A circuit involving a potential of not more than 30 volts and supplied by a primary battery or by a standard Class 2 transformer; or by a suitable combination of transformer and fixed impedance that, as a unit, complies with all the performance requirements for a Class 2 transformer. (A circuit derived from a supply source classified as a line-voltage circuit, by connecting resistance in series with the supply circuit as a means of limiting the voltage and current, is neither considered to be a low-voltage nor an isolated secondary circuit.)
