Class I: These appliances must have their chassis connected to electrical earth (ground) by an earth conductor. A fault in the appliance which causes a live conductor to contact the casing will cause a current flow in the earth conductor. The current should trip either an over current device or a residual current circuit breaker, which will cut off the supply of electricity to the appliance.
Class II: A Class 2 or double insulated electrical appliance is designed in such a way that it does not require (and must not have) a safety connection to electrical earth (ground).
Class III: Designed to be supplied from a 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 electric shock. The extra safety features built into Class 1 and Class 2 appliances are therefore not required.