Sewage pump float switch explained

Sewage Pump Float Switch Explained

Three tether float switches. From left to right: MPM series, KPM series, and JPM series. Due to its shape and size, the MPM series is best suited for sewage water applications.

Figure 1: Three tether float switches. From left to right: MPM series, KPM series, and JPM series. Due to its shape and size, the MPM series is best suited for sewage water applications.

A sewage pump float switch moves up or down with the liquid level in a sewage basin. It is an electromechanical switch that turns the basin's sewage pump on or off, depending on the level of the liquid. This guide further explains this process and discusses how to install, troubleshoot, and repair sewage pump float switches. For more information on float switches, read our float switch overview article.

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What is a sewage pump?

A sewage pump transports waste material from a sewage basin to a septic tank or a sewer system. This pump sits on or very near the bottom of the sewage basin. Different pumps are applicable for residential, municipal, and industrial systems. In residential applications, small submersible pumps can handle soft solids up to 2 inches (50 mm) in diameter.

The difference between sump pumps and sewage pumps is the application. Sump pumps prevent basements from flooding during non-extreme rainy weather conditions. Sewage pumps in residential applications pump wastewater from bathrooms, kitchens, and other plumbing fixtures into a sewage basin. Sewage pumps may be applicable as sump pumps. Check the pump's specifications to see its range of applications.

Sewage pump float switch operation principle

A sewage pump float switch, also known as a tether float switch, fastens to the discharge pipe approximately 100 - 150 mm (4" - 6") above the pump's motor. This ensures that the float switch will float above the pump's minimum liquid level and turn on the pump before the sewage overflows the basin. Typically, tether float switches have an electromechanical reed switch and magnet inside. A simple reed switch has two metal contacts sealed inside a glass tube. In a non-magnetized state, these contacts can be touching (normally closed) or not (normally open). When a permanent magnet moves over the contacts, they will move to the opposite state (closed to open or vice versa). When the magnet moves away, the contacts revert to their normal state.

If the sewage level in a basin rises high enough to tilt the float switch 45° above horizontal, the magnet within the switch slides over and operates the reed switch. In a typical configuration, the reed switch is open until this point. When the reed switch closes, an electrical current can flow through it and signal the sewage pump to turn on. When the sewage level in the basin drops and the float switch tilts to 45° below horizontal, the magnet slides away from and opens the reed switch, signaling the sewage pump to turn off.

Selecting a sewage float switch

There are three aspects to consider when choosing between float switches for a sewage pump:

  • Material: Sewage is highly corrosive. Therefore, choose a float switch with a PVC or Polypropylene casing.
  • Size: Very small float switches are less buoyant and, therefore, less reliable.
  • Power: In a typical configuration, the pump's power flows through the float switch. Therefore, know the electrical load in the system. Ensure that the wattage, voltage, and amperage do not exceed the max capabilities of the reed switch. For example, a 1/2 horsepower pump operating at 115 volts draws a 9.8 amperes load. Multiply these two values together to get the wattage - 1127 watts. None of these three values should exceed the maximum capability of the reed switch.

Sewage float installation

Before installing a float switch in a sewage system, understand these two factors: power connection and application.

  • Power connection: Float switches come with a piggyback plug, or they do not. The switch's piggyback plug can plug into an outlet, and the pump plug can plug into the back of the piggyback plug. A float switch must be hardwired to a control panel and the pump if it does not have a piggyback plug. The control panel's neutral and ground wires connect directly to the pump's neutral and ground wires, respectively. One wire from the float switch connects to the control panel's hot wire, and the other connects to the pump's hot wire. It is normal for the float switch's wires to be white and black. However, consider them both to be black for the purpose of connecting to the hot wires.
  • Application: Float switches have two applications in a sewage system: operating the pump and controlling an alarm. Alarm control switches are typically narrow-angle float switches, meaning they only need to rise or dip 10° above or below the horizontal to sound an alarm.
    • High-level alarm switches: These switches should be normally open down, meaning they must rise 10° to close and activate the alarm. Install these sewage floats higher up the pump's discharge pipe.
    • Low-level alarm switches: Conversely, low-level alarm switches should be normally open up, meaning they close when the switch falls 10°.


What is a sewage pump float switch?

A sewage pump float switch automatically operates the pump by using liquid level information. When the level is high, the float switch turns the pump on. When the level drops down enough, the float switch turns the pump off.

How can a sewage float switch be tested?

If the pump is not running, pull the float switch up. If the pump turns on, the float switch is still operational.

How can a float switch fail?

A float switch typically fails if it is not applicable to the system it is operating in or not rated correctly for the power usage. For example, a metal float switch that does not have high corrosion resistance is not suitable for a sewage system.

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