Causes and effects of partial discharges (PD) inside electrical high-voltage insulation was studied prior to taking measurements with a PDS100 spectralanalyser during excursions to a number of substations in the Swedish electrical grid.This to develop a working-method and manual for workers to use periodically whileon site to be able to identify components on which maintenance is needed. Noprominent signs of PD was measured during the excursions although with materialfrom previous measurements showing PD, provided by the manufacturer of thePDS100, a manual to use in the field could be developed.