A rotating machine is any electrical motor or generator, as opposed to static equipment such as a transformer. They share a problem: the stator insulation works under voltage, temperature, vibration and electromagnetic forces all at once, a combination no other equipment suffers.
Hence diagnostics rest on several complementary measurements. Insulation resistance and the polarization index detect moisture and contamination; dissipation factor measures ageing throughout the volume of the insulation; partial discharge, measured with capacitive couplers or HFCTs, locates the defects that progress; and winding resistance uncovers deteriorated connections and joints.
IEEE 43 sets the insulation criteria and IEC 60034-27 those for partial discharge measurement. On large machines permanent monitoring is the norm, because an unplanned generator outage costs far more than the measuring equipment itself.