This standard applies to surge-protective devices designed for application on the low-voltage ac supply mains (1000 V rms and less, frequency between 48 Hz and 62 Hz). Low-voltage surge-protective devices (secondary arresters) are intended to be connected at locations between, and including, the secondary terminals of the distribution transformer and the main service entrance panel. Surge-protective devices limit transient overvoltages by diverting surge current and subsequently automatically interrupting the passage of follow current.