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Zinc oxide (ZnO) varistor plays an important role in avoiding any electrical or electronic equipment failure caused by dangerous, damaging transients or lightning surges. Numerous investigations of several non-destructive diagnostic techniques have been conducted in order to obtain reliable and accurate condition assessment of ZnO-based protective devices. These diagnostic techniques include the standard 1mA reference voltage, lightning impulse discharge residual voltage, voltage response (VR), polarization or depolarization current (PDC) and Return Voltage (RV) measurements. Among these diagnostic techniques, the return voltage measurement (RVM) seems to be increasingly used as a reliable diagnostic method in monitoring the ageing process of the ZnO materials due to its high sensitivity to smaller degrees of degradation. In addition, RVMs have low sensitivity to disturbances by external noise, a situation that is auspicious for in-field measurements. Nonetheless, the basic interpretation based on the RVM essential parameters - peak RV, time-to-peak RV and initial slope of RV - provides insufficient information of the ZnO material condition since they are inevitably dependent on the measuring parameters such as the charging and discharging times as well as the test object temperature. Hence, this paper focuses on a new way in interpreting the RVM parameters based on dielectric time constants analysis using an equivalent circuit of varistor microstructure, namely the Maxwell-Model. In order to investigate the ageing processes of ZnO varistors, two types of accelerated degradation techniques - impulse and heat degradations - are systematically conducted on different test samples from different manufacturers. Experimental results are presented and discussed in detail according to the underlying physical mechanism. On the basis of this concept, a sensible ageing parameter, p-factor,is used for better characterization of the ageing status of the varistors.