You can download IEC 60071-1:2019 pdf.The title of this standard is Insulation co-ordination – Part 1: Definitions, principles and rules.
IEC 60071-1:2019 applies to three-phase AC systems having a highest voltage for equipment above 1 kV. It specifies the procedure for the selection of the rated withstand voltages for the phase-to-earth, phase-to-phase and longitudinal insulation of the equipment and the installations of these systems. It also gives the lists of the standard withstand voltages from which the rated withstand voltages are selected. This document describes that the selected withstand voltages are associated with the highest voltage for equipment.
The determination of the co-ordination withstand voltages consists of determining the lowest values of the withstand voltages of the insulation meeting the performance criterion when subjected to the representative overvoltages under service conditions.
The co-ordination withstand voltages of the insulation have the shape of the representative overvoltages of the relevant class and their values are obtained by multiplying the values of the representative overvoltages by a co-ordination factor. The value of co-ordination actor depends on the accuracy of the evaluation of the representative overvoltages and on an empirical, or on a statistical appraisal of the distribution of the overvoltages and of the sulation characteristics.
The co-ordination withstand voltages can be determined as either conventional assumed withstand voltages or statistical withstand voltages. This affects the determination procedure and the values of the co-ordination factor.
Simulations of overvoltage events combined with the simultaneous evaluation of the risk of failure, using the relevant insulation characteristics, permit the direct determination of the statistical co-ordination withstand voltages without the intermediate step of determining the representative overvoltages (see Figure 1).