The aim of the research is to investigate and analyse the full discharge events occurring on the oil-pressboard interface that leads to the flow of leakage current. The leakage current is relatively small and insufficient to trip the protection system of a large transformer. The repetition of this kind of discharges could further degrade the barrier board surface. Experimental work to study the characteristics of this leakage current is required to provide understanding and useful information for condition monitoring programs as well as assist manufacturers in improving their design of inter-phase barrier board arrangements. The research is also developing models that are verified against the experimental data.