What are the main insulating oil categories?

The specifications required in terms of the physical-chemical properties of insulating oils are summarised in the international specifications. The most frequently used specifications are IEC 60296 and ASTM D3487, which are applicable to naphthenic or paraffinic mineral oils.

The IEC 60296 specification divides insulating oils for transformers into three categories, based on the levels of antioxidants (also referred to as inhibitors) contained in the oil.

• "Non-inhibited" category (U): antioxidant content = 0%
• "Slightly inhibited" category (T): antioxidant content < 0.08%
• “Inhibited" category (I): antioxidant content < 0.4%

The ASTM D3487 specification divides insulating oils for transformers into two categories, based on the levels of antioxidants contained in the oil. 

• Type I: antioxidant content < 0.08%
• Type II: antioxidant content < 0.3%

Other standards exist which apply to ester-based products:

• IEC 61099: Specifications pertaining to new synthetic organic esters for use with electrical equipment
• IEC 62270: Fluids for electrotechnical applications – new natural esters for transformers and similar electrical equipment