Grape-clusters responsible for an unacceptable capacitance loss in BOPP(Biaxially Oriented Polypropylene) roll capacitors were reproduced in a lab scale test system and then grape-clustering mechanisms were investigated by studying the damage morphology. Experimental observation indicates that the grape-cluster is not a discrete event but a sequential cascade event resulting from an unsuccessful clear which produces damages at its erosion edges. Initiation of the puncturing event may take place at weak spots such as impurities and microgaps. An extensive partial discharge may be one of the causes for the initiation. Propagation and hence a massive electrode erosion takes place by a lateral dissipation of stored electrostatic energy. A sudden release of such energy with energetic ions generated in a discharge event creates damages at its erosion edges when incompletely dissipated. Such damages result in weak spots again with an electric field enhancement and act as initiation sites for the next events.