Discussion on Worst Distance Between SPD and Protected Device

It is commonly known that the longer the distance between a surge protective device (SPD) and a protected device, the worse the protection effectiveness. However, we observed there exists a worst distance between an SPD and a protected device in a low-voltage circuit. If a protected device and an SPD are separated by this worst distance, the protected device will be subject to the most severe surge voltage. The main reason for this phenomenon is that there usually exist two reflection processes, namely, that between the transformer and the SPD and that between the SPD and the protected device. These two refection processes lead to the superposition of voltage wave in a way that is different from that in the high-voltage circuit. This paper presents an analytical formula to estimate this worst distance. It is also shown that the result obtained from the formula has a good agreement with the simulation in PSCAD/EMTDC.