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On Earth, we have a good reason to deploy electric devices with an "Earthing system": this helps to avoid dangerous leak currents on surfaces.

Now how is this solved on a spacecraft?

PearsonArtPhoto
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J. Doe
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    Not all devices on Earth are grounded, see chapter Ungrounded systems in Wikipedia about ground. Automobiles, air crafts, ships and also space crafts don't use an electrode buried into the ground. The frames may be used as a common ground, but this is not the only solution. – Uwe Mar 14 '18 at 15:34
  • The answers at https://space.stackexchange.com/questions/5062/strategies-for-combating-esd-and-ground-plane-potential-shifts-on-spacecraft-cha may be helpful – Erin Anne Mar 14 '18 at 19:29
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    Am I the only one who keeps seeing Earthling System by mistake? – uhoh Mar 15 '18 at 13:48
  • No. No, you are not. – Organic Marble Mar 17 '18 at 00:12

2 Answers2

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The frame of the spacecraft is used as a common ground. See this diagram from this handbook.

enter image description here

PearsonArtPhoto
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  • could be there a situation if some high power devices would outbalance this design? after all the frame has limited capacity. – J. Doe Mar 14 '18 at 15:26
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    They usually have quite a bit of capacity. It isn't that dissimilar from Cars and Airplanes, and those don't really have major grounding concerns either. – PearsonArtPhoto Mar 14 '18 at 15:27
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    Using the frame of the spacecraft as a common ground is not the only solution used, see capter 4.3 of the handbook. Table V mentiones Viking and Seasat as isolated from structure. – Uwe Mar 14 '18 at 15:33
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    @J.Doe The ISS does NOT use the frame as a power return path. It is ground, yes, but all electric loads have a power supply and a power return wire correctly sized to handle the load supplied. But let's say that it used the frame as power return: The ISS has a maximum bus voltage of 160V, and a regulated bus of 124V. The solar panels can produce a maximum of 120kW, and the batteries 158kW. At 124V that's just north of 1,250A. Aluminum can carry 700A per square inch of cross section. At 14 feet diameter, 2 square inches of aluminum cross section would only require 0.000316" thick aluminum. – Adam Davis Mar 14 '18 at 16:26
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    And that's considering just the one outermost shell, and that's less than half the thickness of even thin aluminum foils (range between 0.0008" and 0.001"), which is far less than what the shell of the space station is made of. There's a lot more aluminum in a lot more layers and framework, and aluminum is a good conductor. They don't use it for power return, but if they did, and even if they put the full capacity of the batteries across the entire length of the structure, any gradient would be very, very small. – Adam Davis Mar 14 '18 at 16:31
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    @AdamDavis The batteries wouldn't normally be discharging at full tilt when the panels were close to their maximum output, would they? – Will Crawford Mar 14 '18 at 16:37
  • @Adam Davis: How much current an aluminium conducter may carry depends on cross section, conductor shape and conductor cooling. A rectangular bar may carry more current than a cylindrical conductor with the same cross section. If under certain conditions 700 A per square inch is possible, a round conductor with 4 square inch may not carry 2800 A. Cross section is propotional to the square of diameter, but the surface of the conductor is proportional to the diameter. The heat is transported over the surface of the conductor. – Uwe Mar 14 '18 at 16:45
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    @WillCrawford No, the batteries are charged and discharged with each pass when the orbit leaves the station with no solar power. I used worst case calculations, but neither the solar panels nor the batteries will ever see 1,200A in normal usage. – Adam Davis Mar 14 '18 at 16:50
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    @Uwe Correct, and the space station skin is surely a magnitude of order more thick than aluminum foil, and I also didn't include all the rest of the structure which is even heavier gauge aluminum. I didn't bother to consider it in more detail because even if the station was made of aluminum foil and nothing else, it would be fine. If a more detailed analysis is required, consider submitting a new question. – Adam Davis Mar 14 '18 at 16:56
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What PearsonArtPhoto said in the answer above, but also, if there is a need to ensure the vehicle is "grounded" relative to the plasma environment it's in (i.e., to prevent arcing or corona discharge), a plasma contactor is used.

Tristan
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