Defects and their role in wet and dry ageing of insulation, or, when is a feature a defect?

Electrical ageing is a complex phenomenon and for practical insulation systems the presence of a single deteriorating site is sufficient to cause failure. Against this background the difficulties of examining multifactor ageing are substantial. Thus a practical insulation system is a collection of features: one of the features will be the ultimate breakdown site. Having defined a practical insulation system as a collection of features with a range of sizes and properties there are some additional electrical factors which can influence breakdown to further complicate this simple picture. Examples of experiments where some of the results were unexpected are given to illustrate the theme that electrical ageing at features can be complex even before multifactor ageing is invoked