The 500 V megohmmeter setting does not mean the instrument must reach 500 V before reporting a fault. When it rises only to about 80 V and holds at 0.00, a low-resistance path is loading the tester and preventing its current-limited output from reaching the selected voltage. That 80 V value is operating voltage under load, not a proven winding-breakdown threshold.
For the 10 hp, three-phase, 460 V pump motor that remained half submerged for about two weeks, leave the motor out of service. The test taken from conductors disconnected at the starter may include the motor leads, supply cable, submerged terminations, and winding insulation. Separate those components before condemning only the winding.
Reject the quick restart
Drying the outside, seeing one improved reading, and immediately restarting are the wrong first fixes. Water can enter the stator, terminal box, cable, bearings, and spaces between insulation layers. Contaminated water leaves conductive deposits after visible moisture evaporates.
Do not interpret 0.00 as an exact zero-ohm measurement without checking the displayed unit and instrument range. It may mean the measured insulation resistance is below the tester's display resolution. Either condition is far below what is needed to justify energizing a 460 V motor.
| Observation | Likely mechanism | Deciding check |
|---|---|---|
Tester set to 500 V rises to about 80 V and displays 0.00
|
Heavy leakage or a low-resistance path loads the tester | Verify the tester, then separate the cable from the motor |
| Multimeter shows very low resistance to ground | Water, contamination, damaged cable, terminal leakage, or failed winding insulation | Test cable and motor independently |
| Resistance changes when the multimeter leads are reversed | A DC source or galvanic action is affecting the reading | Measure DC voltage separately before another resistance test |
| Insulation resistance improves while drying | Moisture was contributing to leakage | Continue until readings stabilize, then repeat after cooling |
| Reading falls again after the motor cools | Residual moisture or contamination remains | Stop and send the motor for cleaning, dipping, and baking |
Check before continuing: the motor remains locked out, and nobody is treating the 80 V indication as a trip or breakdown voltage.
Isolate every test point
A megohmmeter applies DC test voltage. Do not connect it while any part of the circuit is energized, connected to a starter, or accessible to personnel in water. Because the winding connections are still underwater, stop field testing until the motor and its terminations can be reached safely.
- Lock out and verify the absence of supply voltage.
- Disconnect all motor supply conductors from the starter so no test voltage can reach the starter, overload, control circuit, or other equipment.
- Identify and secure the disconnected conductors. Prevent their exposed ends from touching each other or grounded metal.
- Inspect the accessible cable route, junctions, and conduit for water exposure.
- After each insulation test, let the tester discharge the circuit and verify that no residual DC voltage remains before touching conductors.
A reading taken only at the starter end is a circuit reading. It cannot distinguish a grounded winding from a wet cable, submerged splice, contaminated terminal board, or water path across insulation.
Check before continuing: the starter is completely outside the test circuit, the tested conductors are de-energized, and all motor-end connections are out of the water and accessible.
Prove the instruments and screen with a multimeter
Confirm that the megohmmeter, leads, battery, selected voltage, and displayed unit are correct. Run the instrument's prescribed lead and self-check procedure. A damaged lead or depleted instrument can produce misleading voltage and resistance behavior, although a repeatable 0.00 reading on a flooded circuit still demands isolation work.
- With the circuit discharged, use a multimeter to check each conductor to the motor frame or equipment grounding point.
- If the resistance is very low, reverse the meter leads and repeat. A significant polarity-dependent change calls for a separate DC-voltage check.
- Select DC voltage and measure from the winding conductors to the frame before reconnecting the resistance function.
- If a small DC voltage exists, treat it as a sign of galvanic action caused by contaminated conductive material between copper and iron. Remove the motor for cleaning; drying alone will not remove salts or deposits.
Never perform resistance measurements on a circuit that is producing voltage. Record the lead polarity, displayed unit, test point, motor temperature, and whether the reading rises or remains fixed.
Check before continuing: the instruments pass their checks, no unexplained DC voltage remains, and the low reading can be reproduced on a clearly identified circuit section.
Separate the cable from the motor
Access the motor terminal box after removing the equipment from the flooded area. Disconnect the supply cable from the motor terminals. If the winding leads can be separated safely and their connection diagram is known, test each available winding-to-frame path; otherwise test the connected winding group without inventing an internal configuration.
- Test the disconnected supply cable conductor-to-ground and conductor-to-conductor.
- Test the motor terminals to the motor frame.
- Compare the results without using the starter-end combined reading as either component's result.
- Inspect the cable ends, glands, terminal board, lead insulation, and frame for water tracks, corrosion, residue, or physical damage.
If the cable remains low with the motor disconnected, divide the cable system at accessible junctions and find the wet or damaged section. If the cable tests satisfactorily but the motor remains low, the motor needs internal cleaning and drying. If both test satisfactorily independently but fail when connected, inspect the termination arrangement and contamination around the terminal board.
Check before continuing: record separate insulation results for the cable and motor, with no submerged splice or termination hidden inside either test.
Remove contamination and inspect the motor
Two weeks of partial submersion calls for more than external drying. Remove the motor from service and compare the cost and outage time of replacement against motor-shop reconditioning. For a 10 hp motor, replacement may be more economical, but make that decision from actual shop and replacement quotations.
A proper recovery includes pulling the rotor, cleaning the stator and rotor, inspecting the winding and leads, and addressing the bearings. Plan to replace bearings after this immersion, including double-sealed bearings; their external appearance does not prove that water or contamination stayed out.
Use a motor-shop cleaning process compatible with the winding insulation. Clean water can remove water-soluble contamination before drying, but uncontrolled solvents, pressure, or abrasion can damage varnish and drive debris deeper into the winding. Where insulation has deteriorated, have the shop clean, dip, and bake the stator rather than relying on heat alone.
Check before continuing: the rotor and bearings have been inspected, visible contamination is removed, and the shop has determined whether the winding needs only controlled drying or a dip-and-bake process.
Dry the winding under control
Dry the stator at 80 to 90 °C under a controlled motor-shop procedure, subject to the construction and materials identified by the shop. Do not guess a drying duration; track insulation resistance until it stops improving and remains stable. Record winding temperature with every reading because temperature changes the value.
Low-voltage DC through the windings can provide internal heating, but apply it only after external warming has produced an improvement in insulation resistance. A winding that still presents an extremely low-resistance path is not ready for electrical heating. Control the DC source to prevent localized overheating, and disconnect and discharge it before every measurement.
If the motor is dual voltage and the winding diagram confirms the available leads, connect all winding sections in parallel for DC drying to reduce voltage stress across the insulation. Do not change connections when the diagram or lead identification is uncertain; use external heat or send the motor to the shop.
Check before continuing: insulation readings have risen into the megohm range, stabilized during drying, and meet the motor shop's acceptance criterion at a recorded temperature. A megohm-range reading by itself is improvement, not automatic permission to start.
Rebuild and verify the complete installation
- Install serviced or replacement bearings and complete the mechanical inspection.
- Reconnect the motor according to its verified lead diagram and voltage rating.
- Retest the motor to frame before attaching the supply cable.
- Retest the cable independently, then connect it and test the complete motor circuit from the starter end.
- Confirm protective devices and grounding are restored before energizing.
- Run the motor under controlled observation. Check rotation, abnormal noise, heating, and phase currents against the motor nameplate and the installation's normal records.
- After the motor stops and cools completely, repeat the insulation-resistance test at the same identified points and record the temperature.
The cold retest matters. A cleaned motor can run while warm yet reveal residual moisture or contamination after cooling. Do not restart it if the cold insulation value falls materially, becomes unstable, or returns to the tester's lower limit.
End-to-end check: retain separate baseline readings for the motor, cable, and assembled circuit. Production is restored only after all three pass the applicable shop or manufacturer acceptance criteria and the cooled motor repeats the result.
FAQ
How do I interpret a megger that reaches only 80 V on the 500 V setting?
The connected circuit is loading the tester, so its current-limited output cannot reach 500 V. The 80 V indication is not proof that winding insulation breaks down specifically at 80 V; isolate the cable and motor to locate the low-resistance path.
How do I tell whether the wet motor or its cable is grounded?
Disconnect the supply cable at the motor. Test the cable conductor-to-ground and conductor-to-conductor, then test the motor terminals to the frame; the starter-end test alone includes both components and any intervening terminations.
How do I dry a water-damaged motor winding?
Remove contamination first, then use a controlled motor-shop drying process at 80 to 90 °C while trending insulation resistance. Low-voltage DC heating may be used only after external warming improves the reading, with confirmed winding connections and temperature control.
When do I stop testing and call official support?
Stop if connections remain underwater, a DC voltage appears from conductor to frame, the meter stays at 0.00 after circuit separation, or the cold reading falls after cleaning and drying. Do not energize the motor when the winding configuration, test result, or drying limit is uncertain. Escalate to the motor manufacturer's or megohmmeter manufacturer's official support channel and a qualified motor-repair shop.