Mauro Diotallevi wrote: > A couple of semi-random questions... > > I was recently watching yet another scifi tv episode where a hit to > a > spaceship causes sparks from electrical equipment. I would think > something as simple as fuses would stop that, but I'm not an > electrician. In the "real world," would fuses stop that kind of > thing from happening?
You see the same sort of effect when the power lines go down, even the lower voltage lines on the pole behind your home. (Well.......your service entrance could be underground, but I'm sure you know what I'm talking about.<G>) I'm an electrician and I've worked at NASA-JSC. I've peered into the innards of some of the mockups which are to varying degrees "exact" copies of the real deals, so I could hazard a few hypothesis based on what I know with certainty and what I've seen and think I understand, at least until someone who knows better speaks up.<G> There are 2 ways to stop short circuits and overloads. You can use a fuse or a breaker. Fuses are good for a single event and then they have to be replaced.This is obviously quite a nusiance if you have an abundance of circuits and it also means you have to maintain a stock of "precious space" eating replacements. My guess is that fuses will only be used for equipment that needs to be protected *very* fast. (Fuses blow faster than breakers trip, can be sized more precisely to a given load, and are much more reliable at protecting circuits. But in general their disadvantages likely outweigh their advantages in a complex machine like a spacecraft or habitat.) Breakers have the advantage of re-useability and that is just great. But there is a dirty lil secret concerning breakers. If a breaker has tripped just one single time all bets are off and it is no longer covered by a garuantee. Sure, experience tells you that you can reset a breaker numerous times and everything will work fine, but the manufacturers will not honor a garuantee beyond the first trip (even though they test them by tripping them at the factory and they come in the "trip" position......go figure). I'm sure aerospace uses the highest quality with upgraded capability but the same physical constraints we have on earth still apply in space and therein lies the problem. When breakers fail, they usually fail to trip. I have tried to trip bad breakers and it looks like I'm welding (This is a good reason to be wary of Federal Pacific [FPE]breakers......they are notorious for this). So you have a physical shock, and a short-circuit/overload situation...... well you could have some sparks for a few seconds, but it wouldn't last all that long. > Also, how do you ground electrical equipment in space, where you > don't have easy access to actual ground? To some degree it doesn't matter all that much. If every part of a craft is electrically tied to every other part you have an "equipotential plane" that effectivly *is* your ground/reference ground. Any voltage you use is above the (even if non-negligable) equipotential plane and 120V is still 120V as far as the craft is concerned. Any difference in potential between the craft and the planet is irrelevant because the systems are isolated from each other by space. If you think about it, every body in the solar system (hell...the universe) has a different electrical potential (not that it ever becomes relevant <G>). Now I'm wondering if there are sparks when a ship docks at the space station because I don't see any way that 2 crafts having travelled in quite different paths will have the same potential at docking. xponent Semi-Expertise Maru rob _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
