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Safety and environmental standards for fuel storage sites
Final report
136
Annex 2 Hardware Fault Tolerance (HFT) calculation to BS EN 61511 (for
sensors, final elements and non-programmable Logic solvers)
NO
NO
NO
NO
YES
0
1
2
N/A
1
2
3
4
SIL
Redundancy
1oo1
1oo1
1oo2 or 2oo3
N/A
HFT
0
0
1
N/A
SIL
1
2
3
4
Is HFT calculated to
BS EN 61511
YES
Is dominant failure
mode safe
See HFT to
BS EN 61508
HFT
0
1
2
N/A
SIL
1
2
3
4
YES
Note 1
Note 1
Is dominant failure mode safe?
See BS EN 61511 Part 1
Section 11.2.11
Note 2
For prior use
See BS EN 61511 Part 1
Section 11.5.3
Note 2
Can prior use be
proven?
Redundancy
1oo1
1oo2 or 2oo3
1oo3 or 2oo4
N/A
HFT
0
1
2
N/A
SIL
1
2
3
4
Redundancy
1oo1
1oo2 or 2oo3
1oo3 or 2oo4
N/A
HFT
0
1
2
N/A
SIL
1
2
3
4
YES
Can prior use be
proven?
Redundancy
1oo2 or 2oo3
1oo3 or 2oo4
1oo4 or 2oo5
N/A
HFT
1
2
3
N/A
SIL
1
2
3
4
1
2
3
N/A
1
2
3
4
HFT
SIL HFT