This guide presents methods for online monitoring and recording of the transient overvoltages in a threephase AC power system when the voltage level is higher than 1 kV. It applies to the qualitative measurement of transient overvoltages in substations@ power stations@ and transmission lines. The results are used for power system operating condition analyses. This guide does not include online monitoring and recording of very fast transient overvoltage (VFTO) caused by operation of GIS disconnectors.