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In the electrical appliance manufacturing industry, the following classes are used to differentiate between the power connection requirements of devices. The official definition of these standards is published in IEC 60536-2:1992 Classification of electrical and electronic equipment with regard to protection against electric shock
Class 0
These appliances are without earthing protection and with simple insulation, intended for dry areas. In many countries sale of such appliances intended to be operated from mains voltage is prohibited as a single fault could cause an electric shock or other dangerous occurrence.
Class I These appliances must have their chassis connected to electrical earth (US: ground) by an earth conductor (usually coloured yellow/green). 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  Double insulation symbol 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).
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 (Separated Extra-Low Voltage) power source. The voltage from a SELV supply is low enough that under normal conditions a human 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
External link - http://www.thomasregister.com/olc/55751051/cordsp3.htm
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