UNE-EN IEC 61125:2018

This method describes a test for evaluating the oxidation stability of insulating liquids in the delivered state under accelerated conditions regardless of whether or not antioxidant additives are present. The method can be used for measuring the induction period, the test shall be continued until the volatile acidity significantly exceeds 0,10 mg KOH/g in case of mineral oils. This value may be significantly higher in case of ester liquids.The insulating liquid sample is maintained at 120 °C in the presence of a solid copper catalyst whilst bubbling air at a constant flow. The degree of oxidation stability is estimated by measurement of volatile acidity, soluble acidity, sludge, dielectric dissipation factor or from the time to develop a given amount of volatile acidity (induction period).In the informative Annex B a test method for evaluating the oxidation stability of inhibited mineral insulating oils in the delivered state by measurement of the induction period is described. The method is only intended for testing the continuity of oil supplies. The results do not necessarily provide information on the performance in service. The oil sample is maintained at 120°C in the presence of a solid copper catalyst whilst bubbling through a constant flow of oxygen. The degree of oxidation stability is estimated by the time taken by the oil to develop a determined amount of volatile acidity. Additional criteria such as soluble and volatile acidities, sludge and dielectric dissipation factor may also be determined after a specified duration.In the informative Annex C a test method intended to simulate the thermal-oxidation behaviour of ester insulating liquids (headspace of air at 150°C for 164h) is described.Additional test methods e. g. IEC TR 62036 based on differential scanning calorimetry may also be used as screening tests, but are not part of this standard.