Transformer oils insulate the high potential differences, impregnate fibrous insulating materials and contribute to preventing any electric arcs. As a dielectric, they prevent discharges between the live elements at voltages of up to 100 kV. The voltage up to which no discharge of sparks occurs under precisely defi ned conditions is termed the breakdown voltage. The breakdown voltage of a transformer, but also of the oil, must be significantly higher than its nominal voltage. This is by far the most important parameter for the electrical insulating property of the oil. Traces of water, which is a good electrical conductor, have a negative impact on the useful life of the system. Fresh oils are thus dried during production with vacuum thin film vaporisers. These oils are delivered in containers under a nitrogen blanket in order to prevent penetration of humidity. Transformer oils also expand at high temperatures and contract at low temperatures. For 1,000 litres of oil and a temperature difference of 50°C, this amounts to 35 litres. Transformers thus have an expansion reservoir. Depending on the type of construction, this is provided with dry cartouches, which extract humidity from the air. In determining the further serviceability of transformer oils, the breakdown voltage and the water content are decisive criteria.