In the electrical appliance manufacturing industry, the following IEC protection classes are defined in IEC 61140 《Protection against electric shock – Common aspects for installations and equipment》 (GB/T 17045-2008《电击防护装置和设备的通用部分》) and used to differentiate between the protective-earth connection requirements of devices. These appliances must have their chassis connected to electrical earth.
Class I等级是指产品的防触电保护不仅依靠基本绝缘，而且还包括接地。
These appliances must have their chassis connected to electrical earth (US: ground) by a separate earth conductor
Class II是指防触电安全等级，标注Class II的电器设备，是指产品的防触电保护不仅依靠基本绝缘，而且还包括附加的安全措施(例如双重绝缘或加强绝缘)，但没有接地或依赖安装条件的保护措施。
A Class II or double insulated electrical appliance is one which has been designed in such a way that it DOEs not require a safety connection to electrical earth (ground).
Class III等级是指产品的防触电保护依靠电源电压为安全特低电压(SELV)，并且其中不会产生危险电压。
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.
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