Up: Corning Pyrex Plain Facts 6 of 8 ·Front Cover & 1 ·Pages 2 & 3 ·Pages 4 & 5 ·Pages 6 & 7 ·Pages 8 & 9 ·Pages 10 & 11 ·Pages 12 & 13 ·Back Cover

WHAT HAPPENS WHEN A SUSPENSION INSULATOR IS HIT WITH A STONE OR RIFLE BULLET? Below a certain energy limit, depending on many factors, nothing happens. Just above that limit the glass, or the glaze, chips off or "spalls." Considerably above that limit any insulator, glass or otherwise, will break. Here, the physical differences between the two commercially important materials are very noticeable. One or more pieces of the skirt usually fall out of type No. 2 insulators -- otherwise, they may seem sound. But often the original cracks slowly extend into the head, during an indeterminate period of time, and cause electrical failure. No visual observation will reveal the fault -- only by buzz stick and high voltage flashover (and sometimes not then) can it be found. This is a maintenance item of appreciable magnitude. In the laboratory a 10,000 volt neon transformer, with voltage between cap and pin, will detect and amazing number of electrical failures in insulators which seem to be essentially sound. PYREX suspension insulators are self-indicating. They resist impact stresses stoutly, but if a deep crack penetrates into the tension zone the stresses are no longer balanced and the insulator quickly drops the skirt which breaks into many pieces. The patrolman has no difficulty in seeing the broken unit. The cracks extend into the head but the head remains in place and will not drop the line because it is encased in metal. Strings of these broken "hub" sections have repeatedly been tested under an 8,000 lb. load while vibrating at 15 to 20 cycles per minutes for hundreds of thousands of cycles and none has ever failed. In a standard M and E test the strength of these "hub" sections exceeds the rated strength of the complete insulator. WHAT HAPPENS IN A "LIGHTNING SURGE" TEST? Many PYREX units have been flashed-over 1,000 times, at 2,000,000 volts, with the steepest available front surge, without puncturing a single insulator. The great majority of competitive units would fail in this test.