Many thanks guys. Paul there is enough here to keep me off the vino this week-end!
My concern is really in the safety testing of electrical equipment in-service, especially relating to insulation resistance testing using either the 500v applied voltage method or the touch current method.
I think it probably the filtering ccts that are causing problems. 500v can damage components but if a resistor is connected across capacitor to earth, a low reading would result. Similarly, with the touch current method, AC applied to a filter cct will show current flow and therefor suggest leakage.
I guess it mau be a case of consulting the manufacturer when in doubt!