Surge protective devices (SPDs) are installed at the boundaries of lightning protection zones (LPZs) to damp the surges to the protection level of the SPD. If the circuit between the SPD and the apparatus to be protected is too long, the propagation of the surges leads to oscillation phenomenon. In this case, the overvoltage can be increased at the apparatus terminals. The increase is taken into account by an enhancement factor which gives the ratio of the maximum overvoltage at the apparatus terminals to the SPD protective level. According to IEC 62305-4, the maximum enhancement factor is considered to be equal to two in the worst case of open-circuit condition. The objective of the present paper is to check this relation for equipment connected to low-voltage (LV) power system. The LV power system is considered as TN-S system with different routings in a three-storey building. The terminals of apparatus are substituted by different loads. All Maxwell's equations are solved by the method of moments and the voltage is calculated at the apparatus terminals. The SPD itself is simulated by a voltage source at the ground floor of the building. The results reveal that the apparatus terminal voltage may overshoot the SPD protection level by a factor of 3.