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\begin{abstract}
things can get very abstract
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\section{Overview}
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\section{Overview of A Burner Controller : Safety Perspective}

\section{Background to the Industrial Burner Safety Analysis Problem}

An industrial burner is a good example of a safety critical system.
It has the potential for devistating explosions due to boiler overpressure, low water, or
ignition of an explosive mixture, and, because of the large amounts of fuel used,
is also a fire hazard. Industrial boilers are often left running unattended 
for long periods of time (typically days).  

To add to these problems
Operators are often under pressure to keep them running. A boiler supplying
heat to a large greenhouse complex could ruin crops
should it go off-line. Similarly a production line relying on heat or steam
can be very expensive in production down-time should it fail.
This places extra responsibility on the burner controller.


These are common place and account for a very large proportion of the enery usage
in the world today (find and ref stats)
Industrial burners are common enough to have different specific standards
written for the fuel types they use \ref{EN298} \ref{EN230} \ref{EN12067}.

A modern industrial burner has mechanical, electronic and software
elements, that are all safety critical. That is to say
unhandled failures could create dangerous faults.

A more detailed description of industrial burner controllers
is dealt with in chapter~\ref{burnercontroller}.


Systems such as industrial burners have been partially automated for some time.
A mechanical cam arrangement controls the flow of air and fuel for the range of
firing rate (output of the boiler).

These mechanical systems could suffer failures (such as a mechanical linkage beoming
detached) and could then operate in a potentially dangerous state.

More modern burner controllers use a safety critical computer controlling
motors to operate the fuel and air mixture and to control the safety 
valves.

In working in the industrial burner industry and submitting product for
North American and European safety approval, it was apparent that
formal techniques could be applied to aspects of the ciruit design.
Some safety critical circuitry would be subjected to thought experiments, where 
the actions of one or more components failing would be examined.
As a simple example a milli-volt input could become disconnected.
A milli-volt input is typically amplified so that its range matches that
of the A->D converter that you are reading. were this signal source to become disconnected
the systems would see a floating, amplified signal.
A high impedance safety resistor can be added to the circuit,
to pull the signal high (or out of nornal range) upon disconnection.
The system then knows that a fault has occurred and will not use  
that sensor reading (see \ref{fig:millivolt}). 
