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Inner cooling of high-voltage generator stator coils1,2 has made possible greatly increased ratings for the active magnetic and conductor materials in large hydrogen-cooled machines. This major advance in cooling. technique has required modification in some insulation design practices and application methods. It is the purpose of this paper to review the insulation of large inner-cooled generators and to re-examine the basic insulation methods and concepts. Test data on models and insulating structures in hydrogen at various pressures are reported. The philosophy for design of creepage distances across insulating surfaces and striking distances through gas is discussed.