Added SRDO and SROVT concepts from caninauto

statistics/statistics.tex

@@ -47,6 +47,28 @@ Impact of boiler going down, no CO2 primary coolant available, possible reactor
 For the Brewery, safety is of the highest importance.
 For the Nuclear power station
 
+
+\section{Terms and Concepts in // Safety Critical Engineering}
+
+\subsection{Safety Relevant Data Object}
+A Safety Relevant Data Object (SRDO)\cite{caninauto}, is a data structure describing the status of
+a particular feature or attribute of a safety critical system.
+For instance, in a burner this could be a flame signal value, or in a nuclear powerstation
+the measure neutron flux.
+\subsection{Safety relevant Object Validation Time}
+Safety times can be given for SRDO's; these are termed Safety Related Object Validation Times (SROVT's)\cite{caninauto}. For instance were
+a flame to fail in operation in a gas burner
+standards state that the gas may not continue to be fed into the
+furnace for more than three seconds \cite{en298}.
+We can say that the SROVT for a flame signal in a gas burner is 3 seconds.
+\subsection{Single and Double Failure Modes}
+A Safety critical system must self check within the relevant SROVT's.
+On detecting a failure mode it must react appropriately.
+Consider the case though where two failures occurr within the
+time windows of their SROVT's. We can term this a double simultaneous failure mode.
+To take an extreme example, were the checking function/mechanism and the object under supervision
+to fail within the SROVT, it may be impossible to detect the failure.  
+
 \section{Interfacing}
 
 Mech - elec - sw