Insulation Resistance and Dielectric Testing
Insulation Resistance and Dielectric Testing covers DC insulation resistance, polarization index, step-voltage, hipot, and dissipation-factor tests. The exam emphasizes correct test voltage, time dependence, guarding, and safe discharge.
Core rules
- Insulation resistance is time-dependent; record 1 min and 10 min values for polarization index.
- PI = R10min/R1min. For dry solid insulation, values below about 2.0 warrant investigation; trend matters more than a single pass/fail number.
- The guard terminal shunts surface leakage; use it on bushings, cables, and dirty equipment.
- Test voltage depends on the equipment rating; do not exceed the manufacturer's limit. Discharge to ground after every DC test.
- Hipot proves dielectric strength but can damage aged insulation; IR and PI are diagnostic tests, not proof tests.
Worked example
A motor winding measures 500 MΩ at 1 min and 1,200 MΩ at 10 min at 500 V DC. PI = 1,200/500 = 2.4, which is acceptable and suggests stable insulation. If the same winding measured 400 MΩ at 1 min and 300 MΩ at 10 min, PI = 0.75; the falling resistance indicates moisture, contamination, or degradation. Discharge the winding, ground it, and investigate before applying a hipot. Temperature correction matters because IR falls roughly by half for each 10°C rise.
Traps
Testing a hot winding and comparing it to a cold baseline; omitting the discharge; using the guard terminal incorrectly; treating high IR as proof of high dielectric strength; testing electronics without isolation.