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This International Standard gives specifications applicable to the following types of transformers for use in resistance welding equipment:— single-phase transformers for a.c. welding, typically operating at 50 Hz or 60 Hz;— single-phase transformers with connected rectifier for d.c. welding, typically operating at 50 Hz or 60 Hz;— single-phase inverter transformers with connected rectifier for d.c. welding, typically operating at 400 Hz to 2 kHz;— three-phase transformers with connected rectifier for d.c. welding, typically operating at 50 Hz or 60 Hz.For the purposes of this International Standard, the term transformer can refer to the transformer alone or with connected rectifier (transformer-rectifier unit).This International Standard applies to transformers built to protection class I or II according to IEC 61140.NOTE The requirements of this International Standard can be supplemented by other resistance welding transformer standards, e.g. ISO 22829 and ISO 10656.