B&R ACOPOSinverter P84, User Manual

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Integrated safety functions
348 ACOPOSinverter P84 User's Manual 2.10
HFT - Hardware fault tolerance and SFF - Safe failure fraction
Depending on the SIL for the safety system, the IEC 61508 standard and SFF value stipulate a special hardware
detected fault tolerance (HFT) in connection with a special Safe Failure Fraction (SFF). Hardware fault tolerance
is the ability of a system to execute the required safety function despite the presence of one or more detected
hardware faults.
The SFF of a system is defined as the ratio between the rate of safe failures and the total failure rate of the system.
Per IEC 61508, the maximum achievable SIL of a system is determined by the hardware fault tolerance (HFT) and
partially by the safe failure fraction (SFF) of the system.
IEC 61508 differentiates between two types of subsystems (type A and type B). These types are based on criteria
set forth by the standard for safety-related components.
SFF Type A subsystem HFT Type B subsystem HFT
0 1 2 0 1 2
<60% SIL 1 SIL 2 SIL 3 --- SIL 1 SIL 2
60% to < 90% SIL 2 SIL 3 SIL 4 SIL 1 SIL 2 SIL 3
60% to < 99% SIL 3 SIL 4 SIL 4 SIL 2 SIL 3 SIL 4
≥99% SIL 3 SIL 4 SIL 4 SIL 3 SIL 4 SIL 4
Measures for avoiding detected errors
System errors in the specification, hardware and software, as well as errors detected during the use and mainte-
nance of the safety system must be avoided to the maximum degree possible. To satisfy these requirements, IEC
61508 defines various measures for avoiding detected errors that must be implemented depending on the required
SIL. These error avoidance measures must cover the entire service life of the safety system, i.e. from the planning
of the system until it is taken out of commission.