This paper focuses on the effects of dielectric strength on transformer oil reliability and on a reproduction of a voltage breakdown simulation during a dielectric strength test. A typical Foster test cell was used in order to measure the dielectric strength of a variety of oils. For a more complete picture, it requires the identification of more parameters but this paper examines only the above one as a result of the operation of a testing apparatus. It is shown that FEM/FDM analysis provides same information for the electrode gap. Bruce electrode provides highly uniform electric field. Factors such as voltage rate, relaxation time between measurements as well as the shape of the testing electrodes influences the dielectric strength of the liquid. © 2021 School of Science, IHU. All rights reserved.