Most people think insulation resistance testing is just: connect the Megger, press test, read the megohms, write PASS. But a single spot reading hides the real story. As a Senior ETO, here is what the Megger is actually measuring — and why the trend matters more than the number.
What the Megger is really doing
The instrument applies a DC test voltage and watches how much current survives through the insulation over time. That current has three stages:
- Capacitive current — high at first, then decays quickly as the insulation charges like a capacitor.
- Absorption current — the insulation’s internal response, decaying more slowly.
- Leakage current — what is left at the end, and the part that tells you about real insulation health.
Why a spot reading is not enough
A good-looking spot value can still hide trouble. Better indicators look at behaviour over time:
- PI (Polarization Index) = IR at 10 min ÷ IR at 1 min. PI ≥ 2 is generally healthy.
- DAR (Dielectric Absorption Ratio) = IR at 60 s ÷ IR at 30 s — a quicker check.
- Trend history — a falling trend over months matters more than one absolute value.
Two things candidates forget
First, correct the reading to a reference temperature — insulation resistance roughly halves for every ~10 °C rise, so without correction your trend is meaningless. Second, always discharge the winding to earth after the test — that absorbed and capacitive charge can bite you.
This is exactly the kind of topic that comes up in the oral. See the full set of ETO oral exam questions on high voltage and grab the free practice set below.
Disclaimer: For exam preparation and revision only. Not a certified STCW course.