Emerson Rosemount 5408 Reference Manual page 142

Safety Instrumented Systems (4-20 mA only)
Table 8-1: Terms and Definitions
Term Definition
λ
Dangerous Detected
DD
λ
Safe Undetected
SU
λ Safe Detected
SD
Diagnostic cov- Fraction of dangerous failures detected by automatic on-line diagnostic tests.
erage
Diagnostic test The time from when a dangerous failure/condition occurs until the device has set the safety related
interval output in a safe state (total time required for fault detection and fault reaction).
FIT Failure In Time per billion hours
FMEDA Failure Modes, Effects and Diagnostic Analysis
®
HART Highway Addressable Remote Transducer
HFT Hardware Fault Tolerance
High demand The safety function is only performed on demand, in order to transfer the EUC (Equipment Under
mode Control) into a specified safe state, and where the frequency of demands is greater than one per year
(IEC 61508-4).
Low demand The safety function is only performed on demand, in order to transfer the EUC into a specified safe
mode state, and where the frequency of demands is no greater than one per year (IEC 61508-4).
Mission time The time from an instrumented system's start-up until its replacement or refurbishment to as-new
condition.
PFD Average Probability of Failure on Demand
AVG
Proof-test cov- The effectiveness of a proof-test is described using the coverage factor which specifies the share of
erage factor detected dangerous undetected failures (λ
effectiveness to detect dangerous undetected faults.
Safety deviation The maximum allowed deflection of the safety output due to a failure within the device (expressed as
a percentage of span).
Any failure causing the device output to change less than the Safety Deviation is considered as a "No
Effect" failure. All failures causing the device output to change more than the Safety Deviation and
with the device output still within the active range (non-alarm state) are considered dangerous fail-
ures.
Note that the Safety Deviation is independent of the normal performance specification or any addi-
tional application specific measurement error.
SFF Safe Failure Fraction
SIF Safety Instrumented Function
SIL Safety Integrity Level – a discrete level (one out of four) for specifying the safety integrity require-
ments of the safety instrumented functions to be allocated to the safety instrumented systems. SIL 4
has the highest level of safety integrity, and SIL 1 has the lowest level.
SIS Safety Instrumented System – an instrumented system used to implement one or more safety instru-
mented functions. An SIS is composed of any combination of sensors, logic solvers, and final ele-
ments.
132
(continued)
). The coverage factor is an indication of a proof-test's
DU
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