Since PCB and transformer oil are miscible in all proportions, and since sometimes the same equipment (drums, pumps, hoses, and so on) was used for either type of liquid, contamination of oil-filled transformers is possible.
Dielektrična trdnost novega neobdelanega olja je 12 MV/m(RMS), po obdelavi pa mora biti> 24 MV/m(RMS).