This part of ISO 21940 specifies procedures to be adopted when balancing medium and large rotors installed in their own bearings on site. It addresses the conditions under which it is appropriate to undertake in-situ balancing, the instrumentation required, the safety implications and the requirements for reporting and maintaining records. This part of ISO 21940 can be used as a basis for a contract to undertake in-situ balancing. It does not provide guidance on the methods used to calculate the correction masses from measured vibration data. NOTE The procedures covered in this part of ISO 21940 are suitable for medium and large machines. However, many of the principles are equally applicable to machines of a smaller size, where it is necessary to maintain good records of the vibration behaviour and the correction mass configurations.