On the Mechanism of Tree Initiation

Although the dc-tree-initiation voltage at a needle electrode in a plastic is very high, a tree can be formed by grounding the needle after charging it with a much lower dc voltage than the tree-initiation voltage. The tree formed by grounding will be called "a grounding tree". Since the formation of a tree under ac voltages is considered to be due to the repetition of charging and discharing, some relationship may exist between the ac tree and the grounding tree. The tree-initiation observed with a voltage composed of dc and ac shows that it is caused by a nearly constant value of the ac component, irrespective of the magnitude of the dc component. This also indicates that the mechanism of the intiation of an ac tree and that of a grounding tree are similar. An explanation is also given as to why the tree shape is branch-like or bush-like with ac voltages.