Float Switches for Simplex Pump Control

Last week, we introduced lift stations and the sensors that power them. You can check out the introduction here. This week, we will discuss a common lift station application: simplex pump control.

Simplex pump control is a relatively simple task. A float switch with a bit of hysteresis will do just fine. There are, of course, continuous level sensors that may be advantageous under the right circumstances.

Controlling pump activity on a simplex lift station is fairly straight forward, but there are a variety of approaches - some of which have great benefits and are relatively unknown. For those who are new to lift stations, simplex is a fancy name for a one-pump lift station. Using one pump makes it cheaper, but it doesn't have the capacity or the redundancy of a duplex lift station, which we will discuss next week. Today we'll discuss 4 different ways to use float switches and the 3 different float switches that can be used.


At the end of the day, all we are trying to do with simplex pump control is to turn on a pump when sewage levels reach a certain point and turn off the pump once levels drop. A single, normally open float switch can make this happen. As sewage levels rise, the float switch is closed, and the pump can be started. As the levels drop, the same float switch returns to its open stage, shutting off the pump.
Mission accomplished, right? Well, not necessarily. Using a single switch causes the pump to be turning on and off far too often. In its worst instance, this is referred to as "chatter," when turbulence causes the float switch to bounce back and forth between open and closed. This is no way to treat a pump, as it greatly increases wear and tear, and causes the pump to overheat. To solve this problem, we need to move to our next option - a system with hysteresis. But before we get any further, let's define the term "hysteresis".


  1. The lag in response exhibited by a body in reacting to changes in the forces, especially magnetic forces, affecting it. Compare magnetic hysteresis.
  2. The phenomenon exhibited by a system, often a ferromagnetic or imperfectly elastic material, in which the reaction of the system to changes is dependent upon its past reactions to change.


Clear as mud, right? To put it simply, hysteresis in pump station control puts an end to chatter by delaying the return to the electrical state indicated by the switch's normal state. It means that once the pump starts, it's not going to stop for a while. This is accomplished with multiple switch points and a relay controller or PLC.


To make hysteresis work with your basic float switch, you are going to need more than one. The first float switch, which is activated at higher levels, is used to start the pump, while a second float, which is activated at lower levels, is used to stop the pump. This configuration provides hysteresis but requires two float switches, which increases cost and maintenance concerns.

The most common maintenance issue for multiple float setups is tangled float cables, often caused by swirling action from the pump running. Tangled float cables impede the floating (i.e. switching) action of the switches, causing the pump to run continuously in a dry well, or causing the pump to not start - resulting in an overflow (SSO). And with two switches you are twice as likely to have a failure due to a malfunctioning switch.

Diagram showing difference between multi-level and single level switches

This is where we like to introduce Kari floats. These float switches are uniquely built to provide multiple switching points in a single float sensor. As the float pivots from hanging straight down, to floating at its highest point, it opens or closes different switches within the float body. It's like having up to 4 float switches in one!


KARI float model KA-2H multi-level cable float switch

One Kari float model, the KA-2H has a switch that is activated when the float is high for turning the pump on. As levels drop, it keeps the pump on until it has reached a low level. This ingenious sensor can provide the necessary hysteresis without using any type of controller or relay contacts! With only one sensor in the lift station, maintenance is simplified, and by eliminating the need for a controller, costs and complexity are cut.


Switch chart of KA model 3Y39 four-conductor, three Normally Open switches in single float

Another Kari float that has clear benefits for simplex pump control is the KA-3Y39. This product can directly replace a common three-float system (off, on, and high level). The KA-3Y39 would be wired to a controller or pump control relay in much the same way as three individual floats would be, but with a single Common wire (wire number 1 in the chart above), instead of one for each float. Now you are getting the benefits of a three float switch system, without the potential problem of tangled floats.


Now, let’s look at a specific Use Case: we’ll focus on wiring a KA series cable suspended float switch for simplex pump control. For this example, we will use the KA-2L, which is simply a dual-level float switch with hysteresis for fill control applications. Here we go:

The contacts in the KA-2L are high-performance miniature switches. Because the float will be immersed in liquid, it is not recommended to switch pump motor currents directly through the float switch. Instead, use a heavy-duty relay, or contactor, between the float switch and the pump.

Wiring the KA-2L sensor is done in only a few easy steps as shown in the illustration below.

Fill control wiring diagram for KA-2L float switch

  1. Connect conductor # 1 (white) to the Line Power.
  2. Connect conductor # 2 (black) to the switch side of relay/contactor.
  3. Connect the power side of the relay/contactor to the Line Power.
  4. Connect the relay/contactor power contacts to the pump.

Note: The relay/contactor R1 should be a Double Pole Single Throw (DPST) type with one contact R1B rated to carry the voltage and current required by the pump’s horsepower rating.

When the liquid is at the low level, the pump will start filling the tank with liquid. When the liquid reaches the full level point, the pump will turn off.  The pump will stay off until the liquid falls back to the low level point where the process will repeat.

As always, let us know if you have any questions! Next week we will take it up a notch, and discuss duplex pump control with float switches, before moving on to continual level measurement.

Have questions about float switches and pump control? Contact our Measurement Experts today.

Explore Kari Float Switches


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