SAFETY MANUAL SIL KFD2-UT2-(EX)*, HID2082
Planning
2011-09
7
2 Planning
2.1 System Structure
2.1.1 Low Demand Mode
If there are two loops, one for the standard operation and another one for the
functional safety, then usually the demand rate for the safety loop is assumed to
be less than once per year.
The relevant safety parameters to be verified are:
■ the PFD
avg
value (average Probability of Failure on Demand) and
T
proof
(proof test interval that has a direct impact on the PFD
avg
)
■ the SFF value (Safe Failure Fraction)
■ the HFT architecture (Hardware Fault Tolerance architecture)
2.1.2 High Demand Mode
If there is only one loop, which combines the standard operation and safety
related operation, then usually the demand rate for this loop is assumed to be
higher than once per year.
The relevant safety parameters to be verified are:
■ PFH (Probability of dangerous Failure per Hour)
■ Fault reaction time of the safety system
■ the SFF value (Safe Failure Fraction)
■ the HFT architecture (Hardware Fault Tolerance architecture)
2.1.3 Safe Failure Fraction
The safe failure fraction describes the ratio of all safe failures and dangerous
detected failures to the total failure rate.
SFF = (λ
s
+ λ
dd
) / (λ
s
+ λ
dd
+ λ
du
)
A safe failure fraction as defined in EN 61508 is only relevant for elements or
(sub)systems in a complete safety loop. The device under consideration is always
part of a safety loop but is not regarded as a complete element or subsystem.
For calculating the SIL of a safety loop it is necessary to evaluate the safe failure
fraction of elements, subsystems and the complete system, but not of a single
device.
Nevertheless the SFF of the device is given in this document for reference.
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