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©ABBGroup
October23,2016|Slide21
BS EN 61511-1 Ed 1 and Prior Use/ HFT
§
Clause 11.4.4
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HFT may be reduced by 1
§
If the hardware is selected on the basis of prior use
§
So SIL 2 has HFT = 0 if ‘prior use’ is met
§
No mention of DC or SFF
SIL
Minimum hardware fault tolerance for sensors, non-
programmable logic solvers and final elements
1
0
2
1
3
2
4
Special requirements apply (see BS EN 61508)
Prior Use – Definition in BS EN 61511 Ed 1
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11.5.3 Evidence is based upon…
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‘Extensive Operating experience’
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‘Performance in similar operating profiles and physical
environments’
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This just acknowledges that different operating and
process conditions will affect instrument performance
©ABBGroup
October23,2016|Slide22