Fire Behaviour of Less-Combustible Dielectric Liquids in a Nuclear Facility

Transformers, modulators and other high voltage electrical equipment are traditionally filled with mineral oil, which serves as a coolant and dielectric insulator. A rising trend is observed globally towards the adoption of less-combustible, bio-degradable transformer liquids at ever increasing voltages and power ratings. This paper presents a study of the fire behavior of five different dielectric transformer coolants: mineral oil, silicone liquid, synthetic ester, and two natural esters. Two types of tests were performed: comparative small-scale tests with two types of pans in the Cone Calorimeter (ISO 5660-1), and intermediate-scale pool fire experiments under the hood of an oxygen consumption calorimeter. The data obtained in this study are used in a fire engineering analysis of the evacuation and smoke removal from the 5900 m² gallery in the planned European Spallation Source in Lund, Sweden. The comparative results indicate a wide range of fire properties for the tested liquids. The heat release rates calculated from the Cone Calorimeter experiments are reasonably consistent with tabulated values, except for the silicone liquid. The latter forms a crust on the liquid surface which significantly impedes combustion. Heat losses from the burning surface to cooler liquid below and pan boundaries have a significant effect on the burning behavior. These scale-dependent phenomena imply that great care should be taken in using small-scale burning data in a fire engineering analysis. Additional work is needed to gain a better understanding of the relation between small-scale tests and large-scale tests, and between the behavior of these liquids in pool fire experiments and that in real fires.