KST – KROMSCHRÖDER SYSTEM TECHNOLOGY
Glossary
Diagnostic coverage DC
Measure of the effectiveness of diagnostics, which may be determined as the ratio between the failure rate of detected dangerous failures and the failure rate of total dangerous failures
NOTE: Diagnostic coverage can exist for the whole or parts of a safety-related system. For example, diagnostic coverage could exist for sensors and/or logic system and/or final elements. Unit: %.
from EN ISO 13849-1:2008
Mode of operation
High demand mode or continuous mode
Operating mode, where the frequency of demands for operation made on a safety-related system is greater than one per year or greater than twice the proof-test frequency
from EN 61508-4:2001
Category
Classification of the safety-related parts of a control system in respect of their resistance to faults and their subsequent behaviour in the fault condition, and which is achieved by the structural arrangement of the parts, fault detection and/or by their reliability
from EN ISO 13849-1:2008
Common cause failure CCF
Failures of different items, resulting from a single event, where these failures are not consequences of each other
from EN ISO 13849-1:2008
Fraction of undetected common cause failures β
Fraction of undetected failures of redundant components due to a single event, whereby these failures are not based on mutual causes
NOTE: β is expressed as a fraction in the equations and as a percentage elsewhere.
from EN 61508-6:2010
B10d value
Mean number of cycles until 10% of the components fail dangerously
from EN ISO 13849-1: 2008
T10d value
Mean time until 10% of the components fail dangerously
from EN ISO 13849-1:2008
Hardware fault tolerance HFT
A hardware fault tolerance of N means that N + 1 is the minimum number of faults that could cause a loss of the safety function
from IEC 61508-2:2010
Mean dangerous failure rate λd
Mean rate of dangerous failures during operation time (T10d). Unit: 1/h.
from EN ISO 13849-1:2008
Safe failure fraction SFF
Fraction of safe failures related to all failures, which are assumed to appear
from EN 13611/A2:2011
Probability of dangerous failure PFHD
Value describing the likelihood of dangerous failure per hour of a component for high demand mode or continuous mode. Unit: 1/h.
from EN 13611/A2:2011
Mean time to dangerous failure MTTFd
Expectation of the mean time to dangerous failure
from EN ISO 13849-1:2008
Demand rate nop
Mean number of annual operations
from EN ISO 13849-1:2008
