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.