An approach to the maintenance of adequate insulation resistance levels on DC machines

The development of more accurate and versatile test instruments and the increasing interest in the extension of rotating equipment life through the use of preventive maintenance have resulted in many changes in rotating machines to facilitate maintenance and improve reliability. Most users of electrical equipment are familiar with at least the more fundamental tests and theories and, as a result, are able to conduct tests and interpret observed data in a manner which will estimate equipment condition and predict its approximate useful life. One of the simplest and most common tests is the reading of insulation resistance to ground values on a regular basis. This is ordinarily done with a hand-cranked, motor-driven, or rectifier-powered ohmmeter capable of reading resistances in the order of megohms. The advantages of this type of meter and testing are: simplicity, low initial investment, and its adaptability to nearly any type of maintenance program. Merely reading insulation resistance is not sufficient, however, unless the readings can be interpreted properly and used beneficially in gaining some reward such as prolonged or improved equipment life.