Side-by-side comparison
| Parameter | Ohmmeter | Insulation Resistance Tester |
|---|---|---|
| Test Voltage | 1.5 V – 9 V (battery) | 500 V, 1000 V, or 2500 V DC |
| Measurement Range | 0.1 Ω to ~20 MΩ | 1 MΩ to 20,000 MΩ (20 GΩ) |
| Primary Use | Continuity, component resistance | Cable/winding insulation quality |
| Typical Instrument | Fluke 117 multimeter, analog AVO | Megger MIT430, Kyoritsu 3121 |
| Display Unit | Ohms (Ω, kΩ, MΩ) | Megohms or Gigohms (MΩ, GΩ) |
| Pass/Fail Threshold | Continuity < 1 Ω; open > range | > 1 MΩ per IS 732; motors > 1 MΩ/kV rating |
| Polarization Index (PI) | Not measured | PI = R10min / R1min; > 2 is healthy |
| Safety Category | CAT III 600 V typical | CAT IV rated; HV warning labels required |
| Circuit Under Test | De-energised low-voltage circuits | De-energised; capacitive discharge needed after test |
| Cost (approx INR) | ₹1,500 – ₹8,000 | ₹15,000 – ₹80,000 |
Key differences
An ohmmeter cannot stress insulation because its 9 V source never drives leakage current through degraded polymer. A Megger at 1000 V will show a healthy motor winding at > 100 MΩ; the same winding reads a misleading "open" or a few MΩ on a multimeter. The Polarization Index — the ratio of 10-minute to 1-minute resistance — is only meaningful at Megger voltages. For switchgear rated above 1 kV, IS 732 mandates 2500 V test voltage, which no ohmmeter can supply.
When to use Ohmmeter
Use an ohmmeter when verifying wire continuity, measuring a resistor value, or checking a fuse on a PCB. Example: a Fluke 117 checks continuity of a 24 V control-panel wiring harness in under a minute.
When to use Insulation Resistance Tester
Use an insulation resistance tester when commissioning a 415 V induction motor, a cable run, or a transformer winding before energising. Example: a Megger MIT430 at 1000 V confirms > 50 MΩ on a 22 kW motor before the first power-on.
Recommendation
For lab practicals and site work, choose the Megger whenever the question involves insulation health, leakage current, or winding condition. Choose the ohmmeter for component-level or continuity checks. Never apply Megger voltage to live or electronic circuits — it destroys semiconductors instantly.
Exam tip: Examiners ask students to calculate Polarization Index from given 1-minute and 10-minute resistance readings and state whether the insulation is acceptable — know PI = R10/R1 and the pass criterion of 2.
Interview tip: An interviewer at a core electrical company will ask you to state the test voltage you would apply to a 3.3 kV motor winding and justify why an ordinary multimeter is insufficient — answer: 5000 V per IEC 60034-1, multimeter test voltage is too low to reveal partial discharge paths.