International Standard Bases and Safety Classification - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 45

It may be supposed that this collaboration
between IEC and IEEE will be considerably
strengthened in the nearest future.

are not category A or B. If a function does not
meet any of the criteria given below, then it shall
be “non-classified.”
IEC 61226 uses the approach based on quali-
tative criteria and not on probabilistic estimates
(though, it indicates that probabilistic estimates
can complete qualitative criteria). Most of the clas-
sification criteria are given in such a way that no
additional analysis is required. For all categories
examples of functions and systems, performing
the functions, are given.
An I&C function shall be assigned to category
A if it meets any of the following criteria:

IEC SAFETY CLASSIFICATION
PRINCIPLES

Categorization of Functions

IEC 61226 (IEC, 2009,b) defines a function
is determined as a specific purpose or objec-
tive to be accomplished, that can be specified
or described without reference to the physical
means of achieving it. This standard extends the
classification strategy presented in IAEA Safety
Guide NS-G-1.3, and establishes the criteria and
methods to be used to assign the I&C functions
of an NPP to one of the three categories A, B and
C, depending on their importance to safety, or to
an unclassified category for functions.
Category A denotes the functions that play a
principal role in the achievement or maintenance
of NPP safety to prevent design basis event (DBE)
from leading to unacceptable consequences. This
role is essential at the beginning of the transient
when no alternative actions can be taken, even if
hidden faults can be detected. These functions play
a principal role in the achievement or maintenance
of the non-hazardous stable state.
Category B denotes functions that play a
complementary role to the category A functions
in the achievement or maintenance of NPP safety,
especially the functions required to operate after
the non-hazardous stable state has been achieved,
to prevent DBE from leading to unacceptable
consequences, or mitigate the consequences of
DBE. The operation of a category B function may
avoid the need to initiate a category A function.
Category C denotes functions that play an
auxiliary or indirect role in the achievement or
maintenance of NPP safety. Category C includes
functions that have some safety significance, but

• Functions required to reach the non-haz-

ardous stable state, to prevent a DBE from
leading to unacceptable consequences, or
to mitigate its consequences.

• Functions, whose failure or spurious ac-

tuation would lead to unacceptable conse-
quences, and for which no other category
A function exists that prevents the unac-
ceptable consequences.

• Functions required to provide information

and control capabilities that allow speci-
ied manual actions necessary to reach the
non-hazardous stable state.

The I&C functions assigned to category A are
necessary for:

• Reactor shutdown and maintenance of
sub-criticality.
• Isolation of containment.
• Provision of information for essential op-
erator action.
• Decay heat transport the ultimate heat sink.

Typical I&C systems are as follows:

• Reactor protection system.

• Safety actuation system and safety support

features.