Interfaces play a major role on charge injection in dielectrics under high voltage. As a consequence, space charge distribution measurements under high voltage may depend on the chosen interface conditions during the tests. In this paper, it is shown that various mountings of an insulating polymer (polyethylene) in a space charge distribution measurement setup develop very different intrinsic dipoles at the interfaces generating in turn an interface voltage that may interact with energy levels between the electrode and the insulator and thus with injection laws. These dipoles are minimal when the insulating polymer has hot-pressed carbon-loaded polymer electrodes or is coupled with silicone oil to carbon-loaded EVA electrodes. On the contrary aluminum coating or direct contact with aluminum in presence of silicone oil for the coupling generates large dipoles compared to gel coupling or with gold coating. The measurements were carried out with an ultra sensible pressure-wave-propagation measurement setup under very low electric field (±30 V/mm at most) for calibration purpose. Aluminum and stainless steel was used for the setup wave-guide in order to check its influence. Interface voltage up to 5 V could be estimated with an uncoated sample in contact with aluminum through silicone oil.