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ELV is used in order to reduce the danger of electric shock. With ELV the danger of serious harm is significantly smaller when compared to normal mains voltage (e.g. 220-240V in the UK).
There are three types of ELV systems: SELV, PELV and FELV.1)
Such voltages can be generated with the use of a safety isolating transformer as defined in the standard BS 3535.
In a separated extra-low voltage (SELV) system the low-voltage output is electrically separated (galvanically) from earth and other systems. Therefore, a single fault cannot create a risk of an electric shock. There should be no provision for earthing of an SELV circuit.3)
In certain locations, e.g. swimming pools or for medical apparatus it is the only measure permitted. However, because there is always a risk of electric shock then the requirements can be even more stringent, e.g. nominal voltage limited to 12 V AC or 30 V DC.
SELV voltage can be generated for instance from a battery.
However, it can be also generated by means of a SELV transformer, but the construction requires high-integrity equipment and materials. This is in order to ensure adequate isolation from the primary voltage (mains voltage) which is much more dangerous. This is achieved for instance by double insulation or reinforced insulation.4)
A SELV transformer must be an isolation safety transformer and must comply for instance with the requirements of EN 61558. The design requires special insulation tests to verify the integration of the construction.
By definition, SELV is an unearthed system, so where required overcurrent devices must be fitted in both live conductors.
In a protective extra-low voltage (PELV) system there is no separation from earth, but otherwise the system satisfies all other requirements for SELV, including the voltage levels.
In a PELV transformer (similarly to a SELV transformer) the magnetic core and the enclosure can be connected to earth (see the image).
A functional extra-low voltage (FELV) system can be used just for functional purposes, for instance for machine control systems.
Protection against direct contact (basic protection) must be provided by insulation, barriers and enclosures - this includes a FELV transformer used for generation of voltage in a FELV system.5)
As seen in the image, in a FELV transformer the magnetic core does not have to be earthed.