For some years, a research project has been undertaken to investigate lightning damage of distribution transformers in Queensland. Previous work has resulted in a substantial reduction in the failure rate, and the current work is aimed at an investigation of the cause(s) of the remaining failures (about 0.3 percent per annum). Distribution transformers and their protecting lightning arresters are exposed to direct lightning strokes, and can therefore be subjected to over-voltages of severity considerably greater than occur in standard laboratory impulse tests. Standards call for tests with impulses of about 100 kV rising in 1 μs — the average rate of rise resulting from a direct strike is about 1500 kV/μs, with an upper limit of about 4000 kV/μs. In this paper, we report on the results of steep-front impulse voltage tests on typical distribution transformer insulation, including oil gaps, inter-twin and inter-layer insulation. Measurements of time-to-breakdown were made in the range 0.05 to 1 μsec, and the resulting volt-time curves are compared with similar measurements previously made on lightning arresters. The steep-front breakdown data are analysed using normal and extreme value statistics.