Underwriters Laboratories, UL™ defines several classes of isolation that are used in modern power supplies.

Safety extra low voltage (SELV) is defined by UL (UL1950 E199929) as a secondary circuit which is so designated and protected that under normal and single fault conditions the voltage between any two accessible parts, or between an accessible part and the equipment earthing terminal for operational isolation does not exceed steady state 42.5 V peak or 60 V_DC for more than one second.