Transformer oil has mainly two purposes, first for insulation, second as cooling of Transformer core and other winding. So while designing Transformer oil use in transformer depends on voltage rating. So testing of oil is done according to voltage rating.
For the purpose of BDV test, oil sample from Transformer is taken in a Sample Bottle. While taking sample of oil from transformer, Sample bottle should be flushed well by Transformer oil and oil in Sample bottle should be vented properly so that atmospheric moisture could not ingress in the sample oil.
Breakdown Voltage is measured by observing at what voltage, sparking straits between two electrodes emerged in the oil, separated by specific gap. Low value of BDV indicates presence of moisture content and conducting substances in the oil. For measuring BDV of transformer oil, portable BDV measuring kit is generally available at site. In this kit, oil is kept in a pot in which one pair of electrodes are fixed with a gap of 2.5 mm. In this video the practical of BDV test is demonstrated for training purposes.