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The reference point for “Normally Open” or “Normally Closed” is based on the state of the electrical circuit when the float is down (at rest, low level). In other words, hold the float switch by the cord so that it hangs directly down toward the ground , and that’s considered to be “at rest”. We have the variation of “Normally Open/Normally Closed” in a single float switch, which is what Sump Alarm dominantly sells – one float switch that can do either application.
A Normally Open (NO) float switch is open when the float hangs down (no power flowing). As the water rises and lifts the float, the float switch closes and power flows . Let’s consider that we have a lift station that we want to keep empty. This is a typical “Normally Open” application. We want the pump off until there is a good amount of fluid present, then we want to run the pump until the vessel is empty, and turn off again. That’s “normally open”
A Normally Closed (NC) float switch is the opposite: the circuit is closed (conducting) when the float is down, and rising water opens the circuit to cut power. NC floats are used for filling a tank or for low-level alarms. So, if we have a tank that is for cattle being fed by a pump in a nearby pond, when the float switch drops, the pump turns on until the float switch raises to a point where the float switch raises, and then shuts off again.
Sump Alarm focuses on “three wire” or “NO/NC” float switches that do both. So each switch comes with 3 wires. There is black wire to which power is connected. When the float is down (at rest) the power is returned on he blue wire, and when the float switch is raised the power is returned on the maroon wire. Why do we do this? Its just not that expensive to include a third conductor, and for our industrial customers, it offers them a “Single SKU solution” where they can stock 1 or 2 SKU’s and use them in multiple applications. It also means there is no confusion on which one a customer needs to purchase – it does both.