Acids in the oil originate from oil decomposition/oxidation products. Acids can also come from external sources such as atmospheric contamination.

These organic acids are detrimental to the insulation system and can induce corrosion inside the transformer when water is present. An increase in the acidity is an indication of the rate of deterioration of the oil with sludge as the inevitable by-product of an acid situation which is neglected.

The acidity of oil in a transformer should never be allowed to exceed 0.25mg KOH/g oil. This is the critical acid number and deterioration increases rapidly once this level is exceed.

Test method: ASTM D974