Enhancement of the SPD residual voltage at apparatus terminals in low-voltage power systems

Surge protective devices (SPDs) are installed at the boundaries of lightning protection zones 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 the open-circuit condition. The objective of this paper is to check this relation for equipment connected to low-voltage (LV) power systems. The LV power system is considered as a TN-S system with different routings in three- and six-story buildings. The terminals of apparatus are substituted by a variety of 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 voltage at the apparatus terminals may overshoot the SPD protection level by a factor of 3.