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Modern insulation systems have to become more compact, less massive and environmentally acceptable. Elastic syntactic foams represent a multifunctional, elastic and compressible hybrid insulation material with a low density and a strong adhesion to polymers and metals. This kind of 'elastic foam' consists of a silicone gel and polymeric microspheres in a range of several 10 μm used as filler, which are often additionally coated with CaCO3. Elastic syntactic foams are increasingly used for power transmission equipment and also dc applications e.g. in cable terminations. This paper deals with the breakdown mechanisms under short- and long-term dc stress. Therefore, electrical and optical investigations, as well as field simulations on this insulation material have been carried out. The results should serve as a basis to develop design criteria for such dc insulation systems. It is found that coating the spheres improves the dielectric strength of this insulation material. Variation of the filling degree also influences the electrical properties. The discharge process itself takes place in the silicone gel due to field displacements caused by the microspheres.