Monitoring Relays in Water and Wastewater Treatment
Where do monitoring relays fit into a water or wastewater treatment plant? They sit between the process instrumentation and the pump or blower motor, watching level, phase sequence, current, or winding temperature, and switching a DIN-rail output relay per IEC 60255 and IEC 60947-5-1 the moment a set threshold is crossed. Miss a phase-loss event on a submersible sewage pump and the motor single-phases against a jammed impeller within minutes, burning the winding before a thermal element even sees the extra current. This article covers lift-station level control, dry-run protection, phase-sequence and phase-loss monitoring for three-phase pump motors, thermistor protection, insulation monitoring on IT-earthed panels, and how to size the relay for the application.
Level Control at Pump and Lift Stations
A wet well runs on two levels: a start level that closes the pump contactor and a stop level that opens it, with a gap between them so the pump does not cycle every few seconds. Conductive level relays sense this through electrodes in contact with the liquid -- current flows between probes when they are submerged, and the relay reads that as "wet." Sensitivity, expressed in kOhm, is adjustable for the conductivity of the process water; raw sewage conducts differently than filtered effluent, and a relay set for one will misread the other.
The gap between start and stop is the same hysteresis concept used across the monitoring relay family, just applied to level instead of voltage or current.
Formula: Level Control Differential — Source: IEC 60947-5-1 (hysteresis/differential setting applied to level control)
Lstop = Lstart x (1 - H)
| Symbol | Description | Unit |
|---|---|---|
| Lstart | Level at which the relay energizes the pump (start/high probe) | % of span or mm |
| H | Hysteresis / differential setting between start and stop levels | % |
| Lstop | Level at which the relay de-energizes the pump (stop/low probe) | % of span or mm |
Dry-Run Protection for Submersible Pumps
A submersible pump depends on the surrounding liquid for cooling. Drop the level below the pump housing and the motor keeps running, but the heat has nowhere to go. Level-based dry-run protection stops the motor before that happens, using a low-level electrode set below the normal duty band.
Some panels back this up with undercurrent monitoring on the motor supply: a pump running dry, or a broken coupling, draws noticeably less current than a loaded one, and a current relay set below normal load trips on that signal independently of the level probes. Two independent signals -- level and current -- catch different failure modes: a stuck float or fouled electrode won't fool a current-based trip, and vice versa.
Phase Sequence and Phase-Loss Protection for Pump Motors
Three-phase pump motors are directional. Swap two supply phases after cable maintenance or a generator changeover and the impeller spins backwards. On a centrifugal pump that can cut flow by more than half and raise motor current at the same time; on some positive-displacement units it risks mechanical damage. A phase sequence and phase failure relay checks the L1-L2-L3 rotation before the starter is allowed to close, and continues to watch for phase loss and asymmetry while the pump runs.
Lose one phase entirely on a running pump and the motor tries to keep turning on the remaining two, drawing higher current on those phases and overheating fast. This is the same single-phasing failure covered in our overload relay phase loss guide, and it is common enough in submersible pump stations, where cable damage and connector corrosion are routine, that most panel builders treat phase-loss detection as a given rather than an option.
What we see in the field: on plants with standby generators, phase rotation on generator power is not always identical to grid power, and a phase-sequence relay wired only to detect grid rotation can let a backwards-rotating pump start during an outage. Checking both sources during commissioning avoids that gap.
Thermistor and Temperature Protection in Submersible Motors
Submersible pump motors often embed a PTC thermistor directly in the winding, closer to the actual hot spot than any external current-based estimate can get. A thermistor motor protection relay trips when the sensor resistance rises to its reference value near 3.3 kOhm per IEC 60947-8, regardless of what caused the temperature rise -- ambient heat, fouled impeller, reduced flow across the motor jacket, or an electrical fault the current relay hasn't caught yet.
This differs from dry-run protection in cause versus effect. Level and undercurrent relays react to the cause -- no liquid, no load. A thermistor relay reacts to the effect, the winding temperature itself, which means it also catches overheating from fouling, blocked flow past the stator, or high ambient temperature in a hot wet well, none of which a level probe would ever see.
Insulation Monitoring on IT-Earthed Treatment Panels
Most treatment plant distribution is TN or TT earthed, where a single earth fault trips a breaker immediately. Some plants run critical process buses -- SCADA power supplies, instrumentation loops, standby lighting -- on an unearthed (IT) system instead, so a first earth fault does not interrupt the process at all.
An IMD injects a small measuring signal onto the IT network and continuously calculates insulation resistance in kOhm. It alarms well before a genuine short develops, giving maintenance a window to trace and clear the fault on their schedule rather than during an unplanned outage. This is the same principle used in hospital medical IT systems, adapted here to keep a treatment process running through a single fault instead of a single power failure.
Selecting and Wiring Monitoring Relays for Water Treatment Panels
Match the relay to the failure mode, not just the parameter being measured. A lift station panel commonly needs level control for start/stop, a separate low-level dry-run cutout, phase-sequence protection ahead of the starter, and either thermistor input from the motor or a conventional thermal/electronic overload sized to the motor nameplate. See our guide on monitoring relay engineering guide for how these functions map across brands and ranges, including our stock of monitoring and control relays from Schneider Zelio Control and ABB CM.
The relay output is a signal, not power -- it drives a contactor coil, not the pump motor directly. Size the downstream contactor to the motor's rated current and duty class, and pair it with motor protection sized to the same nameplate; our motor protection circuit breaker range and thermal overload relay stock cover the short-circuit and overload side of that same starter.
Fail-safe wiring matters more in unattended lift stations than almost anywhere else. A normally-energized output that de-energizes on fault or loss of auxiliary supply means a blown control fuse also stops the pump, rather than leaving it running unmonitored until someone notices the alarm never came in.
Frequently Asked Questions
What type of monitoring relay protects a submersible sewage pump from dry running?
A liquid level relay using conductive sensing electrodes, often paired with the pump's undercurrent detection, stops the motor once the liquid drops below the low-level probe. Some panels combine both signals so the pump only restarts once level and current confirm it is submerged and loaded.
Why does phase sequence matter for treatment plant pump motors?
A three-phase pump impeller is directional; reversed rotation on a centrifugal pump can cut flow by more than half. A phase-sequence relay blocks the starter from closing until the incoming L1-L2-L3 order is confirmed, catching cross-wired supply after maintenance or a generator changeover.
Do wastewater treatment plants need insulation monitoring devices?
Only where the electrical system is deliberately unearthed (IT), which some plants use on critical process buses so a single earth fault does not trip the whole panel. An IMD alarms on that first fault so maintenance can clear it before a second fault turns into a short circuit.
How is dry-run protection different from thermal overload protection on a pump motor?
Dry-run protection reacts to the cause -- loss of liquid or load -- typically through level or undercurrent sensing, before the winding overheats. Thermal overload or PTC thermistor protection reacts to the effect, the winding temperature itself, and trips regardless of what caused the heat rise.
What auxiliary supply do pump station monitoring relays need?
Three-phase supply and phase-sequence relays often draw power directly from the monitored lines and need no separate auxiliary. Single-function voltage, current, level and temperature relays usually need a separate control-voltage feed, commonly 24V DC or 110-240V AC, so check the model before wiring.
Can one monitoring relay cover level, phase and current in a pump station panel?
Multifunction models such as Schneider's RM35 series combine several of these functions in one 22.5 mm DIN-rail unit, which reduces panel space in small lift stations. Larger plants often prefer dedicated single-function relays per parameter for clearer fault diagnosis and easier spare-parts stocking.
Conclusion
Water and wastewater plants combine every monitoring relay function in one place: level control that starts and stops the process, phase and current monitoring that protects the motor driving it, thermistor input reading the winding directly, and occasionally insulation monitoring keeping an IT-earthed control bus alive through a first fault. None of these relays interrupt power on their own -- each one watches a quantity and hands the trip decision to a contactor or a PLC. Getting the hysteresis, delay, and sensitivity settings right on each is what keeps a remote lift station running unattended for months instead of nuisance-tripping every time inflow varies.
