Most controllers incorporate a double-throw relay with three contacts—common (C), normally open (NO), and normally closed (NC). The differential thermostat’s sensor circuit controls the relay. Electricity is always connected to the common terminal. The installer will normally connect the pump(s) to the NO terminal. Wiring the hot, neutral, and ground wires correctly will complete the high-voltage side of the controller wiring. The sensor wiring is on the low-voltage side. If a second control is needed for another pump, valve, or device, but the design calls for only one controller to be activated at a time, the NC terminal can be wired to the input of the second control. This gives the two controls a priority arrangement. When the first control’s sensors energize the NO contact and the pump, the relay disconnects the NC contact. The second control has no power and cannot energize any devices it controls. When the first control is not energized (off due to hitting the high limit, for example), the second control uses the sensors that are attached to it and responds appropriately.