This International Standard is applicable to transformer-rectifier units as used in electric resistance welding machines operating from a power supply with a frequency of 1 000 Hz, and of a rated value of the input voltage equal to or higher than 500 V. These transformer-rectifier units are primarily used in welding guns with an integrated transformer. For these transformer units, this International Standard supplements the requirements given in ISO 5826 and ISO 10656, which remain applicable except where amended by this International Standard.