This paper presents the sensitivity analysis of an earthing conductor under the influence of impulse current arising from a lightning stroke. The approach is based on the 2nd order finite difference time domain (FDTD). The earthing conductor is regarded as a lossy transmission line where it is divided into series connected circiuts and the per-unit length parameters of the circiut are taken as non-uniform and computed using well known expressions. The sensitivity analysis carried out shows that there is limit in the length of the earthing conductor that lowers the transient potential and transient impedance values. This limit is called the effective length and a knowledge of this not
only contributes to the design of an efficient and effective earthing system but also minimizes the cost of earthing conductors and labour.