The composite cross-arm is designed to replace the steel cross-arms of lattice towers to meet the increasing demand for electricity and enable upgrading of transmission systems. The composite cross-arm was developed and installed at a trial site in a service-like environment. The main objectives of the project is to achieve long-term monitoring of leakage current and weather information, interpret data retrieved from the trial site, evaluate performance of the composite cross-arm, investigate effect of long-term low current discharge activities on silicone rubber material and modelling of wide-bodied compression insulator. Several methodologies have been used in this thesis to achieve the objectives of the project. Statistical analysis from six years of trials and experimental studies are performed to evaluate the performance and ageing process of the composite cross-arm under complex weather conditions. Inspections of the trial site investigated the visual change of the composite cross-arm. Experimental work in a fog chamber created low current discharge activities on silicone rubber rod samples and commercial insulators. Based on the findings from data analysis and inspections of the trial site, lumped circuit and FEA modelling approaches have been developed for the simulation of the leakage current, potential distribution and electric field on the wide-bodied compression insulators of the composite cross-arm. The achievements of the study are: the correlation between leakage current and weather conditions has been investigated and mathematic equations are given to predict the leakage; development and distribution of the significant levels of biological growth are given for analysis modelling work, and the effect of biological growth on silicone rubber material is also investigated; the electrical properties of resulting low current discharges are investigated and their impact on silicone rubber materials is analysed; from the modelling work it is suggested that improvement of the field grading device is needed to minimise the low current discharges on compression insulators, to better protect the silicone rubber surface; the long term performance of the composite cross-arm has been evaluated, showing the insulator will be reliable for six years despite organic growth on the upward facing surfaces of the wide bodied horizontal insulators.