The detection rate of on-line monitoring is not 100%, as some instantaneous failures will always escape detection. In the example below, this detection efficiency is estimated at 60%. It is often useful to consider separately major failures and catastrophic failures. The later involves tank rupture, fire, and consequential damage and is to be treated separately because of costly consequences. The breakdown of failure rate in different categories is depicted in Figure 8 with an example of values. This breakdown of failure is useful in the calculation of the potential benefits to be derived from transformer monitoring.