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The paper reviews factors influencing the selection of transformer insulation levels, service experience on the British 132 and 275 kV Grid systems and the testing of transformer insulation. A pattern for the overall insulation strength of a transformer, relative to service conditions, is developed from examination of test procedure, insulation characteristics, variation in voltage waveform, voltage transfer to other windings, and the influence of associated apparatus. This pattern is compared with the characteristics of protective devices, and margins between insulation and protective levels are discussed. Circumstances permitting the adoption of transformer insulation levels below those now specified are considered, suggesting that consequential capital savings for a 132 kV system would be marginal, but at higher voltages they could be significant. A survey of service experience and fault statistics for the British high-voltage networks is made, indicating that there would be no advantage to change the present methods of surge protection. Experience with acceptance tests is reviewed, and suggestions are made for a possible revision of insulation test procedure.