I've got a similar set up as you (i.e. 30g tank w/10g sump).
I've also submerged the output of my return into my main tank, but to prevent flooding during a power outage, I had cut a hole just below the waterline on the submerged output. When power goes out, some water is siphoned back into the sump, but as soon as the waterline of the main tank goes below the hole, the siphon is broken and a flood is averted. Just make sure that the sump isn't too full.
For more detailed info, you probably want to check out
http://www.melevsreef.com/allmysumps.html .
Cheers,
Ben