Their is a primary sensor which uses infrared to detect the water level, if this one fails their is a float switch that sounds an alarm and shuts everything down. Worst case scenario, their is another safety that only allows the system to run for 10 minutes and once this time has passed it sounds an alarm and shuts down. It comes with a small DC bilge pump that you place in a reservoir, this pump does not handle kalk very well, it should last 1-2 years in normal use, cut that in half if you use kalk. The pumps are cheap at about $19 a piece. I like to use it with the kalk dispenser mainly because this way the kalk is mixed in a sealed bottle which prevents a loss of potency from reacting with CO2 in the air. It also protects the pump. The sensors are generally mounted in sump but to prevent a siphon from the reservoir the water should be pumped up to the tank.