KROHNE SU 501 EX EN - Manual (Page 29)

The methods and procedures used during the tests must be
stated and their suitability must be specified. The tests must be
documented.
If the function test proves negative, the entire measuring
system must be switched out of service and the process held
in a safe condition by means of other measures.
In the double channel architecture 1oo2D this applies
separately to both channels.
9.6 Safety-related characteristics
The failure rates of the electronics are determined by an
FMEDA acc. to IEC 61508. These calculations are based on
component failure rates acc. to SN 29500. All numerical values
refer to an average ambient temperature during the operating
time of +40°C (104°F). The calculations are also based on the
specifications stated in chapter "Planning".
The data are also valid for overfill protection (A-mode) as well
as dry run protection (B-mode).
λ
sd
0 FIT safe detected failure (1 FIT = failure/10 9 h)
λ
su
516 FIT safe undetected failure
λ
dd
0 FIT dangerous detected failure
λ
du
100 FIT dangerous undetected failure
SFF >84 % Safe Failure Fraction
T
Reaction
Failure reaction time 0.5 sec
MTBF = MTTF + MTTR 1.52x10 6 h
max. useful life of the measuring system for the safety
function
approx. 10 years
Single channel architecture
The following characteristics are derived from the above
mentioned data:
SIL2 (Safety Integrity Level)
HFT = 0 (Hardware Fault Tolerance)
General data
SU 501 Ex - Signal conditioning instrument 29
Functional safety
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