Extra Low Voltage (ELV)
Extra-low voltage is used where the risks are great: swimming pools, wandering-lead hand lamps, and other portable appliances for outdoor use, etc.
Use of SELV (Safety Extra-Low Voltage)
Safety by extra low voltage SELV is used in situations where the operation of electrical equipment presents a serious hazard (swimming pools, amusement parks, etc.). This measure depends on supplying power at extra-low voltage from the secondary windings of isolating transformers especially designed according to national or to international (IEC 60742) standard. The impulse withstand level of insulation between the primary and secondary windings is very high, and/or an earthed metal screen is sometimes incorporated between the windings.
The secondary voltage never exceeds 50 V rms.
Three conditions of exploitation must be respected in order to provide satisfactory fault protection:
- No live conductor at SELV must be connected to earth
- Exposed-conductive-parts of SELV supplied equipment must not be connected to earth, to other exposed conductive parts, or to extraneous-conductive-parts
- All live parts of SELV circuits and of other circuits of higher voltage must be separated by a distance at least equal to that between the primary and secondary windings of a safety isolating transformer.
These measures require that:
- SELV circuits must use conduits exclusively provided for them, unless cables which are insulated for the highest voltage of the other circuits are used for the SELV circuits
- Socket outlets for the SELV system must not have an earth-pin contact.
The SELV circuit plugs and sockets must be special, so that inadvertent connection to a different voltage level is not possible.
Note: In normal conditions, when the SELV voltage is less than 25 V, there is no need to provide protection against direct contact hazards. Particular requirements are indicated in Requirements applicable to special installations and locations.
Use of PELV (Protection by Extra Low Voltage)
(see Figure F67)
This system is for general use where low voltage is required, or preferred for safety reasons, other than in the high-risk locations noted above. The conception is similar to that of the SELV system, but the secondary circuit may earthed at one point.
IEC 60364-4-41 defines precisely the significance of the reference PELV.
Basic protection is generally necessary, except when the equipment is in the zone of equipotential bonding, and the nominal voltage does not exceed 25 V rms, and the equipment is used in normally dry locations only, and large-area contact with the human body is not expected. In all other cases, 12 V rms is the maximum permitted voltage, where no basic protection is provided.
FELV system (Functional Extra-Low Voltage)
Where, for functional reasons, a voltage of 50 V or less is used, but not all the requirements relating to SELV or PELV are fulfilled, appropriate measures described in IEC 60364-4-41 must be taken to ensure both basic and fault protection, according to the location and use of these circuits.
Note: Such conditions may, for example, be encountered when the circuit contains equipment (such as transformers, relays, remote-control switches, contactors) insufficiently insulated with respect to circuits at higher voltages.