Properties of Safety Important I&C Systems and their Components - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 69

directly performing initiated protective ac-
tions and monitoring the state and opera-
tion of this equipment.

mand withdrawal to - low resistance (or closing)
of output circuit.
Set-points of technological parameters of pro-
tection I&C system can be constant or variable
(depending on other parameters and / or condi-
tions). In the design of protection I&C system
provides means that allow retrieving actual values
of set-points during operation and, if necessary,
changing them within the limits, allowed by the
design. Change of set-points is performed by
personnel of NPP, using specifically provided
hardware and software, contained in the protection
system, all changes are controlled and archived.
During unit operation or in standby modes
the protection I&C system performs described in
Сhapter 1 main and auxiliary functions of moni-
toring, archiving, warning (preventive alarm),
display, diagnostic and archiving:

• Safety systems support features control

and / or control operation of technological
systems and equipment, creating required
operation conditions of protection- and ac-
tuation systems (for example, emergency
power supply, safety equipment cooling,
provision of working environment, etc.)

Protection I&C systems initiate protective
actions in any postulated initiating event, which
can lead to emergency: violations of any of safety
operation conditions; dangerous external effects;
failure of the protection system; exceeding of any
of its controlled parameters (including design
ones) or a specified combination of parameters
beyond the limit, determined by the relevant set-
point. Operational personnel in the main control
room or emergency control room may at any time
issue a command, by which the protection system
initiates performance of protective actions.
The Design and Technical Specification for
Safe Operation for each protection I&C system
the following is specified: conditions for initiation
of protective actions; a list of command receivers
(safety actuation I&C systems and / or actuating
elements of technological safety systems); required
lags of issue and / or withdrawal of commands
(if necessary); compliance of logic conditions of
each discrete output signal (command) with values
of physical quantity, applied for representation of
these states. This compliance is selected on the
basis of qualitative evaluations of probability of
inaccurate interpretation of an output signal by a
receiver (caused by failures of components of a
protection system, communication line, etc.) and
severity of effects, to which such a fault can lead.
For example, in the reactor protection system
issue of commands, initiating emergency power
unit shutdown, corresponds to a high resistance
or output circuit break of this signal source, com-

• Monitors parameters, events and states,
deining speciied conditions of initiating
protective actions.
• Monitors technical state of its components
and adjacent equipment.
• Diagnoses operability failure that may lead
to a situation, when a system will be un-
available to perform safety functions, and
warns operational personnel of detected
disturbances.
• Archives and displays values of controlled
parameters and diagnostic messages of
technical states of its components and ad-
jacent equipment.

In case of operational events the protection
I&C system, having initiated any of the speci-
fied conditions of protective measures, performs
protection functions, warning (emergency alarm),
display and archiving:

• Warns operational personnel of system
actuation and displays information on the
condition (“cause of system actuation.”)