Practical importance to find the solution for the condition assessment of covered conductors due to electrical ageing under moisture and pollution effects is proved on the basis of field experience. For the first time ever the analysis of combined impacts with aspect to construction and material properties on tracking erosion of covered conductor and plastic components of OHL accessories has been performed. High voltage laboratory setup for accelerated ageing tests is performed. The detailed description of the test method is given. The technical condition assessment method based on the leakage current measurement is developed. The given diagnostic parameter provides the opportunity to its wide application within diagnostic procedures for condition assessment during electrical ageing in moisture and pollution environment.