Based on the results of the tests described above, a mean time be- fore failure (MTBF) of 10,000 hours was predicted for this high- voltage transformer. When the transformers were assembled into instruments and put into service, however, failures began to occur at the 5000-hour mark. Physical examination of the failed units identified breakdown in the insulation system in the region
between the 150-megohm resistors. This identification of a failure mechanism precipitated reevaluation of the integrated transformer as well as a literature search for information on
peroxide system failures, subsequently targeted on treeing. The main reevaluation concentrated on treeing as described in the following paragraphs.