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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


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


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