Class 0

These appliances have no protective-earth connection and feature only a single level of insulation and were intended for use in dry areas. A single fault could cause an  electric shock or other dangerous occurrence. Sales of these items have been banned in the UK since 1975.


Class I

These appliances must have their chassis connected to electrical earth (US: ground) by an earth conductor ( colored  green/yellow in most countries, green in the U.S., Canada and Japan). A fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor. This current should trip either an overcurrent device ( fuse or circuit breaker) or a residual current circuit breaker (ground fault circuit interrupter [GFCI]) which will cut off the supply of electricity to the appliance.


Class II

A Class II or double insulated electric appliance is one which has been designed in such a way that it does not require a safety connection to electric earth (US: ground).

The basic requirement is that no single failure can result in dangerous  voltage becoming exposed so that it might 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 two layers of insulating material surrounding live parts or by using reinforced insulation.

In Europe, a double insulated appliance must be labelled Class II, double insulated, or bear the double insulation symbol (a square inside another square).



Class III

A Class III appliance is designed to be supplied from a separated/safety extra-low voltage ( SELV) power source. The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it without risk of electrical shock. The extra safety features built into Class I and Class II appliances are therefore not required. For medical devices compliance with Class III is not considered sufficient protection.