Appliance classes
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In the electrical appliance manufacturing industry, the following IEC protection classes are used to differentiate between the protective-earth connection requirements of devices.
Contents |
Class 0
These appliances have no protective-earth connection and feature only a single level of insulation. They were intended for use in dry areas. In most countries, the sale of Class 0 mains-voltage appliances is prohibited today, as a single fault could cause an electric shock or other dangerous occurrence. The International Electrotechnical Commission is in the process of removing provisions for Class 0 devices from its standards. It can be expected that the Class 0 concept will eventually disappear from the international market, in favour of Class II devices.
Class I
These appliances must have their chassis connected to electrical earth (US: ground) by an earth conductor (coloured yellow/green 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 which will cut off the supply of electricity to the appliance.
Class II
Image:Double-insulated-symbol.png A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require (and must not have) a safety connection to electrical earth (US: ground) (UK Plugs: It has no Earth Wire).
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.
There are also strict requirements relating to the maximum insulation resistance and leakage to any functional earth or signal connections of such appliances.
In Europe, a double insulated appliance must be labelled "Class II", "double insulated" or bear the double insulation symbol (a square inside another square).
See also double switching.
Class III
A Class III appliance is designed to be supplied from a SELV (Safety Extra-Low Voltage) 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.
See also
References
- 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)