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Grounding on Ships, Trains, Airplanes, etc.
Name: Sharik
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
How is Earthing done in trains, ships and aeroplanes?
Replies:
Hello Sharik -
On almost any metal-frame moving vehicle,
the metal shell acts somewhat as a Faraday cage,
so the shell itself is taken as Earth-ground, (safety-ground & noise-ground).
For a Faraday Cage, the voltage difference and current discharges
between the cage and its surroundings do not propagate to things inside the cage,
so it's true that the cage or shell is the effective local ground.
As with Earth grounds, this ground is never used to carry returning currents of a power line,
because resistances between various structural parts might not be low,
and then a dangerous voltage drop might be formed between different parts of the vehicle.
It's also not used to carry small signals (such as for microphones), for a similar reason:
random currents in the ground shell will cause plenty of noise and ruin the signal quality.
On automobiles the whole chassis is taken as local ground,
and used for returning power currents too.
These are the reasons the practice might be acceptable in cars:
usually a car chassis must be very well bonded together,
and low cost is important compared with safety, and the voltage is not very high,
and probably others...
Maybe this practice is one of the reasons that cars are
considered to be fairly noisy electrical environments.
Jim Swenson
Sharik -
Ground does not always have to be connected to Earth. In cars,
airplanes... even portable radios... an artificial ground is established.
It is usually the chassis of the radio or car. It is treated just like a
ground attached to the earth even if it does not have true ground potential.
Larry Krengel
The basic principle of "grounding" or "Earthing" something is to
provide controlled path of very little resistance to any electrical current.
Trains, ships, and airplanes (and cars, buildings, and more)
frequently provide their own means of being grounded in their
construction when in contact with the earth. So long as a metallic
(or conducting) frame is used in the construction, then it can
conduct electricity between 2 points that have a difference in
electrical potential.
Now, the type of grounding that is needed depends upon what kind of
current you might expect to pass through the conductor. If all you
need is something to dissipate a small amount of static charge that
has built up, then a simple wire can suffice. If the purpose is to
prevent damage from a lightning strike, then the conductor needs to
be robust enough to tolerate the extremes of current and heat. I'm
afraid I can't tell you with any authority the exact methods of
grounding each of the vehicles you mentioned. However, I would
imagine the basic idea applies to all of them.
There is one important difference though that I think you can
appreciate. Trains and ships are both in contact with the earth,
but the airplane spends a great deal of time out of contact with the
earth (and traveling through some large electrical potential
differences). While airborne the plane can have its own "common
ground" for any electrical devices inside, but that shared "zero"
potential can be different from the electric potential at the
earth's surface. Once an electrical contact is made between the
plane and the earth there will be some charge transfer and the
potential difference eliminated.
Incidentally it is not the tires on a car that are protecting you
from lightning strikes. The car can still readily be struck by
lightning. However the car's metal frame acts as a large "grounding
wire" and conducts the current from the strike quite well. In
effect the car is a Faraday Cage. Likewise, if you are going to
take advantage of this, you do not want to be touching the metal
frame at different places. It's by being seated inside the frame,
but not making direct contact with it that you are offered
protection. So while it is relatively safe to be inside a car
during a lightning storm (as opposed to standing outside), it's not
safe to be riding a bicycle. The rubber tires offer no protection
contrary to the understanding of many people.
cheers,
Michael S. Pierce
Sharik,
If you mean "earthing" to be "grounding" from an electrical point of
view, then "grounding" means to provide a return path for free electrons
to "ground", or simply, a return path to the negative polarity of a
voltage source. Hence, the metal chassis of a car, train, ship or
airplane is a good "ground" or a return path for electrons to travel.
-Alex Viray
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Update: June 2012
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