Submersible pump trips its MCCB after twenty minutes: reading the measurements correctly
A 7.5 kW, 415 V, three-phase submersible pump with a 15 A nameplate current serves a farm dug well. It is protected by a 32 A MCCB and a contactor with a thermal overload relay set at 16 A. For two weeks the MCCB has tripped after 15 to 25 minutes of running, and the tripping time is getting shorter. The electrician records: line currents 16.2 A, 15.8 A and 11.4 A; phase-to-phase voltages 412 V, 408 V and 415 V; winding resistance between terminals 2.1, 2.1 and 2.1 ohm; insulation resistance to earth 0.5 Mohm on a 500 V megger. The MCCB body is hot to touch and one pole shows discolouration at the cable lug. The pump discharge is normal while it runs, and the overload relay has never tripped. The farmer says the pump is old but fine and asks for a 40 A MCCB so that watering can continue through the week.
1. Interpret the measurements. Which readings rule out a winding fault, and what do the current and voltage values say about the cause?
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Winding. Three equal resistances of 2.1 ohm and normal discharge rule out an open turn, a shorted turn and a mechanical overload. Insulation of 0.5 Mohm is low; the usual minimum for a low-voltage motor is 1 Mohm, so the winding is damp and should be dried and re-tested, but it is not the cause of progressive tripping.
Voltage unbalance. Average = (412 + 408 + 415) / 3 = 411.7 V. Maximum deviation = 6.7 V, so unbalance = 1.6 percent. That is well inside the 2 percent figure at which motors are normally operated without derating, so the supply is essentially healthy.
Current unbalance. Average = (16.2 + 15.8 + 11.4) / 3 = 14.5 A. Maximum deviation = 3.1 A, so unbalance = 21 percent. On a motor with balanced supply and balanced windings, this cannot be a winding or supply effect.
Conclusion. One phase is carrying far less current than the other two. That is the signature of high resistance in that phase path, not a motor fault: a pitted MCCB pole, a burnt cable lug (the discolouration confirms this), a loose termination or a damaged contactor tip. The other two phases carry the extra current, which is why the MCCB thermal element heats and trips after 15 to 25 minutes. The overload relay set at 16 A sees 16.2 A, only 1 percent above setting, which is inside its non-tripping band, so it correctly does not operate; it is not faulty.
2. List the tests you would carry out in order, and the repair you would recommend, before the pump is put back into service.
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Isolate and prove dead first, then test in this order:
- Millivolt drop across each MCCB pole with a low-resistance ohmmeter or a micro-ohmmeter; a healthy pole reads a few milliohms, the discoloured pole will read tens of milliohms or open.
- Inspect and re-terminate the cable lugs; cut back any discoloured copper until bright metal is reached and crimp new lugs.
- Check contactor tips for pitting and measure contact resistance of each pole.
- Clamp the current on each phase at the MCCB, at the contactor and at the motor, to localise where the unbalance starts.
- Insulation resistance of the feeder and the motor separately; dry the winding if below 1 Mohm.
- Thermal scan under load if an infrared camera is available.
Repair. Replace the MCCB, do not merely re-tighten it; a pole that has been heat-damaged cannot be restored. Fit new crimped lugs with correct torque, replace the contactor if tips are pitted, dry the winding and re-test. Then re-measure current balance; the three currents should be within about 5 percent of each other.
Also record. Why the original lug loosened: vibration on a submersible feeder, or a lug sized for a different cable. Correct it so the fault does not repeat next season.
3. The farmer insists on a 40 A MCCB. Explain the consequences and state the protection arrangement you will actually leave in place.
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Why 40 A is wrong. The unbalance, not the rating, is the fault. Increasing the rating raises the current the healthy phases must carry before any trip, so the damaged lug and pole run hotter for longer. The motor, rated 15 A, would then be protected only by the thermal relay, and the feeder cable may be 4 mm2, rated about 30 A, which a 40 A MCCB does not protect at all. The likely outcome is a burnt MCCB, a burnt cable or a winding failure in the well, which is far more expensive than a day of watering.
Correct arrangement.
- MCCB 32 A with a verified thermal-magnetic characteristic, or better an MPCB set to 15 to 16 A.
- Thermal overload relay set to 15 A, class 10, with a single-phasing preventer.
- Earth leakage protection of 100 mA for the submersible feeder if the installation is earthed through a rod.
- Insulation resistance above 1 Mohm before re-commissioning, entered in the log.
Dealing with the farmer. Show him the three current readings and the discoloured pole, explain that a 40 A device would simply move the failure from the MCCB into the cable or the motor, and offer a written statement that the pump is safe to run at 15 A while the replacement MCCB is arranged. Also give him an alternative water source for the two days of repair so the commercial pressure does not force a wrong decision.