How are astronauts in the ISS protected from electric shock?

Question

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?

Answer 1

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.

Answer 2

"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.

Answer 3

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.