The article presents the results of laboratory measurements of U_d breakdown voltages in a high-voltage vacuum insulating system for different pressures, contact gaps, type of electrode contacts and type of residual gas inside the discharge chamber. First of all, the electrical strength of the discharge chamber with a contact system terminated with contact pads made of W ₇₀Cu ₃₀ and Cu ₇₅Cr ₂₅ material was compared for selected values of contact gaps. It was found that below a pressure of p = 3.0 x 10 ^-1 Pa the electrical strength reaches an approximately constant value for each of the set contact gaps d. Analytical relationships were determined to calculate this value for each of the contact pads used. Above a pressure of p = 3.0 x 10 ^-1 Pa, the measured values of U_d breakdown voltages decrease sharply. The values of breakdown voltages in the discharge chamber with residual gases in the form of air, argon, neon and helium were also determined for selected values of contact gaps d. Depending on the residual gases used, significant differences were noted in the values of pressure p at which the loss of insulating properties in the discharge chamber occurred. These values were 3.3 x 10 ^-1 Pa for argon, 4.1 x 10 ^-1 Pa for air, 6.4 x 10 ^-1 Pa for neon, and 2.55 x 10 ⁰ Pa for helium, respectively.