\n
Diagnostic coverage DC<\/b><\/p>\n
<\/p>\n
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<\/p>\n
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: %.<\/p>\n
from EN ISO 13849-1:2008<\/i><\/p>\n
\n<\/i>Mode of operation<\/b><\/p>\n
High demand mode or continuous mode<\/p>\n
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<\/p>\n
from EN 61508-4:2001<\/i><\/p>\n
\n<\/i>Category<\/b><\/p>\n
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<\/p>\n
from EN ISO 13849-1:2008<\/i><\/p>\n
\n<\/i>Common cause failure CCF<\/b><\/p>\n
Failures of different items, resulting from a single event, where these failures are not consequences of each other<\/p>\n
from EN ISO 13849-1:2008<\/i><\/p>\n
\n<\/i>Fraction of undetected common cause failures \u03b2<\/b><\/p>\n
Fraction of undetected failures of redundant components due to a single event, whereby these failures are not based on mutual causes<\/p>\n
NOTE: \u03b2 is expressed as a fraction in the equations and as a percentage elsewhere.<\/p>\n
from EN 61508-6:2010<\/i><\/p>\n
\n<\/i>B10d<\/sub> value<\/b><\/p>\nMean number of cycles until 10% of the components fail dangerously<\/p>\n
from EN ISO 13849-1: 2008<\/i><\/p>\n
\n<\/i>T<\/b>10d<\/b><\/sub> value<\/b><\/p>\nMean time until 10% of the components fail dangerously<\/p>\n
from EN ISO 13849-1:2008<\/i><\/p>\n
\n<\/i>Hardware fault tolerance HFT<\/b><\/p>\n
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<\/p>\n
from IEC 61508-2:2010<\/i><\/p>\n
\n<\/i>Mean dangerous failure rate \u03bbd<\/sub>\u00a0<\/b><\/p>\nMean rate of dangerous failures during operation time\u00a0(T10d<\/sub>). Unit: 1\/h.<\/p>\nfrom EN ISO 13849-1:2008<\/i><\/p>\n
\n<\/i>Safe failure fraction SFF<\/b><\/p>\n
Fraction of safe failures related to all failures, which are assumed to appear<\/p>\n
from EN 13611\/A2:2011<\/i><\/p>\n
\n<\/i>Probability of dangerous failure PFHD<\/sub><\/b><\/p>\nValue describing the likelihood of dangerous failure per hour of a component for high demand mode or continuous mode. Unit: 1\/h.<\/p>\n
from EN 13611\/A2:2011<\/i><\/p>\n
\n<\/i>Mean time to dangerous failure MTTFd<\/sub>\u00a0<\/b><\/p>\nExpectation of the mean time to dangerous failure<\/p>\n
from EN ISO 13849-1:2008<\/i><\/p>\n
\n<\/i>Demand rate nop<\/sub>\u00a0<\/b><\/p>\nMean number of annual operations<\/p>\n
from EN ISO 13849-1:2008<\/i><\/p>\n<\/div>\n
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