Back filter

A filter inserted in the power line feeding an equipment to be surge tested; this filter has a dual purpose: (1) To prevent the applied surge from being fed back to the power source where it might cause damage. (2) To eliminate loading effects of the power source on the surge generator.

Source: UIE Guide to Quality of Electriical Supply for Industrial Installations, Part VI: Transient and Temporary Overvoltages and Currents