuit which would have less resistance that somebody's skin touching the faulty toaster's shell. This protects someone from receiving an electric shock if they touch the toaster's shell. Current only flows through the ground circuit when there is a problem (fault). Normally there is no current through the ground circuit.

Airplanes use their jet engines to generate electrical power while they are in flight (at the gate, they are normally connected to electrical power from the airport). The plane's electrical circuits can still have "grounds" that would prevent an electric shock hazard from faulty electrical equipment on the plane.

Todd Clark, Office of Science
U.S. Department of Energy


Electric systems do not need to be "grounded" in order to work. Rather, the purpose of grounding on earth is to make sure that harmful or undesired electric voltages are not present in the event of a malfunction. Humans and buildings are usually at the electric potential of the earth. Thus, to avoid appliances or other electric devices giving a shock, the outer part of the appliance is connected to ground with a wire. That way, if something goes wrong inside, a person will not get a shock. Airplanes are usually made of metal, so the metal frame is used as a conducting "ground" for all the electrical components inside the aircraft. So "ground" on an airplane is usually the frame. A metal frame also protects the occupants from lightning strikes. Parts of the aircraft that are made of plastics or composites are protected with a metal mesh or fibers.

The aircraft itself becomes charged as it is flying. Snow, dust and rain, as well as ionization of the air by the engines, can cause the airplane to develop a static charge up to several million volts. Because the passengers and the equipment is inside the metal shell, there is little noticeable effect other than communications. Communications are disrupted by the radio frequency static that occurs when corona discharge or electrical leakage occurs.

Bob Erck



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