This standard describes the sampling procedures and methods for the determination of particle concentration and size distribution.Three methods are specified. One uses an automatic particle size analyser, working on the light interruption principle. The other two use an optical microscope, in either the transmitted light or incident light mode, to count particles collected on the surface of a membrane filter.The optical microscope methods are described in ISO 4407.All three methods are applicable to both used and unused insulating liquids.Annex A contains an alternative sampling procedure using a syringe and Annex B reports a reference for the calibration o