extra low voltage (ELV)
In electrical engineering, voltage limits for the various voltage ranges are specified in IEC 60449 and VDE 0100. In general, a distinction is made between extra- low voltage, low voltage, medium voltage, high voltage and extra-high voltage. Low voltages belong to the low voltages.
As far as extra-low voltages are concerned, these are voltages of less than 50 V alternating voltage and 120 V direct voltage. These are voltages that are not life-threatening when touched by adults. However, there are certain areas in which the permissible low voltages must not exceed 25 V( AC) or 60 V( DC). For example in damp rooms. For extra-low voltages, a distinction is made between unearthed safety extra-low voltage( SELV), earthed protective extra-low voltage( PELV) and functional extra-low voltage ( FELV).
The different safety extra-low voltages, protective extra-low voltages and functional extra-low vol tages are all about how the power sources and circuits are separated from each other. Thus, for safety-related SELV voltages and the grounded protective PELV voltages, there is safe separation of the power sources, for example, by a safety transformer. Likewise, the circuits are safely separated. In the case of SELV, they are ungrounded and the user must not intentionally connect to ground or a protective earth conductor. In contrast, in the case of PELV, grounded and ungrounded circuits are possible, and the user may be connected to ground.
The situation is different with function-oriented FELV voltage, where the power sources and circuits have no safe separation. The circuits can be grounded and the user must be connected to the protective conductor.