1For the full text of the General Design Criteria and other sections of the regulations cited in this  guide, see 10CFR Part 50, “Domestic Licensing of Production and Utilization Facilities.”
of the NRC’s  regulations, techniques used by the staff in evaluating specific problems or postulated accidents, and data needed by the NRC staff in its review of applications for permits and licenses.  Regulatory guides are not substitutes for regulations, and compliance with them is not required.  Methods and solutions different from those set out in the guides will be acceptable if they provide a basis for the findings requisite to the issuance or continuance of a permit or license by the Commission.
This guide was issued after consideration of comments received from the public.  Comments and suggestions for improvements in these guides are encouraged at all times, and guides will be revised, as appropriate, to accommodate comments and to reflect new information or experience.  Written comments may be submitted to the Rules and Directives Branch, ADM, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001.  Regulatory guides are issued in ten broad divisions:  1, Power Reactors; 2, Research and Test Reactors; 3, Fuels and Materials Facilities; 4, Environmental and Siting; 5, Materials and Plant Protection; 6, Products; 7, Transportation; 8, Occupational Health; 9, Antitrust and Financial Review; and 10, G
U.S. NUCLEAR REGULATORY COMMISSION              Revision 3December 1999REGULATORY GUIDE
OFFICE OF NUCLEAR REGULATORY RESEARCH
REGULATORY GUIDE 1.105
(Draft was DG-1045)SETPOINTS FOR SAFETY-RELATED INSTRUMENTATION
A.  INTRODUCTION
Criterion 13, "Instrumentation and Control,"1 of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50, "Domestic Licensing of Production and Utilization Facilities," requires, among other things, that instrumentation be provided to monitor variables and systems and that controls be provided to maintain these variables and systems within prescribed operating ranges.
Criterion 20, "Protection System Functions," of Appendix A to 10 CFR Part 50 requires,among other things, that the protection system be designed to initiate operation of appropriate systems to ensure that specified acceptable fuel design limits are not exceeded.
Paragraph (c)(1)(ii)(A) of § 50.36, "Technical Specifications," of 10 CFR Part 50 requires,in part, that, where a limiting safety system setting is specified for a variable on which a safety limit has been placed, the setting be so chosen that automatic protective action will correct the abnormal situation before a safety limit is exceeded.  It also requires, among other things, that the licensee notify the NRC if the licensee determines that an automatic safety system does not
2Copies may be obtained from the Instrument Society of America, 67 Alexander  Drive, Research Triangle Park, NC 20779.
function as required.  The licensee is required to then review the matter and record the results of the review.
This guide describes a method acceptable to the NRC staff for complying with the NRC's regulations for ensuring that setpoints for safety-related instrumentation are initially within and remain within the technical specification limits.  The guide is being revised to endorse Part l of ISA-S67.04-1994, “Setpoints for Nuclear Safety-Related Instrumentation.”2 This standard provides a basis for establishing setpoints for nuclear instrumentation for safety systems and addresses known contributing errors in the channel.
The information collections contained in this regulatory guide are covered by the requirements in 10 CFR Part 50, which were approved by the Office of Management and Budget, approval number 3150-0011.  The NRC may not conduct or sponsor, and a person is not required to respond to, a collection of information unless it displays a currently valid OMB control number.
B.  DISCUSSION
Instrument setpoint uncertainty allowances and setpoint discrepancies have led to a number of operational problems.  Operating experience indicates that setpoints for safety-related instrumentation may allow plants to operate outside the limiting conditions of operation specified in their technical specifications.  Licensees have discovered conflicts between existing setpoints and engineering calculations.  The causes for these setpoint discrepancies were problems with industry practices that led to errors in calibration procedures and a lack of understanding of the relationship of the setpoint to the allowable value.  Additional problems noted included varying setpoint methodologies for engineering calculations, a lack of a consistent definition of allowable value between different setpoint methodologies, and improper understanding of the relationship of the allowable value to earlier setpoint terminology, procedures, and operability criteria.  Further problems were noted when procedures (the setpoint process) (1) failed to provide an adequate margin between the instrument as-left criteria and the values (trip set point or allowable values)required per the technical specifications, (2) did not always reflect current design criteria,and (3) did not ensure that revised instrument loops were verified to the original design requirements or that instrument modifications were evaluated for their effect on setpoint calculations.  It has also been noted that licensees do not typically verify whether setpoint calculation drift assumptions have remained valid for the system surveillance interval.
ISA-S67.04 was revised in 1987 to provide clarification and to reflect industry practice.  The term "trip setpoint" was made consistent with the terminology used by the NRC staff.
The standard was revised further in 1994.  The effects of uncertainty allowances and discrepancies in setpoints, along with operational experience, were appropriately addressed during this revision of ISA-S67.04.  This revision of the standard also reflects the Improved
3Single copies of regulatory guides, both active and draft, may be obtained free of charge by writing the Office of Administration, Attn:  Reproduction and Distribution Services Section, U.S. Nuclear Regulatory Commission,  Washington, DC 20555, or by fax to (301)415-2289, or by email to <DISTRIBUTION@NRC.GOV>.  Copies are also available  for inspection or copying for a fee from the NRC Public Document Room at 2120  L Street NW., Washington, DC; the PDR’s mailing address is Mail Stop LL-6,  Washington, DC 20555; telephone (202)634-3273; fax (202)634-3343.
Technical Specification program (a cooperative effort between industry and the NRC staff)and reflect current industry practice.  This standard provides a basis for establishing setpoints for nuclear instrumentation for safety systems and addresses known contributing errors in a particular channel from the process (including the primary element and sensor)through and including the final setpoint device.
The term "trip setpoint" is retained in ISA-S67.04-1994.  However, Figure 1 in ISA-S67.04-1994 (for convenience, this figure has been reproduced as Figure 1 in this guide) has been revised to depict region "E," "a region of calibration tolerance."  The calibration tolerance uncertainties depicted by region "E" should be defined and accounted for in the licensee’s setpoint methodology.  A trip setpoint value identified to be outside region "E"regardless of direction requires readjustment to satisfy the setpoint methodology and uncertainties identified in Figure 1 (acceptable as-left condition).  It should be noted that this standard does not define "nominal" trip setpoint.  The trip setpoint as depicted in Figure 1 is consistent with the term "nominal" trip setpoint as shown about a defined calibration tolerance band.
Figure 1 of the standard provides setpoint relationships for nuclear safety-related setpoints.  The figure denotes relative position and not direction, but it should be noted that the uncertainty relationships depicted by Figure 1 do not represent any one particular method (direction, combination, or relationship of uncertainty groupings) for the development of a trip setpoint or allowable value.
Section 4 of ISA-S67.04-1994 states that the safety significance of various types of setpoints for safety-related instrumentation may differ, and thus a less rigorous setpoint determination method may be applied for certain functional units and limiting conditions of operation (LCOs).  A setpoint methodol
ogy can include such a graded approach.  However,the grading technique chosen by the licensee should be consistent with the standard and should consider applicable uncertainties regardless of the setpoint application.  Additionally,the application of the standard, using a "graded" approach, is also appropriate for non-safety system instrumentation for maintaining design limits described in the Technical Specifications.  Examples may include instrumentation relied on in emergency operating procedures (EOPS), and for meeting applicable LCOs, and for meeting the variables in Regulatory Guide 1.97, “Instrumentation for Light-Water-Cooled Nuclear Power Plants To Assess Plant and Environs Conditions During and Following an Accident.”3
The industry consensus standard ANSI/ANS-10.4-1987, "Guidelines for the Verification and Validation of Scientific and Engineering Computer Programs for the Nuclear Industry," provides helpful information on the qualification of setpoint methodology software.
ISA-S67.04-1982 has been used by licensees for setpoint methodology and instrument drift evaluations.  ISA-S67.04-1994 provides limited guidance on drift evaluations and uncertainty term development for the evaluation of an instrument surveillance interval.  The
A D
E
B
C
Safety Limit
Analytical Limit
Note:This figure is intended
to provide relative
position and not to
imply direction.
Allowance described in paragraph 4.3.1Allowance described in paragraph 4.3.1Region where channel may be determined inoperable Plant operating margin Region of calibration tolerance (acceptable as left condition)
described in paragraph 4.3.1
A.
B.
C.
D.
E.Allowable
Value
editor evaluating revision(LSSS)
Trip
Setpoint
(LSSS)Normal
Figure 1.  Nuclear Safety-Related Setpoint Relationships