If the siphon breaks, and the pump keeps going, the water that's in the return section of the sump will get pumped into the main tank, and that's it. There's enough room in the main tank to handle that amount of water (I think), but if not, it won't be much that overflows. The biggest issue if the siphon breaks is the pump will burn up. I'm not worried about the siphon breaking, however, the Hurricane overflow works real well. The more important issue is the power going out. When that happens, the siphon still works, and the water keeps draining. The water will keep draining until the siphon breaks, through the overflow, AND the return line. The overflow's siphon will break when the water level in the main tank reaches the bottom of the 'slots' in the part of the overflow that's in the main tank. I've got a small hole drilled in the return plumbing, just under the normal operating water level of the tank, so the siphon in the return line will break when the water level gets below that hole. You just need to make sure that you're sump will handle the water that gets drained from both the overflow and the return line until the siphon breaks. Easiest way to test that is to get your system up and running, and then pull the plug on your pump, and watch.