Electrical control boxes are common on high vapour cloud hazard sites, and in the case of the Buncefield explosion the ignition source was inside such a box, that was sited in an emergency pump house building. There has, however, been relatively little previous research into this type of ignition mechanism and its effect on the explosion severity. Commercially available electrical control boxes measuring 600 mm high, 400 mm wide and 250 mm deep were used to explore the pressure development, venting processes and flame characteristics of stoichiometric propane/air explosions using aluminium foil and the supplied doors as vent coverings. In some tests, the boxes were empty in order to establish a baseline for the effect of the internal congestion of the boxes. In other tests a congestion array was added. It was found that, in both the empty and congested box tests, the door produced a flat petal shaped flame, which differed drastically from the mushroom flame shape and associated rolling vortex bubble venting that is traditionally observed with large orifice vented explosions. Crown Copyright (C) 2020 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.