Symptom
The drive displays a ground fault code at start or while running and stops. The trip may be immediate at every start, occur only after the pump has run for a while, or appear in wet weather.
The short answer
Drive Trips on Ground Fault
A drive trips on ground fault when the sum of its three output currents is not zero, meaning current is leaving the motor circuit through a path to ground. Most trips are a real fault: a motor winding to frame, a cable with damaged insulation, water in a submersible motor or its cable, or a wet terminal box. Some are capacitive leakage on a long or shielded cable at the drive switching frequency, which the drive cannot distinguish from a fault. Lock out, disconnect the motor at the drive, and test the insulation resistance of the cable and the motor; a low reading is the fault, a good reading with the trip persisting points to leakage or the drive.
Key points
- The drive sums the three output currents. Any imbalance is current going to ground.
- Test the cable and the motor insulation with the drive disconnected. A low reading is the answer.
- Submersible motors and their cables are the most common source: water in the cable, a splice, or a motor seal.
- Long or shielded output cables leak capacitively at the switching frequency and can trip the detector without a fault.
- A trip that appears after the motor warms or in rain is insulation breaking down under temperature or moisture.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Motor winding insulation failed to the frame | Insulation resistance test on the motor only; a reading below the manufacturer minimum or a drop from the last test. |
| Motor cable insulation damaged | Test the cable with the motor disconnected; inspect for abrasion, crushing, rodent damage, and conduit fittings that cut the jacket. |
| Water in a submersible cable or splice | The reading falls with time under test as moisture wicks; a splice below the water line is the usual location. |
| Water in the motor terminal box or a wet connection | Trips in rain or after washdown; open the box and look. |
| Motor seal failure on a submersible | A seal leak alarm and a falling insulation resistance; the motor housing has water in it. |
| Capacitive leakage on a long or shielded cable | Insulation resistance is good; the trip occurs at start or at high switching frequency; an output reactor, a lower switching frequency, or a raised threshold where the manual permits. |
| Output filter with a ground reference | Some filters connect capacitors to ground and pass leakage the drive sees as a fault; use a filter rated for the drive. |
| Wiring error after work | A motor lead landed on ground, or a ground landed on a motor terminal; inspect the terminations made since the trips began. |
| Multiple motors on one drive with a fault in one | Disconnect each motor in turn. |
| Drive current sensor or detection circuit failed | The drive trips with the motor disconnected and the outputs open. |
How the drive knows
The three output currents of a healthy motor circuit add to zero at every instant: what goes out on one phase comes back on the others. The drive measures them, and when the sum is not zero, current is leaving through a path the drive did not intend, which is ground. The detector is sensitive, because a real ground fault at a fraction of an amp is a hazard, and that sensitivity is why it also responds to capacitive leakage that is not a fault: every conductor has capacitance to ground, the drive output switches at kilohertz, and current flows through that capacitance on every switching edge. On a short cable it is negligible; on a long, shielded, or submersible cable it can approach the trip level.
The insulation test
- 1
Lock out and disconnect
Drive off, locked out, the motor leads removed from the drive output terminals so the drive is not in the test circuit. An insulation tester applied to a drive output destroys the output stage.
- 2
Test the whole circuit
Insulation resistance from each motor lead to ground at the test voltage the motor rating calls for, commonly 500 or 1000 V DC. Hold for a minute and watch the reading: a healthy circuit rises and steadies; a wet one falls.
- 3
Separate the cable and the motor
At the motor terminal box or the junction, disconnect the cable from the motor and test each alone. The one with the low reading is the fault. On a submersible, the cable and the motor are tested together at the panel first and separated only if the pump is pulled.
- 4
Compare with history
A submersible pump with a record of insulation tests shows a decline before it fails. A reading that has fallen by an order of magnitude since the last test is failing, whatever the absolute value.
- 5
Test phase to phase
For a winding fault that has not yet reached ground, and for balance.
- 6
If everything reads high
The fault is capacitive leakage, a wiring error, a wet box that dried, or the drive itself. Move on to the next section.
When the insulation is good
- Long cable
- Compare the cable length with the drive manufacturer limit for the switching frequency and the cable type. Reduce the switching frequency, add an output reactor or a dV/dt filter, or, where the manual permits, adjust the ground fault threshold. Shielded cable has more capacitance than unshielded; a submersible cable in water has more still.
- Output filter
- A filter with capacitors to ground that is not matched to the drive passes leakage into the detector. Use the filter the drive manufacturer specifies.
- Wet terminal box or connection that dried
- Open every box on the circuit and look for water tracks, corrosion, and insulation that has been wet. Seal the entries.
- Wiring error after work
- A ground wire under a motor terminal, a motor lead on the ground bar, a shield drain touching a phase terminal. Inspect every termination made since the trips began.
- Intermittent fault
- A cable that faults only when the pump vibrates, or a motor that faults only when hot. Test the motor at operating temperature where practical, and flex the cable during the test.
- The drive
- With the motor disconnected and the outputs open, run the drive. A ground fault trip with nothing connected is a current sensor or a detection circuit failure.
Submersible pumps
A submersible pump has its motor and its cable in water, and the cable has a splice where the pump lead meets the drop cable. Water enters through a nicked jacket, a poor splice, a seal that has worn, or a cable that has been pulled and stretched. The insulation resistance falls over months, the seal leak alarm may precede it, and the drive ground fault trip is the final symptom. Utilities that test their submersibles on a schedule and trend the readings replace the pump on the trend; those that do not find out from the drive. When the pump is pulled, the splice and the cable entry are inspected first.
Common mistakes
- Restarting to see whether the fault clears. It clears when the wet spot dries, and returns.
- Applying an insulation tester to the drive output terminals with the motor connected. The drive is the casualty.
- Replacing the drive when the motor was the fault, then faulting the new drive.
- Raising the ground fault threshold to stop trips without testing the insulation.
- Skipping the phase-to-phase test. A winding fault that has not reached ground yet will.
Frequently asked questions
- The insulation reads fine and the drive still trips on ground fault. What now?
- Capacitive leakage on a long or shielded cable, an output filter with a ground path, a wiring error, or the drive. Check the cable length against the drive limit and try a lower switching frequency; inspect every termination; run the drive with the motor disconnected. In that order.
- What insulation resistance is acceptable?
- A common rule for a motor is a minimum of one megohm per kilovolt of rating plus one, so at least 1.5 megohms for a 480 V motor, with new motors reading in the hundreds of megohms or more. The trend matters more than the number: a motor that read 500 megohms last year and 20 this year is failing.
- Why does the pump trip only after running for twenty minutes?
- The insulation breaks down at temperature: a winding with a weak spot that holds when cold and leaks when hot. Test the motor immediately after it has run, or with a heated test where the shop can do it. The motor is at the end of its life.
- Can a ground fault on one pump trip the drive on the other?
- Not through the drive, since each drive measures its own outputs. It can trip an upstream ground fault protection device that covers both, and a fault that puts current into the ground grid can disturb instruments. The drive that trips is the one with the faulted motor.
Related topics
- Drive Faults on OvercurrentAn overcurrent trip on a variable frequency drive: what the drive measured and when, the difference between a trip at start, during acceleration, at steady speed, and at deceleration, the motor and cable faults behind each, the parameter settings that cause nuisance trips, and the tests that separate them.
- Pump Will Not StartA pump that is called and does nothing. The order to check: permissives and mode, control power, the starter or drive, the motor circuit, and the pump itself, with the measurements that settle each one in minutes.
- Variable Frequency Drives in Pump PanelsWhat a drive adds to a pump panel and what it costs: speed control, soft start, reduced inrush, and diagnostics against heat, harmonics, cable and motor stress, bypass, and the minimum speed below which a pump does no work.
- Well Pump ControlControlling a groundwater well: submersible and line-shaft turbine pumps, the pump-to-waste start, drawdown and the low-water cutoff, cycling limits from the well and the aquifer, and what the controller monitors to protect a pump that cannot be seen.
- Duplex Lift Station ControlsThe two-pump station that makes up most of a collection system: what is in the panel, how the sequence works, what is monitored, and where these stations actually fail.
- Drive Faults on OvervoltageA DC bus overvoltage trip on a drive: where the bus voltage comes from, why deceleration and a driving load raise it, the utility-side causes from a high supply and capacitor switching, the deceleration ramp and overvoltage controller settings, braking resistors, and how to tell a regeneration trip from a supply transient.
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