%
% Make the revision and doc number macro's then they are defined in one place
\ifthenelse {\boolean{paper}}
{
\begin{abstract}
This paper looks in detail a the generic `resistor'
as a component in safety critical designs and 
how its potential failure modes are taken into
account in various safety design standards.
A `networked resistor' equivalent to single
resistors is proposed for highly safety critical measurement sensitive
designs, as it has two not three failure modes to consider.
\end{abstract}
}
{
\section{Overview}
Component failure modes are documented for most generic
components~\cite{mil1992}~\cite{rac}. There are some differences in 
the failure modes that we are required to consider for various 
safety standards.
For an example of this we can look in detail at one particular 
type
 of component, and examine potential causes of failure modes and 
and how various standards demands  in terms of  possible failure modes.
For this study the common resistor ahs been chosen.
}



\section{Resistor Failure Modes}


Most common failure OPEN

ENXXX ays only consider OPEN

EN298 considers OPEN and SHORT


\subsection{Physical causes for a resristor short}

* over heating causing a solder bridge

* over heating causing the component to melt and short (wirewound)
\subsection{Physical causes for a resristor open}
* Over current causing the component to burn out (acting like a fuse).

* vacuum conditions can cause solder joints to sublime (space ref jbis)

\subsection{Physical causes for a resristor parameter change}
*the resistor can react to over heating by becoming
damaged so that the resistance track is internally shorted, making the 
part read a lower resistance

*the resistor can over heat and the resistance track and be made thinner leading to a higher resistance

*chemical attack could alter the properties of the resistance track.

*RAC give parameter change as well
this means that the resistor may change its resistance value
as a failure mode.


\section{Properties of a resistor network}

RAC gives a resistor network 
 as having only failure modes of 
OPEN and SHORT.

Let us consider a simple resistoir network.

Examine all causes.
Assume resistors not from same batch.

Show how parameter change is cancelled out, use sum of squares statistics
to show by how much.

\ifthenelse {\boolean{paper}}
{
\begin{figure}
\begin{tikzpicture}[line width=1pt]
    \draw (0,0) -- ++(0,1cm);
    \draw[decorate, decoration=cell] (0,1cm) -- ++(0,1.5cm);
    \draw (0,2.5cm) |- ++(1cm,1cm);
    \draw[decorate, decoration=diode] (1cm,3.5cm) -- ++(1.5cm,0);
    \draw (2.5cm,3.5cm) -- ++(2,0);
    \draw (3.5cm,3.5cm) -- ++(0,-1);
    \draw[decorate, decoration=switch] (3.5cm,2.5cm) -- ++(0,-1.5cm);
    \draw[decorate, 
        decoration={inductor,amplitude=0.35cm, segment length=0.75cm}]
        (4.5cm,3.5cm) -- ++(1.5cm,0);
    \draw (6cm,3.5cm) -- ++(1,0);
    \draw (7cm,3.5cm) -- ++(3,0);
    \draw (8cm,3.5cm) -- ++(0,-1);
    \draw[decorate, decoration=capacitor] (8cm,2.5cm) -- ++(0,-1.5cm);
    \draw (0cm,0cm) -| ++(8cm,1cm);
    \draw (3.5cm,0cm) -- ++(0cm,1cm);
    \draw (10cm,3.5cm) -- ++(0,-1);
    \draw[decorate, decoration=recresistor] (10cm,2.5cm) -- ++(0,-1.5cm);
    %\draw[decorate, decoration=recresistor] (2.5cm,10cm) -- ++(0,-1.5cm);
    \draw (8cm,0cm) -| ++(2cm,1cm);
    \draw[decorate, decoration=ground] (5,0cm) -- ++(0,-1.5cm);
\end{tikzpicture}
\caption{a circuit}
\end{figure}




\begin{figure}
\begin{tikzpicture}[line width=1pt]
    \draw (0,0) -- ++(1cm,1cm);
    \draw[decorate, decoration=recresistor] (1cm,2cm) -- ++(0,-1.5cm);
    \draw[decorate, decoration=recresistor] (2cm,2cm) -- ++(0,-1.5cm);
    \draw[decorate, decoration=recresistor] (1cm,4cm) -- ++(0,-1.5cm);
    \draw[decorate, decoration=recresistor] (2cm,4cm) -- ++(0,-1.5cm);
    %\draw[decorate, decoration=recresistor] (2.5cm,10cm) -- ++(0,-1.5cm);
    \draw (8cm,0cm) -| ++(2cm,1cm);
    \draw[decorate, decoration=ground] (5,0cm) -- ++(0,-1.5cm);
\end{tikzpicture}
\caption{A `network resistor'}
\end{figure}
}
{
electComp not in main thesis director yet
}