These standards and rules apply to converters with mercury vapor and mercury cathode, and in certain parts also to converters with filament, or indirectly-heated cathodes. They apply, in the first place, to rectifiers for a d—c voltage not exceeding 5 000 V and a direct current not less than 100 A; in second place, in applicable parts, to rectifiers for higher voltages and to inverters and frequency changers (see clause 3).
Due to the variety of possible ways in which the circuits tor the various types of converters may be arranged, it has not been possible up to the present time, to establish a completely general terminology or rules covering all possibilities.