What is Safe failure fraction (SFF)?

Safe failure fraction (SFF) is a measure of the probability that a safety-related system or component will fail in a safe state when it is subjected to a specific failure mode. It is a measure of the ability of a system or component to fail in a way that will not compromise the overall safety of the system.

The SFF is typically expressed as a percentage and is determined by analyzing the failure modes of the system or component and determining the probability of a safe failure for each mode. The SFF is used to evaluate the safety integrity level (SIL) of a system or component and is an important consideration in the design and operation of safety-critical systems, such as those used in aviation, nuclear power, and other industries where safety is of paramount importance.

It is generally used together with other safety integrity level requirements such as Probabilistic Safety Assessment (PSA) and Failure Mode and Effect Analysis (FMEA) to evaluate the overall safety of a system.
 

What is HFT - hardware detected fault tolerance?

Hardware Fault Tolerance (HFT) is a technique used in computer systems and electronic devices to ensure that the system continues to function properly in the event of a hardware failure. It is a method of designing and implementing a system that can detect and correct errors in the hardware, allowing the system to continue to operate without interruption or significant degradation in performance.

HFT is achieved by implementing redundant hardware components that can take over the function of failed components, allowing the system to continue to operate. This redundancy can be in the form of duplicate or backup components, or it can be achieved through the use of specialized hardware components that can detect and correct errors.

Examples of HFT are:

+ Dual-modular redundancy (DMR)
+ Triple-modular redundancy (TMR)
+ Error correcting codes (ECC)
+ Error detecting codes (EDC)
+ Redundant Array of Inexpensive Disks (RAID)

HFT is commonly used in critical systems such as aerospace, medical equipment, and industrial control systems, where the failure of a single component could have serious consequences.
 

What is diagnostic coverage?

Diagnostic coverage is a measure of how thoroughly a diagnostic test or procedure can detect faults or failures in a system or component. It is a measure of the ability of the diagnostic test to detect all possible faults or failures that could occur in the system.

Diagnostic coverage is typically expressed as a percentage, and the higher the percentage, the more thorough the diagnostic test. For example, if a diagnostic test has a diagnostic coverage of 90%, it means that the test can detect 90% of the possible faults or failures that could occur in the system.

There are different ways to calculate diagnostic coverage, but in general, it's calculated by comparing the number of faults that the diagnostic test is able to detect with the total number of faults that could occur in the system.

Diagnostic coverage is important in safety-critical systems where early detection of failure can prevent accidents. It's also important in systems where downtime can be very expensive, such as in manufacturing or transportation. In these systems, high diagnostic coverage can minimize downtime and reduce the costs associated with repairs and maintenance.