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Safety of machines and systems
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Table 2-1 Explanation of categories
Category Summary of requirements System behavior Principle to achieve safety
B Safety-related parts of control systems
and/or their protective equipment, as well
as their components, shall be designed,
constructed, selected, assembled, and
combined in accordance with relevant stan-
dards so that they can withstand the
expected influences. Basic safety princi-
ples must be used.
Occurrence of a fault can lead
to the loss of the safety func-
tion.
Mainly characterized by the
selection of components.
1 The requirements of category B must be
met. Proven components and proven safety
principles must be used.
Occurrence of a fault can lead
to the loss of the safety func-
tion but the probability of
occurrence is lower than in
category B.
Mainly characterized by the
selection of components.
2 The requirements of category B and the use
of proven safety principles must be met.
The safety function must be tested by the
machine control system at suitable inter-
vals.
The occurrence of a fault can
lead to the loss of the safety
function between the tests.
The loss of the safety function
is detected by the test.
Mainly characterized by the
structure.
3 The requirements of category B and the use
of proven safety principles must be met.
Safety-related parts must be designed so
that:
– A single fault in any of these parts does
not lead to the loss of the safety func-
tion
and
– The single fault is detected, whenever
this is feasibly possible
When a single fault occurs,
the safety function is always
performed. Some faults but
not all faults are detected. An
accumulation of undetected
faults can lead to the loss of
the safety function.
Mainly characterized by the
structure.
4 The requirements of category B and the use
of proven safety principles must be met.
Safety-related parts must be designed so
that:
– A single fault in any of these parts does
not lead to the loss of the safety func-
tion
and
– The single fault is detected on or before
the next demand of the safety function
If detection is not possible, an accumu-
lation of undetected faults must not
lead to the loss of the safety function.
When a single fault occurs,
the safety function is always
performed. The detection of
accumulated faults reduces
the probability of the loss of
the safety function (high DC).
The faults are detected in time
to prevent a loss of the safety
function.
Mainly characterized by the
structure.