Float Switch These vertical float switches are suitable for smaller-diameter sump basins since they use a float that moves a rod straight up and down to activate the switch. The switch starts and stops the pump automatically when the water reaches a preset level.Sump and sewage float switches have a float that raises and lowers depending on the water level in the sump basin to automatically control switch operation. When water rises to a preset level in the sump basin the switch turns on and the pump starts. When the water is drained down the switch turns off and the pump stops. Sump and sewage diaphragm switches sense pressure against the diaphragm to detect how much water is present in the basin. As the water level rises in the sump basin, the air pressure pushing against the diaphragm b.