Guidance for Addressing Software Common cause Failure In High Safety-Significant Safety Related Digital I&c systems

Concurrent triggering conditions are required to activate a latent defect

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ML20245E561

2.
Concurrent triggering conditions are required to activate a latent defect
a.
A common defect depends on the quality and commonality of the software
Quality of the software is addressed in first principle 1.

Commonality of software for safety-related software must meet the regulatory requirements in
•
10 CFR Part 50, Appendix A General Design Criteria (GDC)”
–
GDC 24, Separation of Protection and Control
•
10 CFR Appendix B, Quality Assurance
–
Criterion III Design Control
b.
A triggering condition depends on system conditions
There are no regulatory requirements for triggering conditions or systems conditions
c.
A concurrent triggering condition depends on timing and commonality of system conditions
There are no regulatory requirements for concurrent triggers or timing. Commonality of system conditions is addressed in first principle ab 3.bb The effects of a software CCF can be reduced by design
a.
The plant systems or components affected by a software CCF can be limited by design
Limiting the impact of a software CCF on plant systems or components must meet the regulatory requirements in
•
10 CFR Part 50, Appendix A General Design Criteria (GDC)”
–
GDC 24, Separation of Protection and Control
•
10 CFR Appendix B, Quality Assurance

DRAFT B - August 2020
© NEI 2020. All rights reserved. nei.org 32
•
Criterion III Design Control
b.
An I&C system can be designed to force a preferred state in the event of a software CCF
Forcing to a preferred state is a condition of quality and testing which are addressed in first principle 1.
c.
Detection of an event or condition due to a software CCF provides an opportunity for response
and recovery
Detection of an event or condition due to a software CCF is related to quality and testing which are addressed in first principle 1.

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