KROHNE SU 501 EX EN - Manual (Page 26)

Safety integrity of the hardware for safety-relating subsystems
of type A (IEC 61508-2, 7.4.3)
Safe failure fraction Hardware
fault tole-
rance
SFF HFT = 0 HFT = 1 HFT = 2
<60 % SIL1 SIL1 SIL2
60 % up to <90 % SIL2 SIL3 (SIL4)
90 % up to <99 % SIL3 (SIL4) (SIL4)
>=99 % SIL3 (SIL4) (SIL4)
9.2 Planning
l
The measuring system must be used acc. to the
application
l
The application-specific limits must be maintained and the
specifications must not be exceeded.
l
Acc. to the specifications in the operating instructions
manual, the current load of the output circuits must be
within the limits.
For the implementation of FMEDA (Failure Mode, Effects and
Diagnostics Analysis) the following assumptions form the
basis:
l
Failure rates are constant, wear of the mechanical parts is
not taken into account
l
Failure rates of external power supplies are not included
l
Multiple errors are not taken into account
l
The average ambient temperature during the operating
time is +40°C (104°F)
l
The environmental conditions correspond to an average
industrial environment
l
The lifetime of the components is around 8 to 12 years
(IEC 61508-2, 7.4.7.4, remark 3)
l
The condition of the output circuit is further processed acc.
to the quiescent current principle
l
The repair time (exchange of the meas. system) after a fail-
safe error is eight hours (MTTR = 8 h)
If the demand rate is only once a year, then the measuring
system can be used as safety-relevant subsystem in "low
demand mode" (IEC 61508-4, 3.5.12).
General instructions and
restrictions
Assumptions
Low demand mode
26 SU 501 Ex - Signal conditioning instrument
Functional safety
27953
-EN
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