The Operating Risk Index exposes problems with operating the transformer given its current conditions. Depending on the state of the liquid and solid insulation, operating outside of the optimal loading conditions may place the transformer in danger of failing. If managed well, the level of Operating Risk will be low even with a poor physical condition (when nearing end of life).
The Operating Risk Index graph gives a succinct view of how the risks scored in each sub-index is affecting the major index.
The Operating Risk Index is assessed with the combination of the following sub-indexes related to the insulation condition as shown in the graph:
Observed Hot spot temperatures .
Observed high instantaneous levels of water content in oil (WCO) .
Suppression of oil breakdown voltage by degradation, impurities, and moisture .
The risk of bubble formation given hotspot temperatures and water content of the paper (WCP) .
Water Droplet Risk
The risk of water droplet formation given hotspot temperatures and water content of the paper (WCP) .
The ratio of carbon oxide gases indicating the risk of accelerated cellulose aging due to contact with hot metal .
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 "Transformer thermal modelling." CIGRE Brochure No. 659, 2016.
 IEC, Mineral Insulating Oils in Electrical Equipment. Supervision and Maintenance Guidance. BS EN 60422:2013, 2013.
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 IEEE, Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers. IEEE Std C57.104-2008 (Revision of IEEE Std C57.104-1991), vol., no., pp.1-36, 2 Feb. 2009, doi: 10.1109/IEEESTD.2009.4776518.
 IEEE, Guide for Interpretation of Gases Generated in Natural Ester and Synthetic Ester-Immersed Transformers. IEEE Std C57.155-2014, vol., no., pp.1-52, 28 Nov. 2014, doi: 0.1109/IEEESTD.2014.6966710.
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