Abstract: The impulse high-voltage method is applied to accurately identify internal insulation deterioration in live porcelain insulators. For a 160 kN normal insulator under both de-energized and energized conditions, the peak echo voltage reaches 50 kV in the de-energized case and decays exponentially to about 10 kV at 200 ms, while in the energized case, the peak remains above 40 kV before decaying. These characteristics confirm the insulator’s normal condition. For a 210 kN zero-value insulator, the echo voltage under de-energized conditions peaks at only 10 kV and decays to zero within 10 ms. Under energized conditions, the impulse voltage rises to less than 15 kV within 1 ms and drops to 0 kV immediately, leaving only residual power-frequency voltage. These results demonstrate that the impulse high-voltage method can accurately distinguish normal and zero-value insulators under both energized and de-energized conditions, providing a new approach for live detection of porcelain insulators in transmission lines.