This International Standard describes specifications and test methods for unused natural esters in transformers and similar liquid-impregnated electrical equipment in which a liquid is required as an insulating and heat transfer medium. Use of natural esters is not recommended for electrical equipment that is open to the atmosphere. The exposure of natural ester to air leads to rapid deterioration of the insulating liquid. Use of natural esters should be restricted to sealed units, or with conservator tank protected from the contact with atmosphere by a membrane or other suitable system In this standard the term natural esters applies to insulating liquids for transformers and similar electrical equipment with suitable biodegradability and lower environmental impact. Such natural esters are vegetable oils obtained from seeds and oils obtained from other suitable biological materials and delivered to an agreed point, at a set date. These oils are comprised of triglycerides. Natural esters with additives are within the scope of this standard. Because of their different chemical composition, natural esters differ from insulating mineral oils and other insulating liquids that have high fire points, such as synthetic esters or silicone fluids. Natural ester-derived insulating liquids with low viscosity have been introduced but are not covered by this standard. IEC 63012 standard covers these liquids.. This standard is applicable only to unused natural esters. Reclaimed natural esters and natural esters blended with other insulating liquids are beyond the scope of this standard. The chemical nomenclature and scientific notations used in the standard are in accordance with the IUPAC handbook (Quantities, Units and Symbols in Physical Chemistry).