Float Switch and 3-Electrode Liquid Level Relay Wiring: Tank Filling vs Dry-Run Protection
Wiring 3-core cable float switches for rooftop tank filling vs sump emptying, AC-3 contactor coil interlocking, and 3-electrode conductive liquid level relay borehole protection.
Cable float switches and conductive 3-electrode liquid level relays protect booster tanks from overflowing and submersible borehole pumps from dry-run burnout. Wide storage tanks and sewage sumps suit mechanical 3-core tethered float switches, whereas narrow deep-well casings—where a float tether would snag—require a DIN-rail conductive liquid level relay with three stainless-steel probes.
> Related Category: Compare level control components in Liquid Level Relays, Float Switches, and Liquid Level Control Electrodes.
3-core float switch wiring: Tank filling vs sump emptying
Inside a standard 3-wire float switch, a rolling steel ball actuates a SPDT microswitch (COM, NO, NC). Because wire color conventions vary across manufacturers, always verify continuity with a multimeter in the hanging-down and tilted-up positions before energizing:
| Operating mode | Control logic | Typical European 3-wire pairing | Unused 3rd core |
|---|---|---|---|
| 1. Water tank FILLING (pump feeds elevated storage tank) | Float drops → contact closes (pump ON); tank fills and float rises → contact opens (pump OFF). | Black (COM) + Blue | Cap and insulate the Brown wire inside an enclosed terminal! |
| 2. Sump EMPTYING / Dry-run cutout (dewatering or suction cistern) | Water rises and float tilts up → contact closes (pump ON); water drops and float hangs down → contact opens (pump OFF). | Black (COM) + Brown | Cap and insulate the Blue wire inside an enclosed terminal! |
Critical safety rules: The blue wire inside a 3-core float cable is a switched live contact, not Neutral—always cap the unused wire so it cannot become live at 230 V when the float flips. Never switch motors larger than 0.75 kW (1 HP) or any three-phase pump directly through the float microswitch; route the float in series with the A1 coil of an AC-3 rated contactor (see Contactor AC-1 vs AC-3 Utilization Categories). Adjust the yellow counterweight along the cable to set at least 25–40 cm of switching differential.
3-electrode liquid level relay wiring for deep boreholes
In narrow boreholes, mount a DIN-rail liquid level relay in the pump panel and suspend three liquid level control electrodes in the well:
- Reference / Bottom Probe (`C` / `COM`): Positioned just above the submersible pump intake screen.
- Minimum Stop Probe (`MIN` / `LOW`): Positioned 30–50 cm above `COM` to cut power before the pump draws air.
- Maximum Restart Probe (`MAX` / `HIGH`): Positioned higher up in the recovery column so the pump restarts only after sufficient water accumulates.
Hysteresis cycle: Once groundwater submerges MAX, the relay detects conduction between `COM` and `MAX` using a safe low-voltage AC probe signal (preventing DC electrolysis) and closes output contact `15-18` to energize the pump contactor. As water is pumped down below `MAX`, the relay latches ON until the level drops below MIN, preventing rapid short-cycling.
Conductivity tuning (kΩ) and fluid limitations
- Adjust the front-panel sensitivity trimmer (5 kΩ – 100 kΩ) higher for low-mineral rainwater/spring water and lower if foam or damp borehole walls cause false latching.
- Non-conductive & hazardous liquids: Deionized/pure water, diesel, gasoline, oils, and flammable solvents cannot use conductive probes; select capacitive, optical, or ATEX float switches instead.
Frequently asked questions
Can a float switch control a 1.5 kW (2 HP) booster pump directly?
No. Motor locked-rotor starting inrush (35–45 A) welds microswitch contacts rapidly. Wire the float switch strictly into the 230 V control coil circuit of an AC-3 contactor paired with a thermal overload relay.
How do I interlock both an upper rooftop tank and a lower well?
Wire the upper tank's filling float contact in series with the borehole level relay's NO output contact (`15-18`) feeding contactor coil `A1`. The pump runs only when the upper tank calls for water *and* the lower well is above minimum level.
Why do conductive level relays output AC instead of DC to the probes?
Applying DC across submerged stainless-steel probes causes electrolytic plating, scaling, and rapid probe corrosion; low-voltage AC eliminates net ion migration.
Can a conductive steel tank use 2 probes instead of 3?
Yes. Bond the relay's `COM` terminal directly to the grounded metallic tank shell and suspend only the `MIN` and `MAX` insulated probes from the top flange.