“Insulation applied to live parts to provide to provide basic protection and which does not necessarily include insulation used exclusively for functional purposes.”
Typically this would be the insulation, usually PVC, coating the live conductors.
“Single insulation applied to live parts, which provides a degree of protection against electric shock equivalent to double insulation under the conditions specified in the relevant standard. The term ‘single insulation’ does not imply that the insulation must be one homogeneous piece. It may comprise two or more layers which cannot be tested singly as supplementary or basic insulation.”
“Independent insulation applied in addition to basic insulation for fault protection.”