The Practical Implementation of Regulatory Guide 1.75 in Nuclear Plant Instrumentation Systems

Implementation of the principles of IEEE 384 and Regulatory Guide 1.75 can help to assure that independence between redundant Class 1E systems is achieved. But designers should not lose sight of the several ways in which this goal can be accomplished. In selecting which method to use in a particular circuit or system, the reliability, maintainability, and cost of each method should be considered along with the independence achieved. The exclusive use of either isolation, separation, or analysis in the multitude of power and instrumentation circuits in a nuclear plant can result in a costly and less-than-optimum design. In determining the most practical approach for implementing IEEE 384 and Regulatory Guide 1.75 the following points should be considered: 1. Initial efforts should be directed toward proper classification of all circuits as Class 1E, Associated, or non-1E. The safety and non-safety function should be evaluated. 2. The use of analysis should be put in proper perspective. It can be used to establish credible faults and criteria for isolation/ separation; it cannot replace the need for proper isolation/ separation. 3. The method of isolating or separating circuits is best determined on a case-by-case basis as opposed to a blind following of blanket criteria. Frequently the best choices are either to separate redundant channel associated circuits or to utilize existing isolating features as the "qualified isolator". 4. Isolation devices, where used, should be specified keeping in mind the intended purpose of the device.