Surge arrester is one of the most important equipment to be considered while performing insulation co-ordination studies. Surge arrester, due to its nonlinear voltage – current characteristics, has the ability to bypass the surges and maintain a safe voltage across the power system equipment. This helps in achieving the necessary protection margin for the system. The insulation coordination study considers the protection level offered by the surge arrester for surges of various rise times and magnitudes. The surges are classified as steep current impulse, lightning current impulse and switching current impulse. The residual voltage of the surge arrester increases as the steepness of the surge increases, hence steep current impulses are more severe as this will increase the stress on the power system equipment. IEC 60099-4 defines the methodology to determine the steep current impulse residual voltage by subjecting surge arrester blocks with surges having front time of 1s. In real life, surges can be even faster. In this paper a simple methodology is defined to compute the protection level offered by a 66kV 10kA Station Low surge arrester while bypassing steep current impulse of 500ns front time. The unknown inherent non-inductive drop offered by the arrester blocks is determined and the inductive voltage drop for the arrester is added, to arrive at the protection level. This will be helpful in providing accurate information to improve surge arrester model while performing insulation co-ordination studies for special scenarios.