You should listen to the compiler and look at what it tells you in the log file. You are getting errors because nesting two math environment this way can't work. Furthermore there is a much smarter way to typeset a case differentiation by the
amsmath package.
Code: Select all
\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools} % loads »amsmath«
\begin{document}
\begin{equation}\label{eq:fault}
\text{CONA}=
\begin{cases}
1 & \text{if IOPQ} < \text{IRTQ}\cdot k; \\
0 & \text{otherwise.}
\end{cases}
\end{equation}
\end{document}
Thorsten