In the electrical appliance manufacturing industry, the following IEC protection classes are used to differentiate between the protective-earth connection requirements of devices.
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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.
As Class 0 but appliance has an earth terminal which is unused since two core cable is used.[1]
These appliances must have their chassis connected to electrical earth (US: ground) by an earth conductor (coloured 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 (CB)) or a residual-current device (RCD) also named as residual current circuit breaker (RCCB), or (ground fault circuit interrupter (GFCI)) or also, residual current operated circuit-breaker with integral overcurrent protection (RCBO). which will cut off the supply of electricity to the appliance.
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 (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)..
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.