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:
nths between calibrations). These discussions considered whether the estimate of total drift that occurs between successive surveillances should be estimated through linear extrapolation of the vendor specification or whether multiple increments of vendor drift specifications should be combined using the square root of the sum-of-the-squares method. ANSI/ISA 67.04.01-2018 has updated Section 4.6 to include methods on considering uncertainties resulting from performance monitoring with respect to the effects of instrument drift and other abnormal equipment performance on AFTs. As described in the footnote in Section 4.6 of the standard, when determining the magnitude of drift to be included when establishing the AFT, licensees should estimate on the low side so as not to potentially mask the ability to detect a degrading instrument during a required surveillance. In addition, when establishing an appropriate allowance for total loop uncertainty between the analytical limit and the limiting trip setpoint, licensees should estimate drift on the high side so as to ensure adequate margin for instrument channel performance in achieving the safety objectives. 2.2.4 Definition of Terms ANSI/ISA 67.04.01-2018 introduces additional definitions. These definitions include AFT, ALT, allowable value, limiting trip setpoint, measuring and test equipment, measuring and test equipment uncertainty, nominal trip setpoint, setting tolerance, tolerance interval, tolerance limit, and total loop uncertainty. These definitions are consistent with the NRC staff’s understanding of these terms. 2.2.5 Illustrative Figure Depicting Relationship of Terms Figure 1 of ANSI/ISA 67.04.01-2018 illustrates setpoint relationships for nuclear safety-related setpoints. The figure denotes relative position and not direction, and depicts uncertainty relationships that do not represent any particular direction, combination, or relationship of uncertainty groupings for the development of a trip setpoint or