Testing, Commissioning and Maintenance
Foundation
Commissioning verifies that equipment is installed correctly and will perform as designed; maintenance keeps it that way. Know the common tests: insulation resistance (megger), transformer turns ratio, winding resistance, contact resistance, and protective relay testing. Understand that insulation resistance is a trend test - a single reading means little without a baseline or a polarization index.
Concepts to master
Know the purpose of a polarization index and how temperature affects insulation resistance readings. Understand transformer oil testing: dielectric strength, moisture content, and dissolved gas analysis. Know what a hi-pot test does and why it is applied with care to aged insulation. Understand the difference between a factory test and a field acceptance test. For breakers, know timing tests and contact resistance. For arresters, know that leakage current testing detects degradation. Recognize that commissioning includes verifying polarity, phasing, and rotation.
Traps
Questions often treat a single megger reading as pass/fail, or compare readings taken at different temperatures. Another trap is confusing turns ratio with impedance testing.
Rule of thumb
Insulation resistance roughly halves for every 10 °C rise in temperature, so correct readings to a common base before comparing.
Worked example
A polarization index (10-minute reading divided by 1-minute reading) below about 2 suggests moisture or contamination; a value well above 2 indicates dry, healthy insulation.