Test assembly of testing of breakdown voltage of insulating oils
IEC 156 semi-automatic oil test set
4.1 Test cell
The volume of the cell shall be between 350ml and 600ml.

The cell shall be made of material that is electrically insulating, transparent and chemically inert, resistant to the insulating liquid and the cleaning agents which may be used.

The cell shall be provided with a cover and shall be designed to permit easy removal of the electrodes for cleaning and maintenance.

Examples of suitable cell designs are given in figures 1 and 2.

4.2 Electrodes
The electrodes shall be made either of brass, bronze or austenitic stainless steel. They shall be polished and, in shape, either spherical (12,5mm to 13,0mm diameter) as shown in figure 1 or partially spherical of the shape and dimensions given in figure 2. The axis of the electrode system shall be horizontal, and at least 40mm below the surface of the test liquid in the cell. No part of the electrode shall be closer than 12mm to the cell wall or stirrer. The gap between the electrodes shall be 2,50mm±0,05mm.

The electrodes shall be examined frequently for pitting or other damage, and shall be maintained or replaced as soon as such damage is observed.

4.3 Stirring (optional)
The test may be conducted with or without stirring. Differences between tests with or without stirring have not been found statistically significant. A stirrer, however, may be convenient especially with apparatus capable of automatic operation.

Stirring may be achieved by means of a two-bladed impeller of effective diameter 20mm to 25mm, axial depth 5mm to 10mm, rotating at a speed of 250 r.p.m to 300 r.p.m. The impeller shall not entrain air bubbles and preferably rotate in such a direction that the resulting liquid flow is directed downward. It shall be constructed so that it is easily cleaned.

Stirring by means of a magnetic bar (20mm to 25mm in length and 5mm to 10mm in diameter) is an acceptable alternative when there is no risk of removing magnetic particles.

The dimensions of the stirring device shall conform to the clearance requirements in 4.2.