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Subsystem with Type B Components
Safe failure fraction
Hardware fault tolerance (see note 1)
0
1
2
< 60 %
not allowed
SIL1
SIL2
60 % - < 90 %
SIL1
SIL2
SIL3
90 % - < 99 %
SIL2
SIL3
SIL4
≥ 99 %
SIL3
SIL4
SIL4
Figure 5 – BS EN 61508-2 Table 3
This tables show that SIL 2 with HFT 0 can be claimed for a Type B instrument if it has SFF > 90%. This
is the methodology used by many manufacturers in claims made on their certificates.
Hardware Fault Tolerance Requirements BS EN 61511 Ed 1 – Prior Use
The process sector have been using instrumented system to reduce the risk from potential
hazardous events for many decades and have examples of systems installed before the modern
functional safety standards were published that have a long history of good performance. BS EN
61511 enables the end user to take advantage of this historical performance; this is termed ‘Prior
Use’.
BS EN 61511 Clause 11.4.4 states that the HFT requirements stated in Table 6 may be reduced by 1 if
the hardware is selected on the basis of prior use. This means that a target SIL 2 SIF may use a single
sensor (HFT = 0) if the prior use requirements are met.
The requirements for selection of components based on prior use are detailed in BS EN 61511 Clause
11.5.3, but the main features of this clause include: -
·
Appropriate evidence shall be available that the components are suitable for the safety
instrumented system
·
In the case of field instruments, there may be extensive operating experience
·
Evidence of suitability shall include
o
Demonstration of the performance in similar operating profiles and physical
environments
o
The volume of operating experience
This just acknowledges that different operating and process conditions will affect the instrument
performance; for example a history of good performance on a gas plant does not guarantee the
same level of performance on an oil platform.
Was the inspector correct?
The information provided to the HSE Specialist Inspector contained references to HFT compliance
involving BS EN 61508 and SFF and BS EN 61511 and Prior Use.
The Failure Modes, Effects and Diagnostics Analysis (FMEDA) report provided by the manufacturer
and certification body contained the following table.
λ
SD
λ
SU
λ
DD
λ
DU
SFF
DC
S
DC
D
0 FIT 461 FIT
1129 FIT
358 FIT 81% 0% 75%