Terms And Definitions; Safety Instrumented Systems - Emerson Rosemount 5408 Reference Manual

Safety Instrumented Systems (4-20 mA only)
November 2017
8.2

Terms and definitions

Table 8-1. Terms and Definitions
Term
BPCS
λ
DU
λ
DD
λ
SU
λ
SD
Diagnostic coverage
Diagnostic test interval
FIT
FMEDA
®
HART
HFT
High demand mode
Low demand mode
Mission time
PFD
AVG
Proof-test coverage factor
Safety deviation
SFF
SIF
SIL
SIS
122
Definition
Basic Process Control System
Dangerous Undetected
Dangerous Detected
Safe Undetected
Safe Detected
Fraction of dangerous failures detected by automatic on-line diagnostic tests.
The time from when a dangerous failure/condition occurs until the device has set the safety
related output in a safe state (total time required for fault detection and fault reaction).
Failure In Time per billion hours
Failure Modes, Effects and Diagnostic Analysis
Highway Addressable Remote Transducer
Hardware Fault Tolerance
The safety function is only performed on demand, in order to transfer the EUC (Equipment
Under Control) into a specified safe state, and where the frequency of demands is greater than
one per year (IEC 61508-4).
The safety function is only performed on demand, in order to transfer the EUC into a specified
safe state, and where the frequency of demands is no greater than one per year (IEC 61508-4).
The time from an instrumented system's start-up until its replacement or refurbishment to
as-new condition.
Average Probability of Failure on Demand
The effectiveness of a proof-test is described using the coverage factor which specifies the
share of detected dangerous undetected failures (λ
a proof-test's effectiveness to detect dangerous undetected faults.
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 failures.
Note that the Safety Deviation is independent of the normal performance specification or any
additional application specific measurement error.
Safe Failure Fraction
Safety Instrumented Function
Safety Integrity Level – a discrete level (one out of four) for specifying the safety integrity
requirements 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.
Safety Instrumented System – an instrumented system used to implement one or more safety
instrumented functions. An SIS is composed of any combination of sensors, logic solvers, and
final elements.
Reference Manual
00809-0100-4408, Rev BA
). The coverage factor is an indication of
DU
Safety Instrumented Systems (4-20 mA only)