The Failing Rural Water Pump
A submersible pump in a village borehole is rated 3 kW and draws power from a solar array through an inverter. For three weeks the pump trips its overload relay after 20–30 minutes of running, especially on hot afternoons. The borehole is 60 m deep and the water level has dropped 8 m since commissioning. The operator reports the pump casing is hot to the touch, the current is 14 A against a rated 12.5 A, and the voltage at the inverter output sags from 230 V to 205 V under load. The solar array is dusty and one panel string shows a lower open-circuit voltage than the others. The relay is set to its factory default.
1. Diagnose the most likely primary cause of the tripping and distinguish it from secondary contributing factors.
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The primary cause is almost certainly mechanical overload from a lowered water level, not an electrical fault. A drop of 8 m increases the total head the pump must overcome, moving its operating point along the pump curve to a higher flow-power demand. The evidence fits: current of 14 A exceeds the 12.5 A rating by about 12%, the casing runs hot, and trips occur after sustained running rather than at start.
The voltage sag to 205 V (about 11% below nominal) and the dusty array are real but secondary. Low voltage raises current for the same power, compounding heating, and a weak string reduces available power, but neither alone explains a delayed thermal trip.
Judgement point: an examiner wants you to rank causes by evidence, not list them. The water-level change is the only factor that alters the hydraulic load; the electrical symptoms are consequences or aggravators. The factory-default relay setting may also be mismatched, but resetting it without addressing head would mask the problem.
2. Recommend a sequence of corrective actions, justifying the order.
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- Verify hydraulics first. Measure static and dynamic water levels and confirm the pump's setting depth relative to the drawdown. If the pump is near the intake, lower it or throttle the discharge to move back along the curve.
- Check and correct the electrical supply. Clean the array, test each string's open-circuit voltage, and confirm inverter output under load. Restore voltage to within tolerance before judging motor health.
- Re-set the overload relay correctly to the motor's nameplate full-load current with the appropriate trip class, rather than leaving the factory default.
- Monitor current, casing temperature, and flow after each change so the effect of each intervention is isolated.
Order matters because adjusting the relay first could allow the motor to run overloaded and burn out. Fixing the load, then the supply, then the protection gives a defensible, evidence-led sequence.