Effective Utilization of Surge Protection Devices

A systematic method is described for selecting the energy rating for surge protection devices when coordinated with a known impedance and subjected to typical low-voltage ac transients. A review of line transient generation characteristics, source impedance determination, and load surge protection device characteristics is given. From this study, an analytical model of the source, line, and load protection device is presented with closed-form mathematical expressions for suppressor current and energy. Results are obtained and compared for a unidirectional voltage impulse, a decaying oscillatory voltage wave, and a unidirectional current wave. From the analysis, the effect that line impedance has on each of these waveforms is shown graphically. In addition, the validity of the analytical model is confirmed experimentally.