Calculation of Ninimum Safety Distances for Live-Line Mintenance - A Statistical Mehod Applied to 765 kV AC Itaipu Lines
This paper presents a statistical method to calculate the miniumm safety distance for live-line maintenance. The method takes into account the complexity of the tower gap configuration and the presence of the lineman working at the line potential. It includes the inherent dispersion of the dieletric characteristics of the air gaps such as the probability of flashover and the tirnes-to-flashover. Laboratory tests were also performed to corroborate the results of the study, checking not only the performance of the various live-line tools but also the effectiveness of the by-passing insulators protective device and the occurrence of broken insulators in the string.
Development of new technology to access the energized phase by walking along the strain string on 500 kV transmission lines and substations
The maintenance of 500 kV transmission lines and substations requires a great amount of manual services on its insulator strain strings, conductors and hardware. The conventional method of gaining access to the hot end of strings uses a long insulated ladder (longer than 6 m), numerous accessories, which involves great effort and the work of many electricians. This paper looks into a method developed by ELETRONORTE to access the energized parts of transmission lines and substations by walking directly on the insulator strain string without having to use an insulated ladder. The results of the experiment show that this method can meet safety standards. This method was further experimented on suspension strings. However, the outcome of such an experiment led us not to recommend its use.
A functional comparison of human Fc-receptor-bearing lymphocytes active in natural cytotoxicity and antibody-dependent cellular cytotoxicity.
Natural (or “spontaneous”) cytotoxicity against the K-562 myeloid cell line and antibody-dependent cellular cytotoxicity (ADCC) against sensitized Chang liver cells were monitored simultaneously in a 4 hr 51 Cr release assay, using the same preparations of normal human peripheral blood lymphocytes as effector cells. Both types of cytotoxic activity were expressed by lymphocytes bearing receptors for sheep erythrocytes and for the Fc portion of IgG. Up to 80% of the total natural killer (NK) cell and K cell (ADCC) activities were recovered in E-rosette pellets separated under optimal conditions. Since the ability to form rosettes with sheep erythrocytes is primarily a marker for human T cells, it seems likely that both NK and K cells are in the T cell lineage. Further, both NK and K cell cytolytic activities were severely depressed after brief exposure of lymphocyte suspensions or rosette pellets to ammonium chloride solutions. Both activities returned to normal values after 24-hr incubation at 24°C. Such sensitivity of human lymphocytes to ammonium chloride solutions may explain why other investigators have failed to observe NK or ADCC activity by E-RFC. When lymphocytes bearing Fc receptors were adsorbed onto plastic surfaces coated with a monolayer of immobilized antigen-antibody complexes made up of TNP and rabbit anti-TNP serum, both NK and ADCC activities were simultaneously depleted. Such depletion was completely blocked when Staphylococcal protein A, which binds selectively to the Fc portion of IgG, was incubated on the antigen-antibody monolayers before adding the lymphocytes. This confirmed that both NK and ADCC activities were mediated by effector lymphocytes bearing Fc receptors. When protein A at concentrations ranging from 0.01 to 100 µg/ml was added to the 51 Cr release assays, ADCC was significantly inhibited, presumably due to binding of protein A to available Fc sites of the sensitizing antibody. However, protein A had no effect on NK reactivity at any of the concentrations tested. When lymphocytes were treated for 30 min at 37°C with purified trypsin or α-chymotrypsin at concentrations from 0.01 to 10 mg/ml, NK activity was significantly inhibited whereas ADCC was unaffected. Other enzymes depressed (pronase, subtilisin) had no effect except at high concentrations (lipase), or stimulated (neuraminidase) both NK and K cell activity similarly, and therefore failed to distinguish between the effector cell types. Thus, our data indicate that effector cells in NK and ADCC are in overlapping, if not identical, subpopulations of Fc receptor-bearing lymphocytes of probable T cell lineage. The cytolytic mechanisms in NK and ADCC appear to be distinct, since we have been unable to find evidence for an “arming” antibody involved in this NK system.