and to even out the internal energy storage which marginalises the dielectric strength of paper insulation. To improve the performance of paper insulated bushings, metallic foils can be inserted during the winding process. These act to stabilize the generated electrical fields, homogenising the internal energy using the effect of capacitance. This feature resulted in the condenser/capacitor bushing. The condenser bushing is made by inserting very fine layers of metallic foil into the paper during the winding process. The inserted conductive foils produce a capacitive effect which dissipates the electrical energy more evenly throughout the insulated paper and reduces the electric field stress between the energised conductor and any earthed material. Condenser bushings produce electric stress fields which are significantly less potent around the fixing flange than designs without foils and, when used in conjunction with resin impregnation, produce bushings which can be used at service voltages over one million with great success. Resin insulation Since the 1965s, resin materials have been used for all types of bushing up to the highest voltages. The flexibility of using a castable form of insulation has replaced paper insulation in many product areas and dominates the existing insulated bushing market. As with paper insulation, the control of electric stress fields remains important. Resin insulation has greater dielectric strength than paper and requires less stress control at voltages below 25 kV. However, some compact and higher rated switchgear designs, have earthed materials closer to bushings than in the past and these designs may require stress control screens in resin bushings operating as low as 12 kV Fixing points are often integral with the main resin form, and present fewer problems to earthed materials than the metal flanges used on paper bushings. However, care must be observed in resin insulated bushings designs which use internally cast screens such that the benefit of electrical stress field control is not off set by increasing partial discharge caused by the difficulties of eliminating micro voids in the resin around the screens during the casting process. The need to eliminate voids in resin becomes more sensitive as voltages increases, and it is normal to revert to resin impregnated, foiled paper insulation for bushings rated over 72.5 kV. Bushing failure Bushings sometimes fail due to partial discharge. This is sometimes due to the slow and progressive degradation of the insulation over many years of energized service; however, it may also be