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Continuous system growth compels the design of transmission links with ever-larger capacities and higher voltages. If increase in cost of facilities is kept linear with voltage while the transfer capacity increases as the square of the voltage, substantial economies can be made in the cost of transmitting electrical energy. Extrapolation of conventional practices at 230 kv and 345 kv and small economies in component equipment would not achieve an economical insulation system for 500-kv service. Different concepts in insulation co-ordination were developed for 500 kv in order to achieve the desired economies. These concepts included: control of switching surges on lines and in the stations, a recognition that some switching surge flashovers are acceptable, just as is lightning, and allowance for diversity among factors which degrade insulation. Application of these principles in the selection of system insulation levels to be used by the Bonneville Power Administration's proposed 500-kv system is herein described.