EN60335-Household and similar electrical appliances Safety —Part 1: General requirements

extra-low voltage
voltage supplied from a source within the appliance that does not exceed 50 V between conductors and between conductors and earth when the appliance is supplied at rated
voltage

safety extra-low voltage
voltage not exceeding 42 V between conductors and between conductors and earth, the noload voltage not exceeding 50 V When safety extra-low voltage is obtained from the supply mains, it is to be through a safety isolating transformer or a convertor with separate windings, the insulation of which complies with double insulation or reinforced insulation requirements.
NOTE 1 The voltage limits specified are based on the assumption that the safety isolating transformer is supplied at its rated voltage.
NOTE 2 Safety extra-low voltage is also known as SELV.

safety isolating transformer
transformer, the input winding of which is electrically separated from the output winding by an insulation at least equivalent to double insulation or reinforced insulation, that is intended to supply an appliance or circuit at safety extra-low voltage

protective extra-low voltage circuit
earthed circuit operating at safety extra-low voltage which is separated from other circuits by basic insulation and protective screening, double insulation or reinforced insulation
NOTE 1 Protective screening is the separation of circuits from live parts by means of an earthed screen.
NOTE 2 A protective extra-low voltage circuit is also known as a PELV circuit.