1908 - The Turning Point

After twenty years of fighting with pin-type insulators, the industry was stuck. Higher Voltage meant larger, heavier insulators. Already the largest insulators were too heavy for most linemen to handle. The mechanical leverage of a taught line on the end of a tall insulator could bend or break nearly any support. Stanislaus Electric Power Co. made the first use of cap-and-pin suspension insulators on their 104kV line that transmitted 40,000 horsepower 134 miles from the power plant site in the Sierra foothills to San Francisco, CA. The 2-part cemented insulators were designed for the line and patented by John Duncan in 1907. They were manufactured by Locke as catalog number 273 and bear a VICTOR stamp. They were used in strings of five, as shown in the drawing and photos.

Duncan suspensions are very rare. Whole strings were dropped from the crossarm and their 14 in. skirts would crush each other, leaving a giant pile of broken porcelain. Occasionally a single unit would be thrown from the tower and would land in brush and soft duff on the downhill side of the right-of-way. We were fortunate to find a mint one (second from the bottom in the photo) in 2001 on a steep hillside about 15 miles from the power plant. We found the three top units along the line as well, but they all needed extensive repair. The bottom unit is an even more exotic Locke 275, which has a longer pin. We found it in July 2003 on the Jordan River line, on Vancouver Island northwest of Victoria, British Columbia.
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