Document: NRC Regulatory Guide
Document ID: bcbe26a0-004f-4616-b7f4-b32b2bfe4909
Document Type: regulatory_guide
Title: Setpoints for Safety-Related Instrumentation (Rev. 4)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2005/ML20055G823.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.105
CFR Part: 
CFR Title: 

Content:
tor the performance of safety-related instrument channels to ensure that the instrument channel performance during operations is consistent with those assumptions. This RG provides guidance to address both the selection of appropriate setpoints and the determination of anticipated limits of setpoint deviation. Lack of adequate allowance for instrument setpoint uncertainty due to drift or the effects of variations in process or ambient conditions has led to operational problems. Past operating experience has indicated that the established setpoints for safety-related instrumentation may also have enabled licensees to operate plants outside the allowable values for the limiting conditions of operation (LCOs) defined in the plant technical specifications. After the establishment of formal setpoint evaluation programs to address this concern, licensees discovered conflicts between the calibration setpoints implemented within the plant calibration procedures and the results of formal engineering calculations used to account for instrument channel uncertainties when establishing appropriate setpoints. These setpoint discrepancies were partially associated with industry practices that led to errors in defining appropriate instrument channel performance test acceptance criteria within calibration and surveillance procedures and a lack of understanding of the relationship of the as-found trip setpoint to the plant technical specification allowable values. DG-1363, Page 6 2.2 Technical Issues Addressed by ANSI/ISA 67.04.01-2018 2.2.1 Technical Specification Allowable Values Historically, the allowable values in many older plant technical specifications were established without the benefit of a consistent, formal analysis of expected instrument channel performance that accounts for all expected performance uncertainties. RIS 2006-17 and TSTF-493, Revision 4, address this concern by identifying the as-found tolerance (AFT) limit as a benchmark for assessing instrument channel