Since the pioneering work of Harrison and Eyre (1986), the existence of secondary or external explosion outside explosion vents is recognized. It was later shown (Proust, 2004(Proust, , 2010) that the phenomenon is rather systematic and that this explosion can be much more powerful than the internal explosion particularly when the mixture is very reactive. But today, the understanding of the formation of the external cloud and its subsequent combustion remains largely outstanding. Very rapid burning was noticed and significant UVCE pressure effects. In some circumstances, a preexisting flammable cloud encompasses the vented vessel, like in Buncefield for instance. What would happen if the cloud is ignited inside the vessel resulting in an external explosion developing inside the preexisting flammable cloud ? In this paper, new information is presented about the physics of the external explosion and the subsequent combustion of outside cloud. Experiments and numerical simulations were performed.