The short answer
Float Switches
A float switch is a contact that changes state when a float rises or falls past a fixed level. It needs no calibration, no scaling, and no electronics, and it fails separately from every continuous instrument in the well, which is why the high level alarm float is wired around the controller to the alarm and the pumps. Its enemies are rags and cables that hang it up, and its rules are simple: mount it where it cannot tangle, wire it fail-safe, rate it for the area, and test it by lifting it.
Key points
- A float switch is a mechanical contact at a fixed level; it does not drift, but it can hang up.
- Wire critical floats normally closed so a broken wire reads as the alarm condition.
- The high level float goes directly to the alarm and to the pump start relays, independent of the controller.
- Mount floats on a stainless bracket, spaced and weighted, away from cables and the inflow.
- Wastewater wet wells are usually classified locations, and the float and its circuit must be rated for it.
Two kinds
- Tilt float
- A sealed float on a cable with a switch inside that closes when the float tilts past horizontal. Hung on its cable from a bracket, weighted so that it pivots at a set level, it is the standard wet well float. The switching level is set by where the cable is clamped; the differential, the distance between switching up and switching down, is set by the free cable length below the clamp.
- Stem or vertical float
- A float that slides on a vertical rod and operates a switch at the top, or a magnet passing a reed switch in the stem. Compact and precise, used in day tanks, sumps, and vessels where there is no room to swing a tilt float, and vulnerable to solids that jam the float on the stem.
Contacts and wiring
A float has a contact that is either closed or open with the float down, and the choice of which decides what a broken wire means. A high level alarm float wired normally closed, opening when the level rises, produces the alarm when the float lifts and also when the cable breaks, the terminal loosens, or the fuse blows. Wired normally open, a broken wire silences the alarm forever. Critical floats are wired so that failure of the circuit produces the safe indication, and the program is written to read them that way. The convention is written on the drawing so that the next person does not reverse it.
Mounting
- 1
Use a bracket.
A stainless steel bracket at the hatch, with the float cables clamped to it at the elevations the levels require, so that a float can be adjusted or replaced without entering the well.
- 2
Space them.
Floats swing. Two floats close enough to touch will tangle, and one held up by another reads a level that is not there. Stagger them and give each its arc.
- 3
Keep them clear.
Away from the pump cables, the discharge pipes, the ladder, and the inflow. A float that lands on a pipe as the level falls stays there.
- 4
Weight the cable.
A cable weight a set distance above the float sets the pivot point and keeps the float from being pushed around by the flow.
- 5
Route the cable.
Through the hatch to a junction box, with strain relief, and with a seal-off where the area classification requires one.
Classified areas
A wastewater wet well is a space where sewer gas collects, and under NFPA 820, the standard for fire protection in wastewater facilities, the wet well and the spaces around it are usually classified locations under NEC Article 500. That decides the float: an intrinsically safe float on an intrinsically safe barrier in the panel, or a float and a circuit rated for the division, and the conduit sealed at the boundary. A general-purpose float from a hardware store in a Class I, Division 1 wet well is a code violation and a hazard. Read the classification for the site before selecting anything that goes in the well.
Using floats with a continuous instrument
| Float | Level | Function |
|---|---|---|
| High level alarm | Above lag start | Alarm and force both pumps to run, independent of the controller |
| Lag start backup | At or just above lag start | Starts the second pump when the controller or the transducer has failed |
| Lead start backup | At or just above lead start | Starts the first pump on floats alone when the continuous level is bad |
| Stop backup | At the stop level | Stops pumps on floats alone |
| Low level cutoff | Below stop | Stops the pumps before they run dry when the transducer has failed high |
The controller uses the floats two ways: as the backup control when it has declared the continuous level bad, and as a check on the continuous level while it is good. A transducer reading below the lead start level while the lead start float is up is a disagreement, and it is alarmed. The wet well level page covers the arrangement in full.
Testing
A float is tested by lifting it. Raise each float by its cable from the hatch, in order, and confirm the indication at the panel and the action it causes: the lead pump starts, the lag pump starts, the alarm dials out. Lower them and confirm the stop. Do it on a schedule and after any work in the well, and record the result. A float that has not been lifted in a year is a float whose state is unknown.
Frequently asked questions
- Why use floats if there is a level transducer?
- Because the transducer, the input card, and the controller can all fail, and floats fail differently. The high level float wired directly to the alarm and the pumps is what keeps the station from overflowing when everything else has stopped.
- Should a float be wired normally open or normally closed?
- For anything critical, so that a broken wire produces the safe indication. A high level alarm float is normally closed and opens on high level, so an open circuit alarms. Write the convention on the drawing and read the input accordingly in the program.
- How far apart should wet well floats be?
- Far enough that their swinging arcs do not overlap, staggered on the bracket so they do not hang in a line, and weighted so the flow does not push them together. A tangled pair reads levels that do not exist.
- Do wet well floats need to be intrinsically safe?
- Usually. NFPA 820 classifies most wastewater wet wells as hazardous locations, and the float and its circuit must be rated for the classification, typically through an intrinsically safe barrier in the panel. Check the classification for the specific site.
Related topics
- Wet Well Level MeasurementHow lift station wet well level is measured, which technology suits which well, the failure modes that flood a station, and why the high level float must stay hardwired.
- Wet Well Level ControlThe measurement that runs a lift station: choosing setpoints, protecting the pumps, validating the signal, and the failures that flood a station.
- Hydrostatic Level MeasurementMeasuring level from the pressure of the liquid above a sensor: submersible transducers, bubblers, and base-mounted transmitters, the density assumption behind all of them, venting, installation, and what makes them drift.
- PLC I/O SystemsDiscrete and analog input and output modules, sinking and sourcing, relay and transistor outputs, wiring conventions, and the module diagnostics that find field faults from the panel.
- UL 698AIndustrial control panels for hazardous locations: what UL 698A adds to UL 508A, a panel in a classified area or a safe-area panel with intrinsically safe circuits, barriers and isolators, the control drawing, and where utilities meet classified areas.
- Radar Level MeasurementNon-contact and guided wave radar, why it handles a wet well better than most alternatives, and the installation details that decide whether it works.
Direct contact
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