The consumer should be supplied with electrical energy, produced in power stations, reliably and economically. Overvoltages cannot be avoided in the required transmission and distribution lines. These voltages are limited by an overvoltage protection to a safe level for the insulating power of the electrical equipment. However pollution layers and aging can reduce this insulating power so far as it is possible to have a flashover already at normal operating voltage. An unavoidable residual risk is kept small by the coordination of the insulation ratings of all operating equipment so far as it is technically possible and economically defensible.
Therefore the insulation of the electrical equipment and plants is to be design in such a way, that this insulation withstands permanently the highest permissible operating voltages and expecting overvoltages. Besides the unfavourable atmospheric conditions at the installation location are taken into account. Insulation coordination is the selection and allocation of the insulation level of the electrical equipment taking account of all the occuring voltage stresses in the power supply system. The aim of the insulation coordination is, if it is possible to avoid faults and defects by overvoltages. The limiting of overvoltages by preventative protective measures or protective equipment is taken into account.