3 Basic materials

3.1 DIELECTRICS

The majority of power transformers in use throughout the world are oil filled using a mineral oil, complying with IEC 60296. In the UK the relevant specification is British Standard (BS) 148, Unused and reclaimed mineral insulating oils for transformers and switchgear which, in its 1998 edition, differs in a number of respects from IEC 60296. More will be said about this later. The oil serves the dual purpose of providing insulation and as a cooling medium to conduct away the losses which are produced in the transformer in the form of heat.

Mineral oil is combustible – it has a fire point of 170°C – and transformer fires do sometimes occur. It is usual, therefore, to locate these out of doors where a fire is ...

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