Statistical techniques for analysing water treeing data and implications for testing procedures

Water trees can have a variety of inception times and growth rates under fixed ageing conditions. It is therefore necessary to use statistics in static or dynamic descriptions of water tree populations. There are two distinct stages in the formation of a water tree: the inception stage which, for a given water tree, accounts for the time between the establishing of conditions suitable for water trees to grow and the tree starting to grow; and, the growth stage in which the tree increases in size. The author presents methods of estimation of the distributions of inception times and growth times for a population of water trees. All the methods of analysis presented rely on data taken at different point in time by sampling populations of trees grown under the same conditions. These samples need to be large enough and taken frequently enough to ensure that accurate population estimates may be made. The specimen geometry used should be representative of `real´ conditions, in particular needles and scratched surfaces should be avoided since they force inception thereby precluding information on the distribution of inception times