The overvoltage protection performance of zinc oxide surge arresters under fast transient conditions is investigated in this work. Double impulse tests have been conducted on 15kV rated zinc oxide surge arresters to investigate impulse preconditioning effects on the zinc oxide protective characteristics. Test results show that the application of double impulses of similar shape and amplitudes with the same or opposite polarities and 10ms or longer time intervals has no significant effect on the protective performance. Impulse voltages and currents up to 6kA were used to test pairs of parallel-connected arresters with both matched and unmatched voltage-current characteristics. The current sharing performance of both matched and unmatched arresters improves as the current increases towards the rated value. The thermal differences between columns significantly change the current balance and may jeopardise the protective performance of multicolumn devices. Asymmetrical configuration has also been proved to affect the current balance in parallel arresters. Changes in the spark gap breakdown characteristics were observed when tested with the arrester in parallel.