User Manual

SAFETY
Safety handbook page - 8-
PNSPO
Category B
Safety related parts of control systems and or their safety devices and their
components must be designed, constructed, selected, assembled and combined in
accordance with the relevant standards such that they can with stand expected
influence.
Category 1
The requirements of B shall apply. Well-tried components and safety principles shall be
used.
The occurrence of a fault can lead to the loss of a safety function, but the probability of
occurrence is lower than in category B
Category 2
The requirements of B shall apply. Well-tried components and safety principles shall
apply. The safety function shall be checked at suitable intervals by the machine control
system.
The occurrence of a fault can lead to the loss of a safety function between checks. The
loss of the safety function is detected by the check.
Category 3
The requirements of B shall apply. Well-tried components and safety principles shall
apply. Safety related parts shall be designed such that:
• A single fault in any of these parts does not lead to the loss of the safety
function.
•
The single fault is detected whenever reasonably practicable
Category 4
The requirements of B shall apply. Well-tried components and safety principles shall
apply. Safety related parts shall be designed such that:
• A single fault in any of these parts does not lead to the loss of the safety
function.
• The single fault is detected during or prior to the next demand on the safety
function, or if this is not possible, an accumulation of faults does not lead to
the loss of the safety function.
3.3 Failure analysis
Failure analysis can be achieved by selecting a category based on the severity of
foreseeable injury, the probability of injury and the frequency of exposure to the hazard.
When determining the category for the safety related control circuit of a machine there
are some cases where the entire control circuit of the machine can be assessed under
one category and other cases where the control circuit is assessed in sections.
The EN 954-1 is describing in the informative Annex B a guidance to select the
appropriate category for the circuit of a safety function.
This simplified method bases on the EN 1050 and is working with 3 parameters to
select and determine the category in the “Risk graph”