Insulation Resistance and Continuity Testing
Domain focus
This domain tests insulation resistance (IR) and continuity measurement: choosing test voltage, interpreting results against recognized minimums, correcting for temperature, and proving protective bonding. Marks are lost by quoting a number without the conditions that make it meaningful.
Insulation resistance rules
- Match test voltage to the equipment: 250 V DC for low-voltage control wiring, 500 V DC for 500 V motors and panels, 1 kV for 1 kV equipment, 2.5-5 kV for medium-voltage windings.
- IEEE 43 minimum for rotating machines: 1 MΩ for windings rated below 1 kV; for windings at or above 1 kV, IR₁min = rated kV + 1 MΩ.
- Polarization index PI = IR at 10 min ÷ IR at 1 min. PI ≥ 2.0 is generally acceptable for class B insulation; PI below 1.5 suggests moisture or contamination.
- Record winding temperature. IR roughly halves for every 10 °C rise, so uncorrected comparisons mislead.
Continuity and bonding
Protective-conductor continuity is proven with a low-resistance ohmmeter delivering at least 200 mA, leads nulled or subtracted, result recorded in ohms. A common acceptance value is ≤ 1 Ω for the whole protective path, but the project or standard limit always governs.
Worked example
A 400 V motor reads 0.6 MΩ after 60 s at 500 V DC. Required minimum = kV + 1 = 0.4 + 1 = 1.4 MΩ. Because 0.6 MΩ is below 1.4 MΩ, the winding fails. The technician dries the motor, retests, and obtains 45 MΩ with PI 2.4 - now acceptable.
Traps that cost marks
- Leaving surge arresters, VFDs, or sensitive electronics connected during the test.
- Not discharging the winding to earth after testing.
- Applying 1000 V to 400 V control wiring and damaging it.
- Reporting a single IR reading with no temperature, duration, or test voltage.
- Nulling leads after the measurement instead of before.