A number of questionable system grounding practices have been identified at nuclear power plants. These practices could result in unforeseen degradation of the ac Class 1E Power System. (Class 1E is the safety classification of the electrical systems and equipment utilized to mitigate accidents at nuclear power plants.) The paper provides an overview of system grounding with emphasis on the challenges faced with ungrounded and high-resistance grounded systems. It explains how design deficiencies are to be treated with respect to the single failure criterion, then gives a few examples of questionable system grounding practices that have been observed at nuclear power plants in the United States, categorizing these as potential design deficiencies.
