Electrical tree: Micro-channels of carbonisation or non-insulation found within insulation that is irreversibly damaged. Electrical treeing will lead to a completed failure path and failure of insulation relatively quickly. Below is an electrical tree that is growing from a water tree.
(https://www.cabledatasheet.com/gallery/74_04_11_24_9_00_35.jpeg)
(https://www.cabledatasheet.com/gallery/74_04_11_24_9_02_16.png)
(https://www.cabledatasheet.com/gallery/74_04_11_24_9_02_53.png)
(https://www.cabledatasheet.com/gallery/74_04_11_24_9_04_17.jpeg)
(https://www.cabledatasheet.com/gallery/74_04_11_24_9_06_27.jpeg)
(https://www.cabledatasheet.com/gallery/74_04_11_24_9_07_19.jpeg)
https://pure.manchester.ac.uk/ws/files/74924313/ICD_2018_Budapest.pdf