Based on field experience, the paper has proved the practical importance of finding a solution for condition assessment of covered conductors due to electrical ageing under moisture and pollution. The analysis of combined impacts with respect to construction and material properties on tracking erosion of covered conductor and plastic components of OHL accessories was performed for the first time ever. A high-voltage laboratory setup for accelerated ageing tests was designed. A detailed description of the test method is given. We have developed a method for assessing the technical conditions, based on measuring the leakage current. The given diagnostic parameter provides the opportunity to its wide application within diagnostic procedures for condition assessment during electrical ageing in moist and polluted environments.