Beta Factor
Relates the probability of common cause failure to the probability of random hardware failure.

Dangerous Failure
Failure which has the potential to put the safety-related system in a hazardous or fail-to-function state.

Diagnostic Coverage (DC)
Fractional decrease in the probability of dangerous hardware failures resulting from the operation of the automatic diagnostic tests.

Hardware Fault Tolerance (HFT)
A hardware fault tolerance of N means that N+1 faults could cause a loss of the safety function.

Safe Failure
Failure which does not have the potential to put the safety-related system in a hazardous or fail-to-function state.

Safe Failure Fraction (SFF)
The ratio of the average rate of safe failures plus dangerous detected failures of the subsystem to the total average failure rate of the subsystem.

Safety Function
Function to be implemented by an E/E/PE safety-related system, other technology safety-related system or external risk reduction facilities, which is intended to achieve or maintain a safe state for the EUC, with respect to a specific hazardous event.

Safety Integrity
Probability of a system satisfactorily performing the required safety functions under all the stated conditions within a stated period of time.

Safety Integrity Level (SIL)
Discrete level for specifying the safety integrity requirements of the safety functions to be located to the E/E/PE safety-related systems.

Safety-Related System
Designated system that both implements the required safety functions necessary to achieve or maintain a safe state for the EUC, and is intended to achieve - on its own or with other E/E/PE safety-related systems, other technology safety-related systems or external risk reduction facilities – the necessary safety integrity for the required safety functions.

Safety Requirement Specifications (SRS)
Specifications containing all the requirements of the safety functions that have to be performed by the safety-related systems.

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from IEC 61508-4