Electric shocks from airplane electronics whilst in flight....!

Discussion in 'Physics' started by HelenR, Mar 4, 2015.

  1. HelenR

    Thread Starter New Member

    Mar 4, 2015
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    I have been looking at the information contained in the text book section on this website on electrical safety, which has been really useful and informative. However, I now have a question that I've not been able to find the answer to by simply google searching - perhaps because the answer is really obvious!
    On this page http://www.allaboutcircuits.com/vol_1/chpt_3/3.html it shows a diagram with an explanation that a circuit that isn't grounded will not cause an electric shock if a person (or bird!) were to touch it (provided no ground fault).
    From further research, I believe that an airplane's electronics are grounded, whilst in flight, by a connection to the metallic body of the aircraft. I am assuming the people in the airplane are not earthed - to 'Earth' or to the metallic body of the aircraft as they would be insulated from the metallic body of the aircraft.
    So, my question is, if people inside they aircraft are indeed insulated from the ground of the aircraft, would it be possible to receive an electric shock from the aircraft electronics, from say exposed wires? My instinct is that yes, of course there would be a shock! However, referring back to the previous link, if there is no complete circuit from the exposed wires in the electronics to the ground through the person then an electric shock would not be received.
    Any help would by appreciated.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I have never been shocked while flying airplanes...

    First off, my airplanes all have a 12V electrical system, like in a car. The negative pole of the battery is tied to the metallic airframe, just like in a car. There are some exposed bus bars that are tied to +12Vdc (the positive pole of the battery). Touching those does not produce a detectable shock (not even a tingle), because 12V is just enough voltage to over come skin resistance, even if the other hand is touching bare metal on the metallic airframe.

    It takes about 30V to produce a detectable "tingle" if you touch the bus bar with one hand while also touching the bare metal airframe with the other. If your other hand was not touching the airframe (local ground), then you can touch any wire anywhere in the aircraft with impunity as long as you are "insulated" (even by a layer of paint), just like the bird on the wire. It is only if the bird (or you) touch two different potentials at the same time that you might feel a shock, and then only if the voltage is high enough to cause current to flow through the flesh under your (dry) skin.
     
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  3. WBahn

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    Mar 31, 2012
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    You are confusing the conflicting uses of the term "ground". In the electrical safety portion of your discussion, "ground" is the physical earth and if a high voltage supply is not ground-referred, then you can touch it (at one point) and the earth all day long and nothing will happen. In general, "ground" is a generic term for a reference conductor. If you were on a large ship the ship's generators are tied to the body of the ship, which serves as the "ground" for the system and touching a high-voltage line and the body of the ship at the same time will get you a jolt. The same is try for an airplane or your car. In small vehicles, the voltages are low enough that they present a low shock hazard even if you grab both terminals of the battery at the same time. But in larger aircraft you often have higher voltage systems as well, such as 115V, 400Hz AC, and if those are ground-referred to the chassis of the plane then you can get shocked if you are touch a hot wire while in electrical contact with the chassis (and there are numerous ways to accomplish that, so it is best to assume that you are in electrical contact with the chassis unless you have taken specific and thorough steps to avoid it -- the same as in your car or on a ship or at home).
     
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  4. HelenR

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    Mar 4, 2015
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    Thank you very much, your comments are very helpful.
     
  5. GopherT

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    Nov 23, 2012
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    You can also walk down the carpeted aisle of your favorite commercial airliner, only touching the plastic armrests of the seats you have not been grounded and you can build a potential that is much different than the airframe as you sluff electrons from the carpet as you walk. When you get to your seat, you touch the metal bracket on the tray which is connected to the metal bracket of the seat in front of you, which is connected to the floor joists, which are connected to the airframe - and suddenly get zapped as the static build-up in your body is suddenly brought to the same potential as the airframe. A shock it not a question of being grounded to earth, it is a question of being electrically insulated and then contacting something that is at a different potential that you.
     
  6. HelenR

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    Mar 4, 2015
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    I feel I am confused again, perhaps I'm missing a link in the chain!
    For current to flow, a circuit needs to be complete, hence not being shocked by electronic circuits if not in contact with the local ground.
    I've experienced shocks from the transfer of charge having been electrically insulated and then making contact with a conductor. Where is the complete circuit from the build up of charge on the person to the metallic frame of the airplane?
     
  7. studiot

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    Nov 9, 2007
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    Like charges repel.
    So a bunch of like charges will want to get as far away from each other as possible.

    A good way to look at static electricity is to think of the charge 'spreading out' over the bod(ies) available to it.

    So if a new body is added (connected) the charge will spread over this as well.

    The spreading causes a temporary current whilst this equalisation happens.
     
  8. WBahn

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    It's a complete circuit, but not on the essentially-instantaneous time scale that you are trying to envision it in.

    You scuff along the carpeted floor and this creates a charge separation. Let's say that your shoes pick up excess electrons (I'm not sure whether the person of the carpet gets the electrons -- I've found references that claim each way -- and it may depend on the carpet and the shoe). That's a localized electric current that lasts for a short while. Then that charge spreads out over your body, which represents another small localized current that only lasts for a short while. Similarly, the surrounding carpet provides electrons to the area where your shoes stole them from. Yet another small current for a short time. So you can see that, over time, the path that participates in this circuit grows. Eventually you touch a wall switch and you get an intense, but very brief, flow of current from your body to the switch plate. These electrons will spread out and eventually take the place of electrons that had moved a ways in order to replace other electrons that had move a ways and on and on to eventually replace the electrons that moved to replace the electrons that your shoes stole. At that point the circuit has completed the entire path. Of course, it's not as simple as that, but the gist is there.
     
  9. #12

    Expert

    Nov 30, 2010
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    Part of the wisdom of this is knowing when the simple, obvious answer is the right one.
    Static electricity is so different from intentional circuits that we haven't even come close to getting a grip on lightning. You can hide from it in a metal shell, but you can't catch it in a jar.
     
  10. MikeML

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    Even more puzzling. If I fly in dry snow flurries, each snow flake carries a charge. When it bumps into the airplane, some of that charge transfers to the airframe (process is similar to a VanDeGraff generator). My Cessna is all metal, so like charges repel, and the entire airframe becomes uniformly charged. As occupant, I am not aware of it because I am sitting inside a metallic Faraday cage.

    Back prior to WW2, airplane tires were made of natural rubber. When a charged aircraft landed, the tires insulated it from the runway/ramp. A line boy would walk out and reach out to open the door, and get zapped, discharging the entire airframe to ground below... A passenger could also get zapped as they stepped out of the airplane.

    Since that time, aircraft tires have been formulated with carbon-black in the rubber, and are slightly conductive. As the airplane lands and rolls toward parking, the charges on the airframe leak to the earth through the tires, so by the time a human bridges the gap between the airframe and the earth below it, the voltage difference is almost zero, so no zap.

    This also is important while fueling aircraft, even though fire and FAA regulations require the aircraft be "grounded" with a wire and clip to the fueling station prior to dispensing fuel. The last place you want a spark jumping is right where the fuel hose is close to the filler neck of the aircraft's fuel tank.
     
  11. WBahn

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    And this is why the static wicks are located on the trailing edges of the control surfaces. The idea is to create sharp points where the electric field intensity grows high enough to discharge back into the air. Without them two bad things can happen. The build-up of charge on the airframe can reach a point that it begins to interfere with the avionics. Also, if allowed to build up to the level that would otherwise be needed to get a discharge there is a chance that the resulting high current that would travel from the airframe and back to the control surfaces might actually be enough to weld a hinge -- which is also why most aircraft have grounding straps between the airframe and the control surfaces.

    Where you still see that happen is with helicopters. Never grab a cable (or even a rope) dangling from a helicopter before it touches the ground first. The resulting static discharge, under the right conditions, can be fatal. And that charge can build up real fast so that if a cable or load is lifted off the ground even momentarily it needs to be allowed to reground before touching it.

    You still see people that think that they should hold the fuel nozzle such that it doesn't make contact with the filler neck. But that is precisely where you WANT it grounded since the rushing fuel coming out of the nozzle and into tank is akin (but not really the same) as a natural Kelvin electrostatic generator otherwise.
     
  12. jpanhalt

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    There is a lot more to it than just having the static wicks. You have to be sure the control surfaces aren't "discharged" while the rest of the airplane is charged. Arching at the articulations and bearings could also cause a problem. Bonding can be complicated and may require an STC.

    Like Mike, I have never been shocked in an airplane; although, I have had so much static charge that the avionics became dysfunctional for a short period.

    John
     
  13. MikeML

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    There is another reason why the static wicks are located on the aft edge of the ailerons (near the wing tips). That is that when charge leaks off the tip of the static wick (lots of very sharp points hanging back in the slipstream made of carbon-impregnated fibers) it creates a corona. Corona makes RFI which can mask VHF reception, and you want the corona to be as far from the whip antennas as possible.
     
  14. studiot

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    This question has led to a most interesting discussion, telling me stuff I hadn't heard of before.

    Thanks folks, keep it up.
     
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  15. WBahn

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    You can often tell whether or not a plane is IFR equipped by the number and condition of the static wicks. In IFR conditions a plane is more likely to build a charge (flying in clouds or dry snow) and the consequences for avionics interference are much more severe.
     
  16. Ajibade Tobi

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    Apr 19, 2015
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    I have learnt a lot pals. Thanks for the comments. You guys are good
     
  17. wmodavis

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    Oct 23, 2010
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    "Grounding" or not is not the primary cause or contributor to receiving a shock! The cause of getting a shock is simply making physical contact with two points wherein exists a difference of potential. That potential depending on its magnitude may cause a resulting shock. Current flows due to difference of potential across a conductive path. If a ground or not was a condition of getting a shock i.e. electrical current passing through your body, explain how avionics in general can function without a ground. It works because there are controlled currents/voltages to do their assigned tasks and walla without even having any earth ground connection.
     
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