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Foam-based internal electrical insulation of hollow-core composite insulators (HCIs) is regarded as a superior solution to gases for not requiring continuous monitoring, with polyurethanes (PUs) being the main solution to date. Potential restrictions facing PUs as well as the need for higher dielectric strengths have prompted research activities into alternative foams. This work follows an extensive experimental and simulative approach to investigate the dielectric strength of a novel polymeric foam, known as dry syntactic foam (DSF). Seven different densities in two thicknesses are manufactured and tested under ac, dc, and lightning impulse (LI) voltages. Two finite-element (FE) models are created based on microscope images to reproduce electrostatic- and conduction-dominated voltage distributions and complement the experiments. Under ac and LI, the dielectric strength is found to rise with foam density, whereas no distinctive effect is observed under dc stress. The simulations indicate that under ac and LI, the highest electrical fields are reached uniformly inside gas pores and, that under dc, certainly arranged pores observe significant higher stresses. The ac and LI results are discussed according to Townsend discharges, whereas a mechanism based on a transient process is suggested for dc. [ABSTRACT FROM AUTHOR]