The protection classes in electrical engineering are defined for all electrical equipment in DIN EN 61140 VDE 0140-0. There are four different protection classes (protection class 0,I,II,III). The respective protection classes differ in the type of protection. Symbols are provided for each protection class to identify the equipment with the respective protection classes. The symbols of the protection classes are also defined in DIN EN 61140 VDE 0140-0. The protection classes describe the different types of protection of equipment and its measures against dangerous body currents.
Protection class 0 no special protection against electric shock besides the basic insulation. There is no connection to a protective conductor system. The protection must be ensured by the environment of the equipment. Devices of this protection class and protection type are not approved in Germany and Austria.
Protection class I (protection by protective conductor) Device with protective conductor connection and basic insulation. With protection class I, electrical devices have a plug connection with the protective conductor contact as a protective measure. The protective conductor connection in protection class I is adjusted in such a way that it is the first to be connected to the plug in case of electrical connection and the last to be disconnected in case of danger in order to comply with the standard against dangerous body currents. The connection line into the device must be strain-relieved so that when the line is torn off, the protective conductor is the last to be mechanically stressed and thus torn off. In case of danger, a live conductor touching the housing connected to the protective conductor will cause a body short, which will trip the fuse or a residual current circuit breaker and disconnect the circuit from the power supply.
Protection class II (protection by protective insulation) Device without protective conductor connection with double or reinforced insulation. Electrical equipment provided with protection class II has double or reinforced insulation between the mains circuit and the output circuit and has no protective conductor connection. In electrical engineering, this type of protection is referred to as protective insulation. Conductive parts such as conductive surfaces of electrical equipment are protected from contact with electrically charged parts by the reinforced insulation. Devices or electrical installations of protection class II do not have protective contact plugs. Protection class II uses either europlugs or contour plugs for electrical installations.
Protection class III (protection by protective extra-low voltage) Equipment in which protection against electric shock is based on SELV supply and in which no voltages higher than SELV are generated. Protection class III is characterized by protective extra-low voltage and requires double insulation during mains operation similar to protection class 2. Devices with this protection class with a voltage above 50 V AC and 120 V DC, for example, require a safety transformer. The reason for this is the missing protective conductor connection of this protective measure.