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re wording SIL stuff

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Robin Clark 2013-08-10 21:12:41 +01:00
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submission_thesis/CH2_FMEA/copy.tex
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@ -28,16 +28,15 @@ The four main current FMEA variants are described and we develop %conclude by d
the concepts the concepts
that underlie the usage and philosophy of FMEA. that underlie the usage and philosophy of FMEA.
% %
We return to the overall process of FMEA We return to the overall process of FMEA and model it using UML.
and model it using UML.
% %
By using UML %we define By using UML
relationships between the FMEA data objects the entities needed to implement FMEA
are defined. % at the start of this chapter. are defined.
% %
The act The act
of defining relationships between the data objects of defining relationships between the data objects
in FMEA raise questions about the nature of the process in FMEA raises questions about the nature of the process
and allows us to analytically discuss its strengths and weaknesses. and allows us to analytically discuss its strengths and weaknesses.
@ -1222,6 +1221,14 @@ FMEDA is a modern extension of FMEA, in that it recognises the effect of
self checking features on safety, and provides detailed recommendations for computer/software architecture. self checking features on safety, and provides detailed recommendations for computer/software architecture.
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% %
%
FMEDA is the fundamental methodology of the statistical (safety integrity level)
type standards (EN61508/IOC5108).
The end result of an EN61508 analysis is an % provides a statistical
overall `level~of~safety' known as a Safety Integrity level (SIL), for an installed system.
%
It has a simple final result, a Safety Integrity Level (SIL) from 1 to 4 (where 4 is safest).
%
These SIL levels are broadly linked to the concept of an These SIL levels are broadly linked to the concept of an
acceptance of given probabilities of dangerous acceptance of given probabilities of dangerous
failures against time, as shown in table~\ref{tbl:sil_levels}. failures against time, as shown in table~\ref{tbl:sil_levels}.
@ -1229,24 +1236,14 @@ failures against time, as shown in table~\ref{tbl:sil_levels}.
The philosophy behind this is that is recognised that no system can have a perfect The philosophy behind this is that is recognised that no system can have a perfect
safety integrity, but risk and criticality can be matched to acceptable, safety integrity, but risk and criticality can be matched to acceptable,
or realistic levels of risk. or realistic levels of risk.
%
FMEDA is the fundamental methodology of the statistical (safety integrity level)
type standards (EN61508/IOC5108).
The end result of an EN61508 analysis is an % provides a statistical
overall `level~of~safety' known as a Safety Integrity level (SIL), for a system.
%
It has a simple final result, a Safety Integrity Level (SIL) from 1 to 4 (where 4 is safest).
%
%There are currently four SIL `levels', one to four, with four being the highest level. %There are currently four SIL `levels', one to four, with four being the highest level.
% %
It allows diagnostic mitigation for self checking circuitry.
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SIL levels are intended to SIL levels are intended to
classify the statistical safety of installed plant: classify the statistical safety of installed plant:
salesmen’s terms such as a `SIL~3~sensor' or other `device' given a SIL level, are meaningless. salesmen’s terms such as a `SIL~3~sensor' or other `device' given a SIL level, are meaningless.
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SIL analysis is concerned with `safety~loops', not individual modules. SIL analysis is concerned with `safety~loops', not individual modules, sensors, computing devices or actuators.
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In control engineering terms, the safety~loop is the complete In control engineering terms, the safety~loop is the complete
path from sensors to signal~processing to actuators for a given function path from sensors to signal~processing to actuators for a given function
@ -1265,9 +1262,11 @@ In EN61508 terminology, a safety~loop is known as a Safety Instrumented Function
% %
FMEDA requires %does force FMEDA requires %does force
the analyst to consider all hardware components in a system the analyst to consider all hardware components in a system
by requiring that an MTTF value is assigned for each base component failure~mode; and requires that an MTTF value is assigned for each {\bc} {\fm};
the MTTF may be statistically mitigated (improved) the MTTF may be statistically mitigated (improved)
if it can be shown that self-checking will detect its failure modes. if it can be shown that self-checking measures will not only detect it within the SIF, but
also react in a safe way.
That is that the SIF can recognise that it has a fault condition and can take appropriate action.
% %
The MTTF value for each component {\fm} is denoted using the symbol `$\lambda$'. The MTTF value for each component {\fm} is denoted using the symbol `$\lambda$'.
% %