For an optimal dimensioning with regard to the grid operation, all components of the high voltage direct current trans- mission have to be rated according to real stresses. Hence, the behaviour of the insulating system has to be investigated not only under DC voltage stress, but also under a superimposed stress of DC and lightning impulse voltage. Especially with dielectric interfaces, the accumulation of surface and space charges has to be considered. Their influence on the dis- tribution of the local electrical field and thereby on the discharge behaviour is proved using the example of insulators for gas-insulated systems with a metallic particle on the surface. It is found that under superimposed voltage, the insulating properties of the arrangement with the defect are determined by the interaction of space charges in the gas and surface charges on the solid insulator. The findings of the investigations also affect the proven procedure of type tests.