Why do some systems use multiple float switches (3 or 4 floats) in one tank?

Complex systems (like duplex pump lift stations) often use multiple floats to manage different stages: typically an “off” float (to stop pumps at low level), a “lead pump on” float (to start the first pump), a “lag pump on” float (to start the second pump if the water keeps rising), and a “high-level alarm” float

In practice, an industrial lift station will use multiple floats: heavy-duty pump floats to start/stop pumps, and possibly a separate float for high-level alarm. If your pump is a “non-automatic” type (no built-in float), you’ll need an external float switch. Use a pump-duty float (or a piggyback style) if wiring directly to the pump’s power; use a control-duty float for sending signals to a control panel or alarm. Using multiple floats allows sequencing of pumps (alternating lead/lag pumps for even wear) and provides redundancy. For instance, many specifications call for four floats in a duplex system so that you get a separate alarm if both pumps can’t keep up.

In smaller systems, you might just see two floats – one turns the pump on at a high level, off at low level, and an optional third for alarm. The reason a single float usually isn’t enough is that you want a difference in levels between pump-on and pump-off to avoid rapid cycling. Multiple floats (or a dual-point float) give you control over that differential. So, more floats equals more precise control and backup signals in an industrial setup.