
\abstract{ This chapter defines what is meant by the terms
components, component fault modes and `unitary~state' component fault modes.
Mathematical constraints and definitions are made using set theory.
}

 
\section{Introduction}
When building a system from components, 
we should be able to find all known failure modes for each component.
For most common electrical and mechanical components, the failure modes
for a given type of part can be obtained from standard literature\cite{mil1991}
\cite{mech}. %The failure modes for a given component $K$ form a set $F$.

An important factor in defining a set of failure modes is that they
should be as clearly defined as possible.
%
It should not be possible for instance for
a component to have two or more failure modes active at once.

Having a set of failure modes where $N$ modes could be active simultaneously
would mean having to consider $2^N$ failure mode scenarios.
% 
Should a component be analysed and simultaneous failure mode cases exit,
the combinations could be represented by a new failure modes, or
the component should be considered from a fresh perspective,
perhaps considering it as several smaller components
within one package.

\begin{definition}
A set of failure modes where only one fault mode
can be active at a time is termed a `unitary~state' failure mode set.
This is termed the $U$ set thoughout this study.
This corresponds to the `mutually exclusive' definition in 
probability theory\cite{probandstat}.
\end{definition}

We can define a function $FM()$ to 
take a given component $K$ and return its set of failure modes $F$.

$$  FM : K \mapsto F $$

We can further define a set $U$ which is a set of sets of failure modes, where
the component failure modes in each of its members are unitary~state.
Thus if the failure modes of $F$ are unitary~state, we can say $F \in U$.


\subsection{Component failure modes : Unitary State example}

A component with simple ``unitary~state'' failure modes is the electrical resistor.

Electrical resistors can fail by going OPEN or SHORTED.
However they cannot fail with both conditions active. The conditions
OPEN and SHORT are mutually exclusive.
Because of this the failure mode set $F=FM(R)$ is `unitary~state'.


Thus

$$ R_{SHORTED} \cap R_{OPEN} = \emptyset $$


We can make this a general case by taking a set $C$ representing a collection
of component failure modes,
We can now state that


$$ c1 \cap c2 \neq \emptyset  | c1 \neq c2 \wedge c1,c2 \in C \wedge C \not\in U  $$

That is to say that if it is impossible that any pair of failure modes can be active at the same time
the failure mode set is not unitary~state and does not exist in the family of sets $U$

 Note where that are more than two failure~modes, by banning pairs from happening at the same time
 we have banned larger combinations as well

%$$ c1 \cap c2 \eq \emptyset  | c1 \neq c2 \wedge c1,c2 \in C \wedge C \in U  $$

%Thus if the failure~modes are pairwaise mutually exclusive they qualify for inclusion into the 
%unitary~state set family.
