De-energized testing focuses on insulation and winding integrity without applying mains voltage. Common tests include insulation resistance (megger), winding resistance (micro-ohm meter), turns ratio (TTR), and continuity checks. Visual inspection of bushings, gaskets, OLTC mechanism, and grounding hardware is also performed. Oil-filled transformers may have samples taken for DGA and dielectric breakdown. These non-energized tests help detect winding imbalances, insulation aging, contact wear, or moisture before energization, reducing failure risk.