Abstract: A test vessel assembly (10) comprises a central test vessel 12 defining a chamber 18 in which a sample to be tested may be stored. A pair of side adjustable electrodes 30, 30a is received in the chamber and immersed in the sample under test. A gap between the electrodes can be adjusted by respective electrode adjusting assemblies, each comprising a shaft 32 connected to a respective electrode, the shaft being moved in and out of the test vessel 12 by rotation of an associated adjusting wheel 60. In use, each electrode 30, 30a is electrically connected to a contact in an insulated horn of a test assembly via the respective shafts, for the supply of a steadily increasing voltage to determine the breakdown voltage of the sample. To prevent the breakdown in air rather than in the sample, care is taken to ensure sufficiently large creepage and clearance distances between the connections to the electrodes and from the connections to the electrodes to a wall of the test chamber.