Classification of IC Systems: A Practical Approach

摘要

The benefit of classifying I&C structures, systems, and components as to their relative importance to maintaining safe operation of the plant has long been recognized in the nuclear industry. Assignment of a system or component to a particular class has historically been done according to a deterministic set of criteria that describe the role the system or component will play in assuring safety. In the United States, this deterministic approach led to two class definitions; class IE for that I&C equipment that is related to the safety of the plant, and non-class IE for everything else. International standards for the nuclear industry establish a graded approach with three categories, A, B, and C, being used to designate relative safety importance of functions that will be performed by the I&C systems and equipment. The systems and equipment are assigned to classes 1, 2, and 3 according to the functions they perform. Other industry segments approach safety classification from a different perspective that is based on the needed reduction in risk that an I&C function can provide as determined by probabilistic risk assessment. This probability based approach to classification also results in a graded set of classes, called safety integrity levels, or SIL, that reflect the relative importance of the functions. Regardless of the approach taken to arrive at the I&C classes, the primary objective is to determine general requirements that must be met to assure that the I&C may be relied upon to perform its safety important functions. This paper describes a blended approach to classification of I&C system that is intended to meet both US and international regulatory needs, while at the same time taking insight from the non-nuclear industry's approach to safety.