In the electrical appliance manufacturing industry, the following protection classes are defined and used to differentiate between the protective-earth connection requirements of devices.
 

Class I

These appliances must be connected to electrical earth (US: ground) by a separate earth conductor (green/yellow colour in most countries, green in the US, Canada and Japan). Earthing is achieved through a 3-core mains cable. The basic requirement is that no single fault can result in exposure to dangerous voltage which may cause an electric shock, and that, in the event of a problem, the power will automatically switch off.
 

Class II

A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground).
The basic requirement is that no single fault can result in exposure to dangerous voltage which may cause an electric shock, and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having at least two layers of insulating material between live parts and the user, or by using reinforced insulation.
In Europe, a double insulated appliance must be labelled Class II or double insulated or bear the double insulation symbol (a square inside another square).