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Functional Safety Manual
M310/FSM, Rev AA
January 2012
Horizontal Float Switch
FIT
FIT is the abbreviation for Failure In Time. One FIT is 1x10
-9
failure per hour
FMEDA
Failure Modes, Effects, and Diagnostics Analysis.
Functional Safety
Part of the overall safety relating to the process and the BPCS which depends on the
correct functioning of the Safety Instrumented System (SIS) and other protection
layers.
HFT
Hardware Fault Tolerance.
Low demand
Mode of operation, where the frequency of demands for operation made on a
safety-related system is no greater than twice the proof test frequency.
PFD
AVG
Average Probability of Failure on Demand.
SFF
Safe Failure Fraction – a fraction of the overall random failure rate of a device that
results in either a safe failure or a detected dangerous failure.
SIF
Safety Instrumented Function – a safety function with a specified SIL which is
necessary to achieve functional safety. Typically a set of equipment intended to reduce
the risk due to a specified hazard (a safety loop).
SIL
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.
SIS
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.