Power grids worldwide are expanding not only driven by ambitious clean energy, but also because of the rising need for reliable energy. Key components of these power grids are transformers. Transformers are traditionally filled with mineral oil, to serve as a coolant and dielectric insulator. Now globally a rising trend is observed towards the adoption of less flammable, biodegradable transformer liquids at ever increasing voltages and power ratings. The objective of this report is to discuss past, current and future attempts to quantify the fire and explosion risk in less flammable liquid filled high voltage transformers. Testing procedure standards that give a reliable assessment of the fire behavior of electro technical insulating liquid based on relevant physical characteristics of the fluids are currently under development, such as IEC 60695-8-3. However more effort is required in order to provide meaningful information concerning the relation between small-scale tests and large-scale tests and that between the tests and failure scenarios in real life applications.