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Treeing is an electrical prebreakdown phenomenon. The term is applied to that type of damage which progresses through a dielectric section under electrical stress so that, if visible, its path resembles the form of a tree . After initiation at a point of high and divergent electric stress, treeing can progress in dry dielectrics by periodic partial discharge; this is called electrical treeing. It may start and progress more slowly at lower electric stress in the presence of moisture without a detectable partial discharge; this is called water treeing. Treeing may also occur rapidly as the result of an impulse voltage. Trees which start to grow from within the insulation and progress symmetrically outward, parallel to the electric stress, toward both electrodes are called bow-tie trees because of their appearance. Electric stress and stress concentration are always required. Treeing has been observed to occur in all solid organic dielectrics tested and was observed in paper/oil insulation systems many years ago. Treeing may or may not be followed by complete electrical breakdown of the dielectric section in which it occurs, but in solid extruded dielectrics it is the most likely mechanism of electrical failures which do not occur promptly but rather appear to be the result of an aging process.
Union Carbide Corporation, Bound Brook, N.J.