Electrical treeing in mechanically pre-stressed 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 behaviour in materials containing fillers and barriers. The internal mechanical stresses produced in resin samples containing cast-in metallic electrodes can affect treeing behaviour 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 pre-stressing the material by the inclusion of fibers in tension; a method akin to that employed in the pre-stressing of concrete.