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.
These appliances must have their chassis connected to electrical earth (US: ground) by a separate earth conductor (coloured green/yellow in most countries, green in the US, Canada and Japan). The earth connection is achieved with a 3-conductor mains cable, typically ending with 3-prong AC connector which plugs into a corresponding AC outlet.
A fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor. For all faults except high-impedance faults, this current will trip either an overcurrent device (fuse or circuit breaker (CB)) or a residual-current device (RCD) also known as a residual current circuit breaker (RCCB), ground fault circuit interrupter (GFCI), or residual current operated circuit-breaker with integral overcurrent protection (RCBO), which will cut off the supply of electricity to the appliance.
Electrical installations where the chassis is connected to earth with a separate terminal.
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).
Insulated AC-DC power supplies (such as cell-phone chargers) are typically designated as Class II, meaning that the DC output wires are isolated from the AC input. The designation "Class II" should not be confused with the designation "Class 2", as the latter is unrelated to insulation (it originates from standard UL 1310, setting limits on maximum output voltage/current/power).
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, and further more-stringent regulations apply to such equipment.
- IEC 61140: Protection against electric shock — Common aspects for installation and equipment. International Electrotechnical Commission. 2001. (formerly: IEC 536-2: Classification of electrical and electronic equipment with regard to protection against electric shock, 1992)
- BS 2754 : 1976 (1999): Memorandum. Construction of electrical equipment for protection against electric shock.