Industry requires that the quality of electricity supply be within certain tolerances, due to sensitive machinery or processes. If the supply is lost, then consumer industries will lose revenue as will the supplier and transmission company, and for this reason continuity and security of supply is paramount. The supplier and transmission company must ensure reliability even as the demand for supply grows, placing greater stress on the existing supply and transmission network. Since the transmission system cannot be easily expanded, due to government legislation, financial constraints and environmental considerations, the only option is to increase the load capacity of the existing network. Thus, protection of the network is of utmost importance. Surge arresters protect against transient faults caused by lightning and switching surges. Premature failure of arresters can compromise the system protection and early fault detection in these devices, by condition monitoring, plays a critical role in ensuring the reliability, continuity and security of the electricity supply. An extensive literature review is given that details arrester construction, characteristics, and causes of arrester failure. Laboratory experiments were used to characterise several arresters in the low-conduction and breakdown regimes where the resistive current dominated the leakage current. An existing arrester condition monitoring system utilising a field probe and current transformer, with third harmonic analysis of the leakage current was evaluated. The method of determining the resistive current, using the third harmonic analysis, was found to be imperfect, and an alternative technique was required. An improved point-on-wave technique was formulated and implemented. It accurately discriminated the resistive and capacitive components of leakage current that flowed through the arrester. As for all techniques, both applied voltage and leakage current flowing through the arrester were required for accurate arrester assessment. Characteristics of new arresters were measured, and then monitored during accelerated ageing tests, which were performed with high levels of leakage current. The point-onwave technique identified the changing characteristics and proved its ability to accurately determine arrester degradation. From the point-on-wave calibrations, an empirical expression was found that gave the resistive current peak values at varying levels of voltage and temperature. Substation implementation of any on-line monitoring technique requires noncontact voltage and current transducers to be used. A non-contact voltage measurement method was assessed, and three capacitive probes were constructed for on-site measurements. These probes gave excellent laboratory results. The voltage measurement method, with the three capacitive voltage probes, was calibrated in a substation, and the initial results were encouraging, indicating that accurate voltage measurement could be achieved.

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