ISO 5826:2014 gives specifications applicable to transformers of IEC 61140 protection class I or II of the following types 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.