The paper reports on an investigation into the mechanisms and development of the breakdown of a triggered water gap plane-parallel electrode system with an electrically isolated trigger pin. A fast (5000 fps) CCD camera was used to observe the gap after a voltage pulse had been applied to the trigger pin The optical observations obtained by the high-speed digital camera show the development and collapse of a bubble formed as a result of the (500ns) trigger pulse Simulation results using QuickField software indicated that the magnitude of the electric field within the bubble increased after the main voltage had been applied across the water gap. Due to the lower electrical strength of water vapour, an electric avalanche was initiated within the bubble
