Flowserve NRS 1-50 For TWO Electrodes - Manual (Page 7)

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Functional Safety acc. to IEC 61508
– continued –
Determination of the Safety Integrity Level (SIL) for safety-related systems
Level electrode, level switch and actuators (auxiliary contactor in safety circuit) are subsystems and
together constitute a safety-related system that executes a safety function.
The specification of the safety-related characteristics
Fig. 1 refers to the level electrode and the level
switch including the output contacts. The actuator (e. g. an auxiliary contactor in the safety circuit) is
installation specific and, according to IEC 6508, must be considered separately for the whole safety-
related system.
Table Fig. 3 shows the dependence of the Safety Integrity Level (SIL) on the average probability of failure
on demand of a safety function for the whole safety-related system (PFD
sys
). The“Low demand mode”
is here considered for a water level limiter, which means that the frequency of demands for operation of
the safety-related system is no greater than one per year.
Low demand mode PFD
sys
Safety Integrity Level
(SIL)
≥ 0 -5 ... < 0 -4 4
≥ 0 -4 ... < 0 -3 3
≥ 0 -3 ... < 0 - 
≥ 0 - ... < 0 - 
Fig. 3
The table in Fig. 4 indicates the attainable Safety Integrity Level (SIL) as a function of the Safe Failure
Fraction (SFF) and the Hardware Fault Tolerance (HFT) for safety-related systems.
Hardware Fault Tolerance (HFT) for type B
Safe Failure Fraction
(SFF)
0  
SIL  SIL  < 60 %
SIL  SIL  SIL 3 60 % - < 90 %
SIL  SIL 3 SIL 4 90 % - < 99 %
SIL 3 SIL 4 SIL 4 ≥ 99 %
Fig. 4