Basic Insulation: Insulation applied to live parts to provide basic protection against electric shock and which does not necessarily include insulation used exclusively for functional purposes.

Class I Equipment: Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for the connection of exposed conductive parts to a protective conductor in the fixed wiring of the installation.

Class II Equipment: Equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions such as supplementary insulation are provided, there being no provision for the connection of exposed metalwork of the equipment to a protective conductor, and no reliance upon precautions to be taken in the fixed wiring of the installation.

Class III Equipment: Equipment in which protection against electric shock relies on the supply from a separated extra-low voltage source, such as an isolating transformer to BS 3535.

Double Insulation: Insulation comprising both basic insulation and supplementary insulation.

Earthing: The act of connecting the exposed-conductive-parts of electrical equipment to the mains earthing terminal of an electrical installation.

Supplementary insulation: Independent insulation applied in addition to basic insulation in order to provide protection against electric shock in the event of a failure of basic insulation.