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by T .Surface conductance of outdoor insulators exposed to polluti on and moisture from rain or fog is related to two aspects of insulator application: (a ) possibility of insulator fl ashover and (b) duration a nd se\·erity of surface arc discharges. This paper will discuss some of the ex perience with monitoring the surface conductance of porcelain and sy nthetic insulation under voltage stress and ex posed to outdoor wea thering in various locations in the U.S. Simple flat surfaces mounted at a 30° angle from the horizo ntal , and so me shed type insulators have been followed in this study. Recordings hav e been made over periods of several years. comparing results with low and high voltage stress. Resu lts of these outdoor tests a re also compared with results of tests of similar surfaces in a sy nthetic salt fog chamber. IntroductionIn mo st of electrical insulation applications, there is a problem oi clefi ning th e environmental a nd service conditions for which elec trical insula tien must be designed. The poorly defined variables are often m a n~·, but so me of the more important ones are: temperature as a function of tim e, mech anical stress, and particularly moisture and/or contamination. Sometimes, but not always, th e most sc\·cre condition is known , but th e duration of this extreme condition or of less severe levels is us ually not defined \"Cry \vell. This situation presents a particular problem in the dc\·elopm ent of new insulation materials or structures, since the de\·elop er may ha\Oe only uncertain criteria for decidin g wheth er the ne\\· sys tem is a deq uate or over designed.Suriace moisture condensation and precipitation and contamination by dirt are th e principal problems with insulators used outdoors. These include bushings, post and suspension , and strain insulators. The cmoironment for such insulators i s, of course, as variable as th e (local) \\Oeath er and local atmospheric pollution conditions. Data on mea n and maximum wea ther conditions, such as tempera ture and precipitationo are available, but data on at mosp heric pollution are only now being gathered b y ecologists, and in such a manner that it is not directly useful to the insulation application oThe influence of surface moisture and contamination on outdoor in sul a tors is through their effect on the insulation characteristics oi resisti\·ity and fla shover voltage . Moisture alone is not as significant as moisture togeth er with an electrolyte type of contaminant. since an electrolyte such as a water soluble sa lt. acid or base is n eeded to provide high surface conductance. Thus; th e surface conductance is a function of both the level of surface electrolqe contamination and of the surface water condensation. The \\·~ttability and roughn ess of the surface is also important. :\ \ \°aler-elec trolyte solution, which stands in disc rete drops on th e surface. gi\Oes a much lower surface conductance than the same electrolyte solution spread uniformly over the surfaceo These effects combine and interact to produce an ...

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