In the electrical appliance manufacturing industry, the following IEC protection classes are used to differentiate between the protective earth connection requirements of appliances.
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 if a fault occurs the supply will be removed automatically (this is sometimes referred to as ADS = Automatic Disconnection of Supply). This is done in a few possible ways.
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 by a separate earth conductor coloured green & yellow. The earth connection is via a 3-conductor mains flex.
A fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor. If large enough, this current will trip an over-current device (fuse or MCB) and disconnect the supply. The disconnection time has to be fast enough not to allow heart fibrillation to start if a person is in contact with the casing at the time. This time and the current rating in turn sets a maximum earth resistance permissible.
To provide supplementary protection against high-impedance faults it is common to use an RCD, (RCCB) or (RCBO) in the supply installation, which cuts off the supply of electricity to the appliance if live & neutral currents are not equal.
Electrical appliance where the chassis is connected to earth with a separate terminal. Historic appliances only.
A Class II or double insulated electrical appliance has extra insulation to prevent shock. This is usually achieved at least in part by having two layers of insulating material surrounding live parts or by using reinforced insulation. These do not require a safety connection to electrical earth. Most Class II appliances are plastic cased, but metal Class II appliances are also in use.
A double insulated appliance should be labelled Class II or double insulated or bear the double insulation symbol (a square inside another square).
Insulated power supplies (eg wallwarts, phone chargers) are typically designed as Class II, meaning the DC output wires are isolated and double insulated 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.