SELV stands for Safety Extra Low Voltage and are considered Class III equipment.
Class III equipment is defined in some equipment standards as that in which protection against electric shock relies on the fact that no voltages higher than safety extra low voltage (SELV) are present. SELV is defined in turn in the relevant standard as a voltage not exceeding 25V ac or 60V dc.
It is interesting to note that current IEC standards relating to safety of luminaires do not recognise Class III equipment since limitation of voltage is not deemed sufficient to ensure safety of the consumer. All light fittings that are capable of mains connection must be classified as Class I or Class II.