Abstract-Switching surge, impulse, and 60-cycle tests performed
on the APS 500-kV tower-insulation system proved the adequacy
of the design which consisted of twenty-four 53/4 X 10-inch insulators
in V-string configuration with an 1 1-foot 2-inch minimum strike
distance. The minimum switching surge withstand voltage is 965 kV.
Minimum impulse critical flashover voltage is 1890 kV, and the
minimum 60-cycle flashover and withstand voltages are 707 kV
and 660 kV rms, respectively. The switching surge test results from
the full-scale tower agreed closely with those predicted from simulated
tower-insulation tests. The switching surge flashover probability
curve tends to follow the equations for a normal cumulative
distribution curve to about the 2 to 5 percent probability level,
whereas previous test results showed correlation only to the 5 to
10 percent probability level.