Do you have a water heater? and if so, does it have an electric heating rod in it?
I retrofitted a heat rod to my heater and installed a switch similar to this with a red pilot
light so I would know when it is turned on. Reason: You will ruin the heat rod should the tank be empty and the rod turned on.
Go and look at your water heater on the outside and if you see wires attached where the drain should be, you have a rod installed.