The inverse problem in 3D eddy current imaging is ill-posed and non-linear. The most commonly used image reconstruction algorithms for electrical imaging are based on a linear approximation (the Jacobian Matrix). We start with an initial conductivity distribution. The forward problem is solved and the predicted voltages compared with the calculated voltages from the Finite Element model. The conductivity is then updated using a regularised inverse of the Jacobian. The process is repeated until the predicted voltages from the finite element method agree with the calculated voltages from the finite element model. In this study we present the validity range of a linear inverse problem based on analysis of the Jacobian matrix.