Answer / shridhar awasthi

The oil is used is mineral oil,synthetic oil(Chlorinated diphenyl/askarel) and silicon oil.The synthetic oil is used to reduse the risk of fire.it is heavier than water,non-oxidising and has higher dielectric strength than mineral oil.the synthetic oil contains PCB (polychlorinated benzyne)which is hazardous for human being and handling of this oil require great precaution,which is why this oil is banned in many countries.
Silicon oil offers nearly all the advantage of synthetic oil and does not contain PCB also but ten times cost than mineral oil.
Silicon oil is an option where the fire is also a criterion.

Mineral oil in good condition appears clear and transparent.
It degrades faster due to oxidation,so anti oxidant called inhibitors are mixed with the oil to prolong it's life.
Dielectric strength of treated oil should be 50KV

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