Single failure

A Single failure is a term used to describe system redundancy standard. Such that a system should be designed to be able to work even if a single failure were to occur. The reason for redundant trains of equipment is to assure that even if 1 component randomly failed then it won't prevent the system from performing its safety function.

Single failure is defined in 10 CFR 50 Appendix A, GDC for Nuclear Power Plants As follows:

A single failure means an occurrence which results in the loss of capability of a component to perform its intended safety functions. Multiple failures resulting from a single occurrence are considered to be a single failure. Fluid and electric systems are considered to be designed against an assumed single failure if neither (1) a single failure of any active component (assuming passive components function properly) nor (2) a single failure of a passive component (assuming active components function properly), results in a loss of the capability of the system to perform its safety functions.

It is important to note that the "single failure criterion" is meant to be in terms of the system design and not in terms of day to day operation. The ability to fulfill a safety function only depends on existing operable equipment. This is important to remember when considering reporting a loss of safety function.

See also

• 10 CFR 50 Appendix A
• ML060260236