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Designing a Safety Function Using the Level Switch
April 2017
Each subsystem must be checked to assure compliance with minimum Hardware Fault Tolerance
(HFT) requirements. When using the level switch in a redundant configuration, a common cause
factor of at least 5% should be included in the safety integrity calculations.
The failure rate data listed in the FMEDA report is only valid for the useful lifetime of the level switch.
Failure rates increase after this useful lifetime period has expired. Reliability calculations based on
the data listed in the FMEDA report for mission times beyond the lifetime may yield results that are
too optimistic, i.e. the calculated SIL will not be achieved.
3.5
SIL capability
3.5.1
Systematic integrity
The Mobrey level switch has met manufacturer design process requirements of Safety Integrity
Level 2 (SIL 2). These are intended to achieve sufficient integrity against systematic errors of design
by the manufacturer.
A Safety Instrumented Function (SIF) designed with the Mobrey level switch must not be used at a
SIL higher than the statement without "prior use" justification by the end-user, or verification of
diverse technology in the design.
3.5.2
Random integrity
The Mobrey level switch is a type A device according to Table 2 of the standard IEC 61508-2.
Using Route 2H assessment criteria, the device Random Capability has been determined as SIL 2.
3.5.3
Safety parameters
The failure rates given here are valid for the useful lifetime of the product, as described in the section
"Useful lifetime" on page
Note
The FMEDA report is available from the Mobrey Magnetic Horizontal Float Switches web site page at
Emerson.com/Mobrey. In the Documents section, there are SIL documents including the FMEDA
report and this safety manual.
Table 3-1. Failure Rates for Level Switch, 4-contact Versions – Types D and P
Failure category
Fail Safe Detected
Fail Safe Undetected
Fail Dangerous Detected
Fail Dangerous Undetected
No Effect
1.
FIT is the abbreviation for Failure In Time. One FIT is 1x10
8
13.
Maximum detection
0
88
0
243
34
-9
failure per hour.
Functional Safety Manual
(1)
Failure rate (FIT)
Minimum detection
Designing a Safety Function Using the Level Switch
M310/FSM, Rev BA
0
117
0
214
34
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