Applies to all "radio apparatus, electro-acoustic apparatus and combined radio and electro-acoustic apparatus" intended for operation from public or private electricity supply mains having a rated voltage not exceeding 300 volts. It applies to all associated devices including "power units" for supplying current and/or applying voltage to "radio apparatus, electro-acoustic apparatus and combined radio and electro-accoustic apparatus", power transformers for heating the cathodes of thermoionic valves, devices connected to the supply mains such as "line filters" for the reduction of noise interference, power transformers and associated apparatus for furnishing energy to battery chargers, loud speakers, head telephones, control equipment, etc., whether incorporated in the "radio apparatus, electro-acoustic apparatus and combined radio and electro-acoustic apparatus" proper or whether used in conjunction with it as a separate unit or units.