When water enters the bilge and reaches a certain height, 2-3/4" (70mm), a sensor turns the pump on. After the water is pumped out, another sensor shuts the pump off. The sensors' unique “field effect” technology is environment friendly because they only sense the dielectric constant of water. In the event of an onboard oil spill, straight motor oil that enters the bilge will not turn the pump on. Please note that this is not an oil overspill system or a safety measure device.