How to Verify Wet-Well Float Backup Switches

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Confirm Float Elevations and Free Movement

Start at the drawing and confirm each float's set elevation. The backup pump-control float, where one exists, is set above the analog lead pump-on so it only acts if the analog control has already failed to start the pump. The backup high-level float is set above the analog high-level alarm and below the lowest incoming invert, so it catches a level the analog path missed but still annunciates before sewage backs up the collection system. A float set at or below the analog setpoint it is meant to back up is useless, because it trips at the same time as the path it is protecting.

Physically confirm each float hangs freely and tilts through its full arc without snagging on the pump cables, the guide rails, the wall, or another float. A tethered float switches on the angle it reaches, so a float that catches on a cable never reaches its switching angle and never acts. Confirm the cable is secured at the correct length at a bracket that will not slip, because a float that slides down its cable over time drifts to a different, unknown elevation and quietly defeats itself.

Prove Independence From the Analog Path

The whole value of a backup float is that it works when the primary control does not, so the verification has to break the primary path. Simulate a failed analog sensor by forcing the analog level input to a fixed value, or by disconnecting the transmitter, so the controller believes the well is at some benign level and will not act on rising water. This puts the station into exactly the failure the floats defend against.

With the analog path blinded, physically lift each backup float in turn and confirm the intended action happens through wiring the analog controller cannot influence. The high-level float should sound the alarm and, where designed to, start a pump, even though the controller still thinks the well is low. If lifting a float does nothing while the analog input is blinded, the float's action is routed through the same logic as the primary control and it is not a true backup. That independence check is the single most important part of this procedure and the one most often skipped, and it complements the design intent described in the wet-well float backup control guide.

Restore and Confirm Normal Handoff

Return the analog sensor to service and confirm the station resumes normal analog control cleanly, with the floats reverting to their standby role. Watch one full pump-down under analog control to confirm the floats are not interfering with normal operation, which happens when a backup float is set so low it acts during ordinary cycling. A backup that trips on every cycle is not a backup, it is a second control that fights the primary.

Confirm the alarm from a float override reaches an operator. A backup float that starts a pump silently has saved the day but told nobody the primary sensor failed, so the underlying fault runs undiagnosed until the floats too are overwhelmed. Route the float-override and high-level-float conditions to the site's notification path so a person is dispatched to fix the real sensor, which ties into the station's SMS alerts in SCADA.

Verifying the Result and Common Mistakes

A verified backup float scheme has each float at its drawn elevation, hanging free, acting on the correct rising level, and doing so through wiring that survives an analog sensor failure, with every override annunciated. Record the as-left float elevations and the independence-test result so the next inspection has a baseline. On a monitoring platform, a float that has taken over is visible as a divergence between the analog level and the pump state, which is a useful early warning that the primary sensor has failed even before a person reaches the site.

The most common mistake is setting a backup float at or below the analog setpoint it protects, so both trip together and the backup adds nothing. The second is verifying the float with the analog sensor still live, which cannot prove independence because the analog control might be doing the work you attribute to the float. The third is a float fouled on a cable or slipped on its tether, which reads fine on a drawing while never reaching its switching angle in the well.

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