A novel insulating cross-arm (ICA) has been developed for new and existing overhead transmission lines of up to 400 kV. The cross-arm consists of four insulating members, end fittings, field grading devices and a nose connection for the attachment of the conductor. The two main structural elements of the assembly have a unique non-cylindrical geometry which gives them improved mechanical characteristics compared to conventional overhead line insulators. The profile for the compression insulator has been designed. After examining six profile variations, it was determined that the lateral orientation which would give the best performance would be with the flat face of the core facing upwards and tilted by 6o. Using the results obtained from performing flashover tests on a conventional 145 kV insulator, the elevation angle for the compression insulator was set to 6o. The dimensions of the compression insulator were calculated based on the assumption that the ICA would be used to uprate an OHL with L3 towers from 275 kV to 400 kV. The optimal insulator profile was determined to be an alternating profile with three different shed sizes, an arcing distance of 3083 mm and a creepage distance of 12470 mm. Electric field grading devices for the ICA were designed. For the LV end, a grading device resembling a ring which follows the general shape of the cross-section of the insulator was designed. For the HV end, an iterative process yielded two designs. First, the ‘butterfly’ grading device was a unibody piece of cast aluminium for all four ICA members. FEA simulations and tests in the laboratory showed that it could effectively control the electric field at voltages of up to 132 kV. The design was patented and the device was used on six cross-arms installed on a live line in Scotland in August 2013. Second, the ‘M-W’ grading device, was a solution made out of four components for managing the field at voltages of up to 400 kV. The device was designed to be easy to install and service, easy and cheap to manufacture and to have minimal visual impact. The compression insulator and the cross-arm assembly were subjected to a multitude of tests adapted from international standards and the Technical Specifications of National Grid. The performed tests aimed to test the electrical characteristics of the cross-arm and the quality of the materials and manufacturing process of the compression insulator. All of the tests were completed successfully except from the corona extinction test for which the appropriate equipment was not available at the time. Two trials were commissioned to examine how the cross-arm performs in a service-like environment. The snow and ice accretion patterns recorded at the mechanical trial site were used for optimising the profile of the compression insulator. The results after a year of continuous monitoring of leakage current and weather conditions at the live trial site showed that there were humidity and visibility thresholds, above 93% for the former and below 400 m for the latter, which increased the average leakage current by 15% on the tension insulators and by 20% on the compression insulators. It was found that when the longitudinal axis of the cross-arm was perpendicular to the weather the leakage current was higher because more of its surface was exposed. The performance of the novel compression insulators was found to be as good as that of the industry standard tension insulators, reaffirming the potency of the design. Finally, on-site observations showed that the ‘butterfly’ grading device could not effectively manage the electric field on the cross-arm at 400 kV, confirming the results of the FEA simulations and testing.