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This paper presents the results of measurements of conductor corona losses on two open-circuited 400 kV transmission lines, one 85 km long and the other 143.2 km long. The results are compared with those obtained under test conditions in a local corona cage. From these it is shown that the loss predictions from the cage excitation functions (for the relevant conductors and conductor surface gradients) agree tolerably with the losses measured in both wet and dry conditions on the two lines. The overall errors and uncertainties in the CVT, CT and wattmeter measurements are also assessed.