ELV (extra low voltage)
voltage which does not exceed 50 V a.c. r.m.s. or 120 V ripple free d.c. between conductors, or
between any conductor and earth
SELV (safety extra low voltage)
ELV in a circuit which is insulated from the mains supply by an insulation not less than that
between the primary and secondary circuits of a safety isolating transformer according to
IEC 61558-2-6 or equivalent