Environmental Engineering Reference
In-Depth Information
Table 1. Correlation between classes of I&C
systems and categories of I&C functions
I&C safety class is reasonably set according
to the following rules:
Categories of I&C
Functions Important to
Safety
Corresponding Classes of
I&C Systems Important to
Safety
• Deine a complete list of system functions,
in realization of which this component is
participating, and the category of each
function.
• Refer all components that are participating
in I&C function realization to the same cat-
egory as this function.
• Determine component safety class, cor-
responding to the highest of the catego-
ries of these functions that it performs.
Components that are participating in real-
ization of one or several category A func-
tions (and, possible, of other categories)
are referred to the highest safety class.
A
(B)
(C)
1
B
(C)
2
C
3
ries important to safety. Each class is characterized
by a specific set of requirements for system features
and capabilities, and also for design, production
and quality of components. Satisfaction of these
requirements allows considering a system as a
proper one for performance of functions of a
specific category. A system can perform or par-
ticipate in performance of one or several functions
of the highest category, corresponding to its class,
and any number for functions of a lower catego-
ry.
If one component, for example, a sensor is
included in several systems, it is referred to the
highest of safety classes that it could have as a
component of each of the systems separately (Fig-
ure 6). Hardware that is directly connected with
components of a specific safety class is referred
to the same class. An example could be signal
galvanic isolation devices (also called “isolators”),
providing a possibility to use the same sensors
in the systems of different safety class, as shown
in Figure 6. Communication lines, connecting
components of one safety class, are referred to the
same class; connecting components of different
safety class — to the highest of the classes. The
same rule is applied to equipment of data com-
munication channels and local networks.
A typical classification of I&C systems is
given in Table 2.
The requirements for the function with the
highest safety category determine the class of the
system.
Before classification of components notes that
functions of I&C components (SHC, HW, and
SW) are offered to be considered as components
of those I&C functions, in performance of which
they are participating. The classification of I&C
systems component is based on the principle that
each system function can be assigned to unam-
biguously specific set of components required and
sufficient for realization of this function.
Table 2. Typical classification of I&C systems according to IEC
Class 1
Class 2
Class 3
Not Classified
Plant automation and control systems
X
X
X
HMI systems (class 1 HMI may be restricted to a few
critical indicators and push-buttons)
X
X
X
X
Protection system and safety actuation system
X
Emergency power actuation system
X
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