Other generators have only the terminal A, which only permits checking the thrust-bearing insulation to ground. This bearing insulation test should be performed annually on all Reclamation generators. If the insulation resistance is abnormally low, the cause of the trouble should be investigated. Before concluding that the insulating sheets under the bearing supports are causing the low resistance, check the RTD leads, temperature device tube, or high pressure oil connection to the bearing shoes. The thrust-bearing insulation resistance should measure from about 10,000 ohms to infinity.
Low resistance can indicate mechanical damage or damp insulation from leaky cooling coils.
Dampness in the oil pot, due perhaps to a very slow cooling-coil leak, can cause serious corrosion damage or saturation of insulation, if allowed to persist. These problems are difficult to detect except by this test. When there is very high resistance with the machine running, it is a good indication that the bearing surfaces are free of high spots or roughness, and the bearing is not grounded. This test can also be used when establishing an oil film with a high pressure lubrication system or to determine how quickly a complete oil film is established on starting a unit, or how long it persists on stopping. When operating at normal speed the oil film resistance should be from 100,000 to 500,000 ohms.