Electrical treeing in mechanically prestressed insulation

For several years it has been apparent that the mechanical characteristics of insulating materials have a pronounced influence on their electrical breakdown by treeing. As a consequence of this discovery, it is possible to look again from a different perspective at the treeing behavior in materials containing fillers and barriers. The effect of temperature on treeing can be interpreted in terms of the effect of temperature on the various mechanical properties of the material, and of any particulate or solid barrier inclusions. The internal mechanical stresses produced in resin samples containing cast-in metallic electrodes can affect treeing behavior both positively and negatively, depending on whether this stress in the material is tensile or;compressive. In the light of this experience it has been found possible to enhance the treeing resistance of electrical insulation by using these mechanical effects in the most advantageous way, viz, by mechanically prestressing the material by the inclusion of fibers in tension a method which is akin to that employed in the prestressing of concrete.