The decarbonisation of the electrical power system makes the expansion and conversion of the existing electrical energy networks unavoidable. In some cases, for example the connection of offshore wind-farms to the existing grids, the high voltage direct current (HVDC) transmission proves to be advantageous. Nevertheless, the integration of new transmission technologies has to continue to provide the high security of electrical power supply. Therefore, all electrical components have to withstand several tests. In addition to the high voltage test with pure loads, such as DC and impulse voltages, testing with composite voltages, e.g. DC and lightning impulse (LI) voltage, is state-of-the-art for HVDC components. Special test circuits with blocking and coupling elements for the different voltage sources are used to generate these composite voltages. The blocking and coupling elements influence the shape of the composite voltage, thus affecting the reproducibility of the voltage waveform, which is necessary for standardised tests. This study presents the results from measurements of composite voltages consisting of DC and LI voltages with different blocking and coupling elements. The influence of the elements on the front and on the shape of the composite voltage is examined. Derived from the results, recommendations for the test execution are given.