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Electrical utility improvements and plant expansions have the available short-circuit interrupting current up to, and in some cases over and above, the 25000 A, 1500 MVA interrupting rating of some of existing 35 kV oil circuit breakers (OCBs). The authors discuss the process of replacing these OCBs with sulphur hexafluoride (SF/sub 6/) breakers. They address the entire project from the discovery of the problem through the investigation, analysis, engineering, purchase, and installation phases. >

The haustorium of a mycoparasite, Piptocephalis virginiana, was found enclosed in an electron-dense sheath 30 h after inoculation into its host Choanephora cucurbitarum. The sheath was not present around the young haustorium 18 h after inoculation, and its progressive development is described. High resolution autoradiography showed that [3H]N-acetyl-glucosamine was incorporated preferentially into the host cell wall and in the sheath zone. Cell fractionation of label-fed hyphae showed that 83% of the label was in the cell wall and specifically in the chitin portion of the cell wall. Polyoxin D, a specific inhibitor of chitin synthetase, suppressed the label incorporation in the cell wall and sheath zone and resulted in a decrease of electron density. These results suggested that the sheath zone around the mature haustorium contained host cell wall material, i.e. chitin. Significance of cell wall material deposition in the sheath zone of the haustorium is discussed in the light of host resistance.