Ok, I'll jump into the fray. I liked maintenanceguy's battery example..that's the first example that popped up in my head when I read Cindy's question. Redsy's comments were right on too. Instead of the 'center-tap' secondary situation..let's simplify things and just think of a single winding for the secondary, with the "neutral" gounded (technically its not a neutral, just the "grounded conductor", like the code describes it). Now, the secondary current will only go out and back from that one winding..and it will take the path of least resistance. The current has no reason to go to ground as long as the wires are intact and/or the ground doesn't link both ends of the secondary. If someone were to connect a ground connection at the load end of the circuit, but still at the "neutral" side of the load, then the current would divide a little, with most of the current going through the "neutral" conductor, since it has the least resistance,and the rest going through ground..the earth and the "neutral would be in parallel. If the "hot" side of the circuit accidentally connected to ground, then we'd have the ground fault situation that causes things to blow up and melt things..unless the breaker or fuse beats it to the punch.
Ok, so, since we're talking ac current, it would seem that the selection of which end is the "neutral" vs which end is the "hot" would be somehow dangerous. What would happen if you picked the "wrong end"..and connected the "hot" to ground? It just wouldn't matter. The secondary acts like its own little alternator..pushing and pulling ..and if there is an external loop of conductor and resistance..current will flow. It wouldn't inject electrons into, or pull electrons out of, the earth unless the opposite end of the winding was also connected to earth.. or the "neutral" was paralleled with the earth.
That is the basic stuff..we haven't touched on statics and capacitive coupling and charging currents of the primary and the derived system..all that is stuff that MAY have some bearing on Cindy's question..but I can't tell at this point.
I'm glad Cindy had the guts to ask this question..I suspect that there are lots of lurkers who are curious about the same thing.
