Mineral Oil Insulated Transformer Installation

Normally indoor transformers will be dry type. - cast resin -

Yes! There is a clear cut stipulation in the Indian Electricity Rules, 1956. Vide Rule 64 (2) (e) (iv): "... only dry type transformers shall be used for installations inside residential/commercial buildings"

Many years ago you could install mineral oil insulated transformers indoors. However the transformer had to be surrounded by a barrier that would hold all the oil if it leaked out. There were other requirements also. In general there were too many problems with using mineral oil transformers indoors and they were rarely used.

The specifications for using mineral oil transformers indoors can probably be obtained from "NEMA" "Electrical Standards & Products Guide". (USA)

You could also look in ANSI standards for requirements on using mineral oil insulated transformers indoors.

It depends upon the local regulations. In 1970s in Srilanka we used mineral oil filled transformers indoors with carbon di oxide fire protection. In 1980s in Saudi same system was used but for auxiliary transformer inside substations silicone oil filled transformer was used with carbon di oxide fire protection. In 2000s I have seen consultants specifying dry type transformers to be supplied by contractor for indoor use- in tender documents- but Utility use mineral oil filled transformers widely.