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A quantitative research report is made here, based on tests of EHV (extra-high-voltage) insulator strings. Impulse, switching-surge, and a-c tests on vertical, horizontal, and 45-degree V-strings were carried out, using both positive and negative unidirectional impulses and 250- to 3,250-Â¿sec (microsecond) switching surges. The switching-surge and a-c tests were made, both dry and wet, under ASA (American Standards Association) specifications. Research was instrumented in the CESI high-voltage laboratory in Milan, Italy. The full name of this laboratory is: Centro Elettrotecnico Sperimentale Italiano Giacinto Motta, the last two words referring to the late Dr. Motta, to whom the center is dedicated (see Figs. 1 through 3). The study included radio-influence (RI) measurements, determination of voltage distribution along the insulator strings, photographs, and cathode-ray records of each test. Interesting and useful data pertaining to the design of EHV lines were collected.