Article written by: Jason Danaher Ph.D.
In aquatic facilities, a float switch (Figure 1) is used to detect the level of water in a tank or sump. While it is a simple device, a float switch is fundamental to control the flow of water into and out of the tank and ensures the water level remains within a safe range for the aquatic organisms. The float switch can activate a pump to start or stop and thus prevent overflow events from occurring or avoid over draining a tank if the operator has to step away and lose visual on the system.
Figure 1. A float switch typically used in an aquatic facility to drain or fill a tank.
Most float switches operate with a simple internal contact. Understanding the difference between a normally open and normally closed contact is crucial for selecting the appropriate float switch type for specific applications, ensuring both functionality and safety for the aquatic system.
Whether a float switch is normally open or normally closed depends on what the contact in the float switch is doing when it’s in a normal or “resting” state. In our example, normal state means the float switch is hanging straight down at the time of a low water level. In this instance, an easy way to picture the float’s resting position is to hold the float switch by its cord and let the float hang toward the ground. It would resemble a position similar to Figure 1. An open contact means the circuit is off and electricity is not flowing while a closed contact means the circuit is on and electricity is flowing. Often the electrical connection between the float switch and pump is a simple piggyback plug allowing the operator to connect the pump and float switch into one another and the same outlet. The electrical load for the float switch must be matched to the pump it will service and electrical outlet supplying the current.
A normally open (NO) float switch is open when the float is hanging down, so no power is flowing (Figure 2). As the water level rises and lifts the float, the contact inside the float switch closes and activates the pump by turning the power on. This setup is commonly used to drain tanks or places like effluent sumps, where you want the pump to stay off until the liquid level rises and there’s sufficient effluent volume to pump. Once the effluent level reaches a predetermined level in the sump, the pump turns on, runs until the effluent level is decreased or completely drained, and then shuts the pump off again. A NO or “pump down” float switch activates the pump as liquid rises with the normal state being when the float is down.
Figure 2. Activated and normal state for a normally open (NO) contact switch.
A normally closed (NC) float switch works the opposite way (Figure 3). When the float is hanging down, the contact is closed and activated allowing the pump power to be on. As the water rises in a tank, the float lifts and opens the circuit, shutting the power off. These switches are often used for filling tanks or to activate low-level alarms monitoring water depth. For example, in a closed-recirculating system where make-up water is supplied by a pump, the pump turns on when the water level drops. As the production system fills and the float rises at the monitoring location, as a result of an increase in water depth, the pump shuts off at the water’s source to prevent overflow. A NC or “pump up” float switch stops the pump as liquid rises with the normal state being when the float is down.
Figure 3. Activated and normal state for a normally closed (NC) contact switch.
Choosing between a NO (pump down) and a NC (pump up) float switch depends on the specific application and desired control over the water level in a tank. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com for additional information on which type of float switch may be appropriate for your aquatic system scenario and operation.