The introduction of large air-cooled gas turbine generators, and the ever-larger hydrogen has caused the re- engineering of the GCM for application in air-cooled machines [17]. The device is installed in the air-cooling system in a totally enclosed fan cooled machine, and a cloud chamber principle is used to detect the particulates. Most insulation not produce until they are high temperatures In rotor and stator windings, if the groundwall insulation is at such a high temperature, it is likely that a ground fault will follow within minutes or hours. This is in contrast to stator cores, where the lamination insulation can burn locally, but this does not imply that the machine will fail even within the next few years. Consequently, GCMs are usually not able to give much warning of overheating problems in the rotor and stator windings.