EMC Implications of Using a Class II Power Supply in a Class I Application
The majority of power supplies used in commercial or industrial applications today fall into one of two construction categories, Class I or Class II (not to be confused with Class 1 and Class 2). These classes are associated with safety, and specifically indicate the constructional method used to provide end users with the an appropriate level of protection from hazardous voltages. In general, all electrical appliances must provide users with at least two levels of protection from hazardous voltages (greater than 42Vac or 60VDC). Class I and Class II power supplies differ in how they achieve these two levels of protection, specifically in whether or not they utilize a connection to earth ground. One can identify a Class I supply by looking for a third pin on the AC inlet (Earth Gro und), if the device has only two blades, it is a Class II.
While safety is the primary purpose of any Earth Ground connection, this connection has evolved over the years to be used for more than just protection. Earth Ground connections are widely used in Class I applications as a sink for high frequency conducted disturbances that would otherwise degrade EMC performance, and potentially prevent compliance with FCC or CISPR regulations. This breeds a common misconception: If Class II devices meet conducted emissions standards without the use of an Earth Ground connection, then adding an earth ground connection between my design, and the grid can only better my conducted emissions performance…. Read More (view pdf)>>