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2. Safety
Abbreviation
PL
EN
MooN
(M out of N)
architecture
PFD
avg
T
or T
i
proof
PFH
D
SFF
MTTF
D
B
10d
n
op
β factor
10
Description
Performance level; The international standard EN ISO 13849 1
defines five discrete performance levels (PL a to PL e). Each level
corresponds to an ability of safety-related parts of control systems to
perform a safety function under foreseeable conditions.
The architecture describes the specific configuration of hardware
and software in a system. N is the number of parallel channels and
M defines how many channels must be working correctly.
Average probability of a dangerous failure on demand of the safety
function in the operating mode with low demand rate
Interval of the proof tests (in hours, typically one year (8,760 h)).
Following this interval, the proof test will be carried out.
Average frequency of a dangerous failure on demand of the safety
function in the operating mode with high demand rate
Safe failure fraction
Mean time to a failure to danger
Number of cycles till 10 % of the components have failed
dangerously
Average number of operations per year
Factor for failure due to common causes, in terms of the interaction
of several channels
WIKA operating instructions DPGS40TA with component testing
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