Oxidation stability of insulating oil

The limitations of any accelerated oxidation test carried out under a single set of conditions are such that it may give a good rating to insulating oils known to have a poor performance in service. The BS 148 and IEC oxidation tests have these limitations. The service limits of the three factors materially affecting oil oxidation have been determined and used to design a test procedure which will not misrate oils in this way. An oxidation-stability-assessment procedure designed at CERL uses four sets of conditions: one basic, and three designed to show the susceptibility of oils to oxygen, copper and temperature, respectively. Test criteria and limits are proposed which eliminate oils showing adverse susceptibilities to these factors. When an oil formulation has been accepted by this assessment procedure, further supplies can be checked using the basic conditions only. Comparison of the results, on a series of oils, from the BS 148 and IEC oxidation tests and the CERL procedure and examination of the effect of adding aromatics to two base oils have shown that the CERL procedure selects only those oils which are expected to have a good service performance.