simultaneous fault modelling with PLD

logic_diagram/logic_diagram.tex

@@ -791,12 +791,50 @@ errors of ommission are automated in the FMMD tool.
 
 \section{Double Simultaneous Fault Modelling}
 
-TO DO:
-matrix diagram
+
+Some deterministic based safety standards are specifying
+that not only single component failure modes must be considered in 
+analysis, but that the possibility of two component failing
+simultaneously must be considered.
+EN298 states that if a burner controller is in `lock out' (i.e. has detected a fault
+and has ordered a shutdown) a secondary fault cannot be allowed to put the equipement under control (the burner) into a dangerous state.
+To cover this rigourously we are bound to consider more than one fault being active at a time.
+\paragraph{Covering Double faults in a PLD Diagram}
+Because we are allowed to repeat contours in a PLD diagram 
+we can arrange them in a matrix like configuration as in figure \ref{fig:doublesim}.
+Note that we have here all the single and double failure test cases in one diagram.
+
+\begin{figure}[h]
+ \centering
+ \includegraphics[width=400pt,bb=0 0 677 527,keepaspectratio=true]{./doublesim.jpg}
+ % doublesim.jpg: 677x527 pixel, 72dpi, 23.88x18.59 cm, bb=0 0 677 527
+ \caption{Double and Single fault modes for a Functional Group with 3 failure modes}
+ \label{fig:doublesim}
+\end{figure}
 
 
 \section{N Simultaneous Errors}
 
+There are systems where it may be necessary to model for N simultaneous failures.
+This can be achived in a PLD diagram by enclosing a test case with
+all the failure modes to be modelled simultaneously, see figure \ref{fig:allfour}.
+
+For instance, a 747 Aircraft with four engines, could suffer from
+volcanic ash intake, affecting all engines.
+Obviously the symptom of this multiple failure would be loss of propulsion and more importantly
+the loss of ability to maintain altitude.
+% and maybe even the APU !
+The test case AFE provides the system modeller to introduce this
+possibility into the design.
+\begin{figure}[h]
+ \centering
+ \includegraphics[width=400pt,bb=0 0 349 236,keepaspectratio=true]{./allfourengines.jpg}
+ % allfourengines.jpg: 349x236 pixel, 72dpi, 12.31x8.33 cm, bb=0 0 349 236
+ \caption{PLD diagram showing a testcase where four fault modes are active}
+ \label{fig:allfour}
+\end{figure}
+
+
 TO DO:
 Venn N example