On Earth, most of the electrical appliances having exposed metal parts, such as electric iron, are grounded, to protect the user from electric shock when an uninsulated-wire accidentally comes into contact with the metallic part. The ground (earth) wire is connected to a good conductor like copper and the latter is buried in the ground (earth), which acts as an infinite source/sink of charges to keep the electric potential of the meatal in safe limits, to avoid electric shock.

But, in the ISS, obviously, there must be earth cable, but not connected to ground (earth) however, as it is impossible to join ISS and earth with wire due to its motion.

Then, How are astronauts in the ISS are protected from electric shock? They may not be using electric iron, but the entire station is full of electrical appliances for life support, experimentation, station keeping, etc, with exposed metallic parts. I initially thought the earthing must be connected to the metallic hull of ISS, but then realised, the structure is not large enough (compared to earth) to act as an infinite source/sink of charges. So how do engineers solve this kind of problem in space?

  • $\begingroup$ The answers to this question are relevant. $\endgroup$ Oct 15 '19 at 17:50
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    $\begingroup$ Boats have the same problem, and solution. $\endgroup$
    – Ron Beyer
    Oct 15 '19 at 18:12
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    $\begingroup$ @RonBeyer I'd assume aircraft do as well. I mean, really even cars do, since their only contact with the ground is through non-conductive rubber tires. $\endgroup$ Oct 15 '19 at 20:58
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    $\begingroup$ No. The rubber compound used in tires has additives that make the tires conductive enough to not build up a static charge on the car. $\endgroup$
    – Hobbes
    Oct 16 '19 at 6:58
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    $\begingroup$ It is worth noting that the mechanism of electrical earthing in aeroplanes is slightly different (as far as I know). Aeroplanes have many small sticks with sharp tips, these are to minimize the static charge buildup on the plane's fuselage due to its motion in air as well as due to electrical earthing. These sticks help due to the principle of corona discharge. Thus they help in keeping the plane at nearly the same potential as of air. But in the space station, we can't use this since corona discharge requires a fluid medium. $\endgroup$
    – Vishnu
    Oct 16 '19 at 7:24

It isn't the actual level of charge (potential) that causes electric shock. but being connected to two things (like your iron and the ground) that are at different levels. Hence why birds can sit on a 750kV overhead line and not fry. The earth wire in a domestic system exists to keep all exposed metal at the same potential. Grounding everything to the frame of the ISS should work just fine for astronauts, except perhaps when an incoming vehicle needs to dock, then they would need to make sure there wasn't a large potential difference between the two. I don't know how that is done.

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    $\begingroup$ @Intellex, Vacuum a a very good insulator, and the light flash is largely ionized gas producing light so two space craft at different potentials touching will not produce the same flash two aircraft or similar would. That said there certainly would be a discharge and accidentally welding yourself to the ISS would be awkward. $\endgroup$ Oct 15 '19 at 7:52
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    $\begingroup$ @Intellex: NASA looked into this. Discharges happen at approximately 7 mm separation, and it appears the discharge is a result of the two plasma sheats merging. This seems to avoid a localized arc discharge. $\endgroup$
    – MSalters
    Oct 15 '19 at 11:52
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    $\begingroup$ Re, "...birds can sit on a 750kV overhead line..." Have you ever seen a bird on a 750kV line? I'm pretty sure I never have. There's guys who get dropped off by helicopter to "work hot" on those lines. You can watch on You Tube, and see how they dress: Both the lineman and the pilot wear full-body Faraday-cage suits to protect from the AC electric field surrounding the wire. Without the suit, just coming within a few meters of the wire would induce uncomfortable/dangerous electric currents inside their bodies. Birds are smaller (less body capacitance), but still... $\endgroup$ Oct 15 '19 at 20:56
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    $\begingroup$ @SteveLinton source? To my knowledge all power grids in Europe (and everywhere else?) use AC power lines to reduce losses. $\endgroup$
    – Baldrickk
    Oct 16 '19 at 12:18
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    $\begingroup$ @Baldrickk actually DC is more efficient for long-distance power transfer because of the skin effect; resistance/impedance from the wire is effectively lower for DC. The reason why power lines were traditionally AC is simply because the high voltage is what increases efficiency (because P=I^2R), and traditionally there was no way of transforming high voltage DC to low voltage DC and vice versa, whereas with AC it's trivial with transformers. Nowadays we have power electronics that can do that voltage conversion, so DC is therefore preferable. $\endgroup$
    – Muzer
    Oct 16 '19 at 12:23

"Earth" doesn't work the way you might think.

In any case, on a vehicle of almost any kind, "earth" is replaced by "metal chassis".

Your question is based on a very common misconception: that electricity wants to return to earth. Actually, electricity wants to return to source.

  • For instance, electrons at a battery's negative terminal want to return to its positive terminal.
  • The positive terminal is electron-deficient, but it hungers for electrons from the negative terminal; not any of the phases from the isolated 3-phase delta generator nearby.
  • Current from one of the phases from the generator wants to return to another phase, not the battery and not the station chassis or that big blue marble down there.

All this is confused by a technique used in mains wiring: where the largely-isolated AC power systems are intentionally bonded to earth. I have run a normal 120/240V AC power system fully isolated. But it can float up to unexpected voltages; indeed I had a 120V leg jump up to 240V above ground, and neutral floated at 120V above ground. Imagine if it had floated to 2000V above ground, say, due to a transformer leak? To prevent this, we add an equipotential bond to clamp the system to a near-earth voltage. It's cheapest to bond directly to one system wire, and that wire is labeled "neutral".

Because of this equipotential bond, "hot" still wants to return to source (neutral or another hot) - but earth will do, only because it's connected to neutral. That is where that misconception comes from.

On a vehicle, the idea of "earth" is replaced by "chassis". The choice to either bond or fully isolate is made on a case-by-case basis. Generally you bond one system and isolate the other(s). (Cars, diesel locomotives: low-voltage; electric subway cars: high voltage because you have to).

And on vehicles, the chassis is often also used as a normal current return for the bonded system, effectively merging the function of ground and neutral.

An isolated system typically has one connection to chassis: via a "ground-fault relay". Any leakage current to chassis will attempt to return via the ground fault relay, tripping it.

I assume spacecraft will be like aircraft; the choice to bond vs isolate will be decided on both safety and weight considerations. Lower voltages need thicker wires to carry the same effective power, so they benefit most from a massive chassis as a current return.

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    $\begingroup$ I think this is a nice answer once I got a ways into it, but the beginning kind of threw me. I think you could have led with: 'On a vehicle, the idea of "earth" is replaced by "chassis"' rather than discuss where electricity wants to return. In my view, charge carriers go where the local electric potential tells them to go. I'm not sure the concept of them wanting to return somewhere is so useful. $\endgroup$ Oct 15 '19 at 20:28
  • $\begingroup$ @WaterMolecule Good suggestion. Agreed, the semantics of describing potential are difficult. Some people object to describing it as desire, but then, you have "potential telling them", so not a lot of good options. $\endgroup$ Oct 15 '19 at 20:44
  • $\begingroup$ I'm kind of a fan of the Hydraulic Analogy. Picture the poles as two ends of a river, with of course one end at the highest elevation, and the other at the lowest. The water just wants to go downhill. $\endgroup$
    – T.E.D.
    Oct 16 '19 at 18:32
  • $\begingroup$ Yes! An excellent real-world example (which predates the broad use of GFC/RCD devices) is the British BS4573 shaver outlet. These are common in UK wet rooms and are powered by a simple isolation transformer. By going through the isolation transformer the bond to earth is broken and electrocution path to ground with it. The only way to get a shock from such an outlet is to complete the circuit back to the other pole (and not by dropping the shaver into your tub...). $\endgroup$
    – J...
    Oct 16 '19 at 19:08
  • $\begingroup$ The article linked in comment on the other answer says that the spacecraft is biased to negative voltage when it's solar panels are lit, suggesting that the “neutral” (negative as it is DC) terminal is bonded to the chassis. $\endgroup$
    – Jan Hudec
    Oct 16 '19 at 20:04

The answer is given in the thread linked in @KaushikGhose's comment:

The space station solar arrays operate at 160 VDC. When the arrays are producing power, the station structure will also tend to float to a voltage close to the array voltage. Under these conditions, the space station could be subjected to problems like arcing from its surface to the surrounding environment, or arcing to an astronaut. To avoid these problems, the structure has been grounded with a Plasma Contactor Unit (PCU). To protect the astronauts from shock hazards, the PCU is operated during all spacewalks.

The PCU acts as an electrical ground rod to connect the space station structure to the local environment and harmlessly dissipate the structure charges. Glenn [NASA Glenn Research Centre - ed.] engineers designed, manufactured, tested and installed the hollow cathode assembly, which is the critical component of the PCU. The Hollow Cathode Assembly performs this function by converting a small supply of gas into ions and electrons and discharging this stream to space. The stream carries with it the excess electrons that created the surface charge.

From NASA Factsheet PS-00537-0811, "Powering the Future".

That covers operations on the ISS and during EVA's. The question remains what to do about differently charged vehicles about to dock. While docked, the international docking standard specifies a

Ground Safety Wire [that] provides bonding ground connection between vehicles

However, I couldn't find any information on whether or not all incoming vehicles are required to run a PCU. If not, the just mentioned ground cable might possibly be overloaded. Especially, I'd expect the docking standard to specify some sort of maximum potential difference or alike, on which I also couldn't find any information.

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    $\begingroup$ About the last paragraph: There is no way to "overload" a cable. The stored charge is so low, even if the vehicles are at 5kV different potential, the resulting shock is about as much as you can get from an electric fence (the ones used around meadows). $\endgroup$
    – asdfex
    Oct 15 '19 at 20:57
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    $\begingroup$ @asdfex Cables can be overloaded, but yes, it would be quite surprising if they could be overloaded by a discharge of static electiricity. Overloading a cable usually takes quite a high current for several seconds (at least). $\endgroup$
    – user253751
    Oct 17 '19 at 8:48

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