Effects of Built-In Varistors With Low Protection Voltages on Surge Protection Performances in Low-Voltage AC Power Systems

Voltage and current surges originated by lightning is one of the most common cause of degradation, upset, failure, and/or damage of susceptible equipment in low-voltage (LV) ac power installations. Surge protection systems realized with surge protective devices (SPDs) placed through LV power installations are used for protection of equipment against surges. However, modern electronic devices have very low withstand insulation level due to ongoing trend of miniaturization and high integration of components. Therefore, their manufacturers usually install built-in varistors inside devices' power supply units in order to provide their self-protection against overvoltages. These varistors have low protection voltages, but also low energy absorption capability. The comprehensive analysis of effects of built-in varistors with low protection voltages on surge protection system's performances is given in the paper. As analysis will show, it is very difficult to provide energy coordination of built-in varistors and installed SPDs within LV power installations. In most cases, this will lead to thermal damage and failure of the varistors due to energy overstressing, what causes overvoltages with values higher than withstand insulation level of the equipment to be protected. Possible solution to overcome these problems and to ensure adequate overall surge protection system's performances is proposed.