NRTL listing for residential use is meaningless, for example, if the device is used for more severe commercial or industrial purposes. Information about NRTL approval limitations is found on the approval label. Its composition and thickness are selected for that particular application. It cannot be modified without invalidating the NRTL listing or approval. Double-insulated tools and appliances have two, independent layers of insulations, one of which may be the exterior case.
Insulation is reliable unless it is damaged by impact damage, heat, chemicals, or tampering.
Most insulation will not protect against shock if the device gets wet. If you detect any deterioration to outer jackets, cases or cabinets, or you suspect that the insulation or barriers have been impaired, immediately take the device out of service. It requires a third conductor in the appliance cord and in any extension cord and branch circuit into which it is plugged. Ground prongs on plugs lead a hard life. Since ground conductors are not needed for the device to operate, there may be an undetected open ground in an appliance, its cord, or in a branch circuit.