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Functional Safety Manual
IP101/FSM, Rev AA
January 2012
Horizontal Float Switch
SIL Capability 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.
Random Integrity
The Mobrey level switch is classified as a type A device according to Table 2 of the
standard IEC 61508-2. Therefore, based on an Safe Failure Fraction (SFF) of between
0% and 60%, a design can meet SIL 1 with Hardware Fault Tolerance (HFT) = 0 and
SIL 2 with HFT = 1 when the level switch is used as a single component in a sensor
subsystem.
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 12.
NOTE:
The FMEDA report is available from the Safety quick link at www.mobrey.com.
In the right-hand panel, there are SIL documents including the FMEDA report.
Table 3.
Failure Rates for Level Switch
4-contact Versions – Types D and P
Table 4. Failure Rates for Level
Switch 6-contact Versions –
Types D6, P6, H6, and B6
Table 5.
Failure Rates according to
IEC 61508, 4-contact Versions –
Types D and P (FIT)
Table 6.
Failure Rates according to
IEC 61508, 6-contact Versions –
Types D6, P6, H6, and B6 (FIT)
Failure Category
Failure Rate (FIT)
(1)
(1) FIT is the abbreviation for Failure In Time. One FIT is 1x10
-9
failure per hour.
Maximum Detection Minimum Detection
Fail Safe Detected 0 0
Fail Safe Undetected 87 89
Fail Dangerous Detected 0 0
Fail Dangerous Undetected 195 193
Residual 34 34
Failure Category
Failure Rate (FIT)
(1)
(1) FIT is the abbreviation for Failure In Time. One FIT is 1x10
-9
failure per hour.
Maximum Detection Minimum Detection
Fail Safe Detected 0 0
Fail Safe Undetected 167 169
Fail Dangerous Detected 0 0
Fail Dangerous Undetected 275 273
Residual 34 34
Device
SD
(1)
(1) FIT is the abbreviation for Failure In Time. One FIT is 1x10
-9
failure per hour.
SU
DD
DU
SFF %
Maximum Detection 0 87 0 195 30.9
Minimum Detection 0 89 0 193 31.6
Device
SD
(1)
(1) FIT is the abbreviation for Failure In Time. One FIT is 1x10
-9
failure per hour.
SU
DD
DU
SFF %
Maximum Detection 0 167 0 275 37.8
Minimum Detection 0 169 0 273 38.3