Cantilever-Loaded Insulators for Isolated-Phase Bus [includes discussion]

Previous tests and experience have shown that the reliability of isolatedphase busses was not reduced in changing the insulator loading from compression only to compression-tension. Mathematical studies and laboratory tests of the shielding effect of the enclosure1 show that the reduction in force on the insulators is of considerable magnitude. This makes the use of the cantilever loaded insulators a very practical solution to the problem of reducing the number of insulators in the bus. Tests have shown that the isolated phase bus with cantileverloaded insulators will withstand the maximum momentary currents now available with ample factors of safety. An isolated-phase bus was tested up to 350,000 amperes, rms, with insulators spaced 8 feet apart. Even at this high value of current the load on the insulators was 26 per cent under the actual breaking load of the insulator assembly as determined by test. The load on the insulator was less than 15 per cent of the load which would have appeared if the enclosures were not present. At 200,000 amperes, the insulator was loaded to only 77 per cent of the National Electrical Manufacturers Association (NEMA) rating. This is powerful proof of the shielding effect of the isolated-phase enclosures, and that the cantilever insulator is more than adequate to meet present and future momentary requirements.