Testing of breakdown voltage of transformer oil - Sampling
IEC 156 oil dielectric test set
7.1 Sample containers
Sample size should be approximately three times the capacity of the test cell.

Appropriate sample containers shall comply with IEC 475. An amber glass bottle is the preferred container. Clear glass bottles may be used but they shall be shielded from direct light until ready to be tested. Plastic containers which are not attacked by the liquid to be tested
may be used, but these shall not be used more than once. For sealing, screw caps with polyolefine or polytetrafluoroethylene insert are preferred.

Containers and caps shall be cleaned by washing with a suitable solvent to remove residues of an earlier sample. Containers shall next be rinsed with acetone, traces of which shall be removed by blowing with warm air.

After cleaning, containers shall be immediately capped and kept sealed until used.

7.2 Sampling technique
Sampling of new and used insulating liquids shall be carried out in full compliance with procedures detailed in lEG 475.

When sampling, containers should be almost filled with sample, leaving about 3% of the container volume as free air space.

Breakdown voltage is extremely sensitive to the slightest contamination of the sample by water and particulate matters. Special reference is made to precautions necessary to avoid contamination of the sample and the need for trained personnel and experienced supervision.

Unless otherwise required, the sample is taken where the liquid is likely to be most contaminated, usually at the lowest point of the container holding it.