Duplex Pump Control With A Single Float Switch

Rapidly changing liquid levels can overwhelm a single electrical pump, and cause excessive wear and tear. To prevent this, additional pumps can be added to control the liquid level.

A common example of this is a duplex lift station; liquid is pumped from a well to a higher elevation, where gravity can carry the liquid to the next station. Lift stations are used primarily in the wastewater industry, to carry liquid waste to the treatment plant.

A duplex lift station uses two pumps: a lead pump and a lag pump. When needed, the lag pump turns on in support of the lead pump.

The KA-4H5E is an example of a float switch built for duplex pump control, and we will cover how to wire the pump float for this application. This particular model’s function is for duplex pump empty control with an isolated high alarm. Duplex pumps are used in many more applications than lift stations, and are often used for fill control as well.

The contacts in the KA-4H5E are high performance miniature switches. Because the single float switch will be immersed in liquid, it is not recommended to switch pump motor currents directly through the KA-4H5E. Rather use a heavy duty relay or contactor between the KA-4H5E contacts and the pumps.

Wiring the KA-4H5E sensor is done in only a few easy steps as shown in the illustration below.

Note: All wires of the KA-4H5E are black with the conductor number printed in white on the wire jacket.

1. Connect conductor # 1 (common) to the Line Power.
2. Connect conductor # 2 (Lead Pump Relay Switch) to the switch side of relay/contactor R1 and the common side of Relay contact R2A.
3. Connect conductor # 3 (Lag Pump Relay Switch) to the switch sides of relay/contactor R2 and Relay contact R2A.
4. Connect conductor # 4 (Alarm Power) to the Alarm Power.
5. Connect conductor # 5 (Alarm Relay Switch) to Alarm Bell or Light.
6. Connect the power side of the relay/contactors R1 and R2 to the Line Power.
7. Connect the relay/contactor power contacts R1B and R2B to the pumps.  If the pumps use three phase power, use heavy duty three pole switch contacts.

Note: An Alternator control is usually added to Duplex Pump Control Circuits. This control automatically alternates the order in which the two pumps come on. If this is not done the user will eventually have a worn lead pump and a barely used lag pump.

With the KA-4H5E properly wired, it will direct the two electrical pumps to empty the tank or well. This particular float switch is normally open, so when the float is hanging vertically, both pumps are off. As the level rises, the first contact closes at the low level, and the lead pump is turned on. If the lead pump float is insufficient to keep the level from rising further, the lag pump is also turned on with the closing of the second contact at the high level.

Because the float switch has hysteresis built in, both Duplex pumps will stay on until the level drops below the first contact at the low level, all the way to the off position known as the low low level. If both pumps are unable to control the rising liquid level, the final contact is closed, setting off the alarm at the high high level.

As noted, a pump alternator switches the roles of the two pumps, so they take turns as lead and lag.

Still have questions about float switches and pump control? Contact our Measurement Experts today. Using a single float switch such as the KA-4H5E will streamline your pump float control, and prevent the hassle of using multiple floats for the same application.