When a cable is subjected to a considerably higher test voltage than what it typically sees in service, any defects in the cable will see higher stress levels that may grow within the insulation. This is a phenomenon known as “treeing” and these trees arise at stress enhancements where there are voids, protrusions, contaminants, or water trees. The term “treeing” stems from their branch-like structure resembling a tree.

Electrical trees are channels of carbonization that arise from partial discharge activity within the insulation. Once an electrical tree grows big enough and bridges the electrodes of the cable system, a breakdown of the cable insulation is created. Water trees are tree-like structures that form from the electrochemical interaction of the electric field and water ingress within the cable. Their growth is extremely slow, but they act as stress enhancements, which can help to initiate an electrical tree. Below is a photograph of a water tree growing into an electrical tree.