Some mechanisms of insulation failure

The paper reports recent investigations into mechanisms of insulation failure. After a review of recent progress emphasizing that failure depends on a number of distinct factors which can be separated by suitable techniques, experimental results are given covering some theories recently established. These include: (a) Electrochemical attack on capacitor dielectrics, with results of life tests on capacitors stabilized with a hydrogen acceptor. (b) Discharge inception in oil-impregnated paper insulation, with results showing that the inception voltage is raised by addition of a hydrogen acceptor to the oil. (c) Failure in thin polystyrene films due to the action of oily particles. (d) Local thermal instability in high-permittivity ceramics due to stress concentration at electrode edges. At least five general causes of failure are now clearly distinguishable, and there is strong evidence for two others which are under investigation. The conclusion is drawn that working stresses in insulation can usually be increased when the cause of failure is known in detail and the design modified accordingly. Attention is drawn to the need for close collaboration between laboratory workers investigating causes of failure and design or development staff.