8

What are all the wire shields grounded to on a system that has no contact with earth, like an airplane? I found this document that says all shields and chassis and other grounds should be connected at a single point next to the main power source ground. What if the power source is a battery and the lightning strikes with a lot of voltage and current, won't that blow up the battery? My guess there should be some big resistor or something that will quickly and orderly convert all that electricity into heat, but that is just my guess. Surely I can't just connect it to the negative on the battery, or can I?

Arthur Tarasov
  • 562
  • 9
  • 16
  • Ground would be the same as on cars and boats, I would imagine - the metal chassis/body. – Lundin Mar 20 '20 at 07:42
  • Well that's what's bothering me. What if chassis is the source of interference. Say lightning strikes the chassis. Wouldn't that energy have to go somewhere? It can't go to earth or water up there – Arthur Tarasov Mar 20 '20 at 07:47
  • 4
    Can you edit to explain your thinking on how a lightning strike on the airframe would "blow up the battery"? – Transistor Mar 20 '20 at 07:47
  • @Transistor I just don't know where all the energy will go. Can modern lithium ion batteries absorb it all? Maybe there should be a massive capacitor to smoothen it out somehow? I am very fuzzy on this but I just can't imagine I can treat a battery negative the same as earth – Arthur Tarasov Mar 20 '20 at 07:51
  • 3
    The airplane chassis is pretty much a Faraday cage. Lightning strikes for the purpose of evening out the number of electrons between two bodies, not for some purpose of "delivering energy". So I guess that's all that would happen: electrons jumping to/from the plane chassis. – Lundin Mar 20 '20 at 07:51
  • 1
    Having one terminal of a battery "lifted" by a lightning strike doesn't cause any damage in the same way a bird can sit on a high-tension cable. It's only if the battery negative and positive are forced to different voltages (<> battery voltage) that damage will occur. – Transistor Mar 20 '20 at 07:56
  • 1
    @Lundin so if it is not an airplane but something that is mostly made of plastic and has very little metal in the chassis, then the wires and electronics will be doing the job of the Faraday cage and something like lightning strike would fry it? The metal chassis is the key then? – Arthur Tarasov Mar 20 '20 at 07:57
  • 4
    Yes, they have been testing airplanes built from other materials, but they had to put a copper mesh everywhere just underneath the top surface layer. – Oldfart Mar 20 '20 at 08:24
  • 1
    That single connection point is most likely used when being filled with fuel... sparks due to static charge created by moving fluids is a real hazard. – Solar Mike Mar 20 '20 at 08:25
  • Anything on an aircraft that is susceptible to lightning is tested for that scenario; aircraft earth is the body of the aircraft but everything powered from the primary power source(s) is isolated from it. Details of the required tests are normally found in DO-160 or equivalent (Airbus has its own standard but it is almost identical). – Peter Smith Mar 20 '20 at 09:11
  • 1
    For protection against the specific case of lightning strikes, electronics on planes are fitted with TVS (transient voltage suppression) diodes to clamp any high transient voltages apparent on the circuit. – BenAdamson Mar 20 '20 at 11:21
  • 1
    If lightning hits a plane, it travels through and goes out somewhere else to continue its journey, which is where the energy goes. Look for photos of "lightning striking plane". – Andrew Morton Mar 20 '20 at 11:53
  • 1
    Those Boeing 737 are grounded... – Huisman Mar 20 '20 at 13:13
  • Relevant: https://en.wikipedia.org/wiki/St._Elmo%27s_fire – Hot Licks Mar 20 '20 at 17:31
  • 2
    It's floating, as all good planes should. ;) – Steve Mar 21 '20 at 01:22
  • Consider a car is exactly the same - there is no earth connection. – Criggie Mar 22 '20 at 00:38

2 Answers2

21

I think you need to know the difference between "ground" and "earth".

Ground is the term EEs use for a reference point. In principle we're completely free to choose any reference point we like (crazy example: the + terminal of the 9 V battery lying on my desk). Some choices of Ground are more sane than others, my crazy example is a pointless reference but the metal chassis of an amplifier, a car or an airplane is a good one. Also the - pole of a battery might be a good choice.

Earth is usually an actual connection to our planet earth like a metal pin sticking in the earth. This is used to make a "safe to touch" connection in a mains (120 V or 240 V AC) setup like in your house. You would not want the get an electric shock when you touch the metal frame of your washing machine. So we connect it to earth.

At the same time we often also use this earth as the ground (reference) of our circuit so then earth = ground. They're connected and they're the same.

It is difficult to answer what happens when lightning strikes because many different scenarios are possible and it heavily depends on what happened exactly. Also, a direct lightning strike contains a lot of energy, it isn't always possible to make the electronics survive such a hit without damage.

Bimpelrekkie
  • 80,812
  • 2
  • 94
  • 185
  • 1
    In British English zones, we say 'earth' for what Americans call 'ground', noun and verb, and we stand on both. – Michael Harvey Mar 20 '20 at 17:38
  • 1
    @MichaelHarvey Americans use both terms interchangeably. – Charles Mar 20 '20 at 18:20
  • 4
    The + terminal on a battery is actually a pretty good (and common) choice of ground point, especially with things like older op-amps that needed + and - power supplies, so you had two batteries in series, and where the two connected to each other was the ground. – Jerry Coffin Mar 20 '20 at 19:19
  • @Charles In the context of mains electricity, that's true. But in electronics, ground need not be earth-ground. – MooseBoys Mar 21 '20 at 02:39
  • @MooseBoys - and ground need not be ground-ground either. – Michael Harvey Mar 21 '20 at 08:29
10

Airplanes have static discharge wicks. They're often on the trailing edges of the wings or empennage (tail control surfaces). Strictly speaking, this isn't a real "earth" connection, but a way to bleed off the kind of static that comes from lightning strikes or even airflow over the airplane.

The wicks are typically pointed to help facilitate charge transfer. They serve to avoid plasma damage to other parts of the airframe by concentrating the discharge. They're on trailing edges to help sweep any generated plasma away from the airframe, and because the best aerodynamic place for pointed objects is on trailing edges (for subsonic airplanes).

https://en.wikipedia.org/wiki/Static_discharger

O. Jones
  • 201
  • 1
  • 5