<p>High-voltage (HV) rotating machinery that have been in operation over extended periods are prone to insulation degradation, particularly in critical areas such as the slot ends of the stator windings. One of the most vulnerable components is the outer corona protection (OCP) tape that covers the surface of the groundwall insulation. Mechanical and thermal stress over time can lead to damage of the OCP layer and at the overlap region between the OCP and the endwinding corona protection (ECP) layer. Additionally, contamination by dust, moisture, and conductive particles near the core and endwinding surfaces can further exacerbate partial discharge (PD) activity in these regions. PD pattern analysis revealed surface discharges occurring at the slot ends, which were attributed to insulation deterioration caused by frequent starts and stops over the years. Such degradation can lead to damage of the OCP or delamination at the overlap region of OCP and ECP. As a result of the poor PD evaluation, rewinding of the stator was recommended, and further tests including withstand voltage and dielectric breakdown testing were carried out. This paper analyzes the correlation between insulation diagnostic results and breakdown testing, emphasizing the role of PD patterns in identifying critical failure points in aged HV rotating machinery.</p>