This paper presents a comprehensive study on the quality and reliability of commercially available medium voltage surge arresters, with particular emphasis on their electrical performance and structural degradation under aging conditions. Samples from four manufacturers were subjected to 8/20 μs aging impulses and accelerated durability tests. Electrical analysis revealed that some products exhibited excessively high residual voltages (Ures) exceeding declared values by an average of 7.5% and high partial discharge (PD) levels. The resistive component of the leakage current (ILCR) indicated that medium voltage (MV) surge arresters possess higher thermal resistance to aging as compared to the low voltage (LV) ones. Structural analysis performed using powder X-ray diffraction (pXRD) confirmed the presence of ZnO as the primary phase, along with intergranular δ-Bi2O3 and Sb2O3-based spinel and pyrochlore phases. Study of the powder X-ray diffraction patterns using theWilliamson–Hall method revealed the microstrains exhibiting significant variations in residual lattice strains. This suggests that during varistor ceramic processing distinct sintering protocols have been utilized. Elevated microstrain values (εWH) correlated with increased porosity indicated that volatilization ZnO was promoted by high sintering temperatures. Aging processes led to dopants diffusion into grain boundaries and growth of the spinel phase (Zn7Sb2O12). Dielectric spectroscopy measurement revealed changes in the real part of varistors’ permittivity (ε) up to 50% under thermal aging. This provides insight into the thermal stability and long-term reliability of medium voltage surge arresters under test.