Before any operating voltage touches a transformer's windings, we must be confident in its insulation. The insulation resistance test with a megger is the first line of inspection: quick, simple, and able to reveal moisture and weakened insulation before they turn into a failure.
Why a Megger and Not an Ordinary Ohmmeter?
Healthy insulation resistance is measured in hundreds and thousands of megohms, and an ordinary ohmmeter, with its tiny voltage, can neither measure such values nor stress the insulation enough to reveal weakness. A megger applies a high test voltage (500V, 1000V, or more depending on the procedure and the transformer's voltage rating) and accurately measures these high resistances.
The Three Measurement Points
| Measurement | Between | Reveals |
|---|---|---|
| HV–LV | The primary and secondary windings | Weakened mutual insulation between the two windings |
| HV–E | The primary winding and the transformer body (earth) | Leakage from the high side to the frame |
| LV–E | The secondary winding and the transformer body | Weakened insulation on the low side |
Interpreting the Readings
- Practical references state that values above 1000 megohms are considered a good indicator in common contexts — but the final judgment rests with the manufacturer's specification and the transformer's voltage rating.
- The reading is affected by heat and humidity: compare readings taken under similar conditions, and apply temperature correction when needed.
- The trend over time matters more than a single value: a gradual decline over the years is an early warning.
- For a more accurate assessment of insulation condition, add the Polarization Index (PI) guide.
After the test, the insulation carries a stored charge from the megger voltage. Discharge the charge to earth before touching the terminals — forgetting to discharge is one of the most common testing accidents.
Sample answer: For two reasons: healthy insulation resistance is very high (hundreds of megohms or more) and requires a device capable of measuring such values, and more importantly, the megger applies a high test voltage (500V or 1000V or more) that genuinely stresses the insulation, revealing weak points and moisture that wouldn't show up with the small voltage of an ohmmeter. The stored charge on the insulation must be discharged after measurement.
Taking the reading immediately upon applying the voltage without waiting the standard time (e.g., one minute), or comparing readings taken under different temperature and humidity conditions. Readings should be compared under similar conditions and with a consistent measurement time.
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