Properties of Safety Important I&C Systems and their Components - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 82

For protection against unauthorized access,
the following is provided: administrative mea-
sures (access restriction in premises); physical
protection (seals on door locks, cases - safes,
etc); software methods (use of passwords, access
restriction through external interfaces and service
equipment); location of programs and information
in write protected memory spaces; alarm, warning
access obtaining in the device (for example, open
of case doors) and / or attempt of unauthorized
modification of programs and information. If ac-
cess in a device can be required, for example, for
maintenance, recovery, reconfiguration, adjust-
ment of software, such a possibility is provided
without decreasing of protection efficiency against
unauthorized access.

its generation) and false operation (identified by
availability of an output signal without conditions
for its generation). As a reliability measures for
“false operation” failures, failure flow parameter
is taken, for “nonoperation” failures availability
factor is standardized. For one and the same
function several types of failures can be defined,
which differ by cause of occurrence and / or ef-
fects, which they cause; in these cases failure
criteria and reliability measures are determined
individually for each type of failures.
Required value of reliability measures of each
function is determined on agreement between a
designer of I&C system (developer of SHC or
peripheral equipment) and NPP (customer), in
this case function category is considered. Values
of these measures for functions, performed by
updated or new I&C systems and SHC, are usu-
ally significantly exceed measures of the best
prototypes, earlier operated in NPP.
During standardization and estimation of reli-
ability function, reliability of all devices, taking
part in its performance (including peripheral
equipment, removable component parts of SHC,
data transmission facilities and connecting lines,
power sources, etc.) is considered. For this purpose
failure criteria and required values of reliability
measures for devises, used in the I&C system, are
regulated. Failure criterion is non-compliance,
inaccurate meeting or non-conformance of speci-
fied properties of at least one of required func-
tions of device, this fact required its recovery or
substitution. Reliability measures for devices,
restored directly on operating site , is a MTBF
or failure intensity, for non - restorable devices -
mean time to first failure. Required values of reli-
ability measures are determined for operationing
conditions. Aging, deterioration, common cause
failures, including software failures and personnel
errors, are considered in case of availability of
approved methods and initial data, which allow
numerically estimate their probability and influ-
ence to reliability.

Dependability Measures

Dependability measures define reliability, main-
tainability and durability of I&C systems and
their components.
Reliability is standardized and estimated for the
main functions, performed by I&C systems, SHC
and peripheral equipment, and for components
of I&C systems (except software) and remov-
able component parts of SHC and peripheral
equipment.
Failure criteria and reliability measures are
determined considering function character (con-
tinuous or discrete) and type of possible failures.
Continuous functions are monitoring, achieving,
display, register (analogue) and regulation, discrete
- warning, digital register, protection, limitation,
interlocking, discrete and remote control.
Criteria of continuous function failures may
be non-compliance, inaccurate meeting or viola-
tion of specified requirements for characteristics
of performed function; as a reliability measures
mean time between failures (MTBF) is taken. For
discrete functions failure criteria are nonoperation
(a failure type, where an output signal is absent,
despite occurrence of conditions specified for