Treeing is a long term failure mechanism for extruded dielectrics. Sometimes it takes decades to fail. In addition, the formation of trees does not mean the dielectric will fail. Often, trees will begin formation, slow down and then stop formation, so the dielectric will remain operative for an indefinite period of time. Voltage stress and exposure to water accelerate the tendency of the dielectric to tree.
Trees forming due only to high electrical stress are electrical trees.
Trees forming due to exposure to water are electro-chemical or water tress. On exposure, water molecules penetrate the dielectric material. To evaluate the performance of different methods of coping with treeing, one can do an Accelerated Cable Life Test. In the ACLT, test conditions accelerate the growth of trees. These conditions can include subjecting the cable to water at various temperatures, using high electrical stresses, filling the strands of the conductor with water, and the use of varying frequencies.