Coordination of insulation

COORDINATION of system insulation is the selection of insulation in proportion to the anticipated overvoltages as limited by a coordination gap. A review of the development of coordination during the past few years is presented briefly in this paper, as a basis for discussing the means at present used for obtaining coordination of system insulation together with the advantages and disadvantages of various schemes. The 2 classes of overvoltage which may appear on a system, namely, switching surges and arcing ground voltages in the first class, and lightning voltages in the second class, are considered in respect to the selection of the coordination gap. It is pointed out that it is not logical to have steps or levels of insulation within a station, but that a uniform level of at least 10 per cent above the strength of the gap seems logical.