Recent experiments under X-ray examination have shown that a transition from planar front to equiaxed growth is likely to occur in the case of faceted interfaces. Such a transition is suspected to be at the origin of the sudden occurrence of deleterious small grains, among large columnar grains, observed in photovoltaic silicon ingots. A model is presented for the occurrence of equiaxed grains observed ahead of a planar faceted interface. Simple expressions are obtained which predict when the equiaxed structures should develop, in the case of rough (thermal dendrite) and of faceted equiaxed grains. These models provide a semiquantitative basis to discuss the Faceted front to Equiaxed structure Transition (FET). Then, it is applied to the case-study of photovoltaic silicon. Further developments are proposed to improve the model.