Protection of the internal insulation of a static transformer against high-frequency strains

Most forms of lightning-arresters, especially of the multigap type, protect transformers against high-frequency even better than against low-frequency strains so far as these strains exist between either the high- or the low-potential windings, or between either of these windings and any other part of the transformer; because the high frequency causes a greater condenser charging current to flow across the gaps at the line end of the arresters, thereby reducing the resistance of these gaps. They do not, however, always prevent strains far in excess of normal being thrown upon the adjacent turns and layers of the windings; because, although the high frequency strain may be less than the working voltage, the frequency, and consequently the wave-length may be, and generally is, such as to concentrate this strain on a relatively small portion of the turns at the end of the windings connected to the line.