Fault finding: the stuck Lead float
A duplex sump has Stop, Lead and Lag floats. The Lead float arm gets stuck in the closed position. What does edge-triggered alternating logic do on the next fill cycle? Predict it, prove it in the simulator, and fix it.
Loads the panel and the ladder program from the video. Press RUN PLC and try it.
The question
In a duplex pump station the Lead float starts the lead pump, the pump runs until the level is below the Stop float, and the Lag float starts the second pump if the water keeps rising. The pumps alternate on every rising edge of the Lead float. Now the Lead float arm sticks in the closed position, and the sump drains below the Stop float. What happens on the next fill cycle?
The answer
- The pump still stops at Stop, because the run rung needs the Stop float too.
- There is no new edge. The Lead input never goes off, so it can never rise again. The toggle bit freezes and the same pump is chosen every time; the other pump never runs.
- The pump starts at the Stop level, not at Lead. The start condition (Lead) is already true, so the pump starts as soon as the rising water closes the Stop float, pulls the level just below Stop, stops, and starts again. This is short-cycling: many starts per hour with very little water per start, which overheats the motor and wears the contactor.
Note that it depends on the alternation design. Many alternating relays toggle on the falling edge of the pump call instead; with them the pumps keep alternating, but both short-cycle.
Prove it in the simulator
Starting from the alternating pump solution (tank LOW = Stop float, HIGH = Lead float), add a selector switch called LEAD STUCK wired in parallel with the HIGH switch, so switching it on holds the Lead input on, exactly like a stuck float. With INFLOW on, pump A switches on and off several times a second at the LOW level, M0.0 never changes, and pump B never runs.
Three fixes
| Rung | Contacts | Output | Fix |
|---|---|---|---|
| 1 (changed) | N M0.2 (falling edge: a pump stopped) | M0.1 | toggle on pump stop, so alternation can never freeze |
| 3 (changed) | (M1.1 OR M0.2) AND I0.0 | M0.2 | start only when allowed |
| 7 (new) | NOT M0.2 | TON T0, 2 s | rest timer while the pumps are off |
| 8 (new) | I0.1 AND T0 | M1.1 | start allowed: Lead, and rested |
| 9 (new) | I0.1 AND NOT I0.0 | S M1.0 | float-order alarm: Lead on while Stop is off is impossible |
| 10 (new) | M1.0 | Q0.3 | FLOAT FAULT lamp |
With the fixes, a stuck Lead float no longer freezes the alternation, each pump rests 2 seconds before a restart (minutes in a real station), and the FLOAT FAULT lamp tells the technician exactly which float to check. A real duplex station would also run on Stop and Lag only once the fault is detected.
How to find this fault on a real site
- Look at the input LEDs: a Lead input that stays on while the level is below Stop is impossible for a healthy float.
- Count pump starts per hour; short-cycling shows up as a high count with short run times.
- Compare the run hours of the two pumps: one pump with all the hours points to frozen alternation.
Practise more faults like this in the PLC troubleshooting practice challenges.
Try it yourself
Build it from scratch in the free PLC Sim Lab Playground, or open the finished solution and change it:
Loads the panel and the ladder program from the video. Press RUN PLC and try it.
Frequently asked questions
What is pump short-cycling?
A pump that starts and stops far more often than designed, each time moving only a little water. It overheats the motor windings and wears the starter contacts.
How can a PLC detect a stuck float?
By checking float order: with healthy floats the Lead float can only be closed when the Stop float is also closed. Lead on while Stop is off is impossible, so it is a reliable alarm condition.
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