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©ABBGroup
October23,2016|Slide19
Two Types of Instruments in BS EN 61508-2 Ed 2
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BS EN 61508 has two tables relating to hardware
fault tolerance
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Used by the manufacturers to demonstrate
benefits of improvements in technology
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Type A instruments – simple instruments
where all failure modes can be predicted
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Type B – complex, usually programmable
instruments incorporating intelligence
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The Radar Level transmitter was a Type B
instrument
©ABBGroup
October23,2016|Slide20
BS EN 61508-2 Ed 2 HFT Type B Component
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Need at least 90% SFF to reduce HFT to 0 for SIL 2
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Used by many manufacturers on certificates
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
BS EN 61508-2 Ed 2 Table 3