Signs (visual inspection)
Signs of the criticality only become apparent through an internal inspection of the transformer, following a failure for example. Where the criticality is present, grey deposits are typically observed on copper conductors (copper sulfide) or silver contacts (silver sulfide). On the other hand, copper sulfide contamination shows up on insulating papers as grey spots and streaks.
Representative sampling
Should it be decided to carry out an internal inspection of the transformer, following a failure or in order to carry out a thorough inspection, it is strongly recommended to take samples of the insulating papers in accordance with relevant protocols and procedures. In particular, it is advisable to select the paper at the top, bottom, and middle of the both primary and secondary windings for each phase, taking multiple paper samples in areas with greatest darkening or embrittlement of the papers themselves.
During external inspection of the transformer it is necessary to take samples of insulating oil in accordance with the reference norm and the operating instructions attached to the sampling kits.
