6 CONCLUSIONS number of non-destructive and destructive diagnos-A tic techniques for assessing the condition of MOSA have been discussed in this paper. The results indicate a comparable degree of degradation for each of the different techniques. The new polarization techniques have shown an excellent correlation with a number of existing techniques such as 1 mA reference voltage and discharge residual voltage measurement. They have shown relatively large changes in ratio characteristics on MOSA after the application of current pulses which could provide more information of the MOSA insulation condition. The microstructure observations showed that degraded varistors were found to have smaller average grain size, changes in the diffraction peak position and different element ratio compositions compared to new samples. A possible explanation is the non-uniform temperature distribution in the material due to the development of localized hot spots during the current impulse and the dissolving in some other phases. The reduction in average grain size and change in the lattice parameters could affect the electrical properties by lowering the resistance and increasing capacitances ofbulk material at low voltages. Hence it changes the leakage current. These findings suggest that the microstructure of a varistor has a strong correlation with its electrical characteristics. The reduction in its average grain size and larger concentration of bright spots in particular after the current pulses could significantly change the electrical characteristics of a varistor by increasing its conductivity. In conclusion, the modern non-destructive electrical diagnostics based on polarization methods such as return voltage and polarizationrdepolarization current measurement have shown good indications of the ageing level of MOSA. The non-destructive diagnostics and the destructive diagnostic results obtained from this study have also proven that there is a strong correlation between electrical properties and microstructure characteristics of MOSA. Although this study was conducted on distribution class arresters, the techniques could be applied to substation class high voltage arresters. This will be performed in a continuing research project and findings will be reported in a future paper