The axial electric stress necessary to cause external flashover of the interrupter porcelain of various designs of circuit breaker has been determined as a function of the severity of an artificial salt fog, related investigations have been conducted at a marine testing station. The much inferior performance of the horizontal head to that of the vertical one is explained by using a simple mathematical model to analyse the influence of saline drainage from the porcelain. The findings are discussed from the viewpoint of trying to realise the full potential of the outdoor SF6 circuit breaker on a 400kV system -- in particular, for the synchronising function. The need for further research to enhance the pollution flashover strength of a horizontal barrel insulator is clearly shown. The experimental results are analysed with regard to the validity of IEC document 815.
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