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This document provides guidelines for the assessment of torsional natural frequencies and component strength, under normal operating conditions, for the coupled shaft train, including long elastic rotor blades, of steam and gas turbine generator sets. In particular, the guidelines apply to the torsional responses of the coupled shaft train at grid and twice grid frequencies due to electrical excitation of the electrical network to which the turbine generator set is connected. Excitation at other frequencies (e.g. subharmonic frequencies) are not covered in this document. No guidelines are given regarding the torsional vibration response caused by steam excitation or other excitation mechanisms not related to the electrical network. Where the shaft cross sections and couplings do not fulfil the required strength criteria and/or torsional natural frequencies do not conform with defined frequency margins, other actions shall be defined to resolve the problem. The requirements included in this document are applicable to a) steam turbine generator sets connected to the electrical network, and b) gas turbine generator sets connected to the electrical network. Methods currently available for carrying out both analytical assessment and test validation of the shaft train torsional natural frequencies are also described. NOTE      Radial (lateral, transverse) and axial vibration of steam and/or gas turbine generator sets is dealt with in ISO 20816-2.