What is Askarel transformer oil?

Askarel is not actually oil but a synthetic, non-flammable liquid that was historically used as an insulating fluid in transformers and capacitors. Askarel is a trade name for a group of polychlorinated biphenyls (PCBs), which were used because of their excellent fire-resistant properties. However, due to their environmental toxicity and potential health hazards, the use of Askarel and other PCB-based insulating fluids has been banned or heavily restricted in many countries.