Tree-definition stage

These must be very carefully spelt out to minimize the chance of a systematic error at the definition stage. Fault trees and/or trip logic diagrams should be used to achieve the best possible clarity. 

Initiation and propagation of water trees depend on a multitude of parameters electric field strength, mechanical stress, type and concentration of ions in the water, constitution of the polymer and its additives, etc. At this stage there is no satisfactory model that explains the process of water treeing. In a comprehensive review, Crine (1998) carefully considers the various types of possible mechanism in relation to a vast amount of experimental investigation, but is unable to come to any definite conclusion about primary mechanisms. Rather surprisingly, oxidation appears to play no major role (Bulinski et al., 1998). It seems only likely that ingress of water, aided by the presence of... 

Section 2 addresses the strategy of defence in depth and the importance of fulfilling the safety functions (SFs) to achieve the objectives for the different levels of defence. Section 3 provides a detailed description of the approach for making an inventory of the defence in depth capabilities of a plant. Section 4 discusses the applications of the approach and how to use the approach for assessing defence in depth. Section 5 presents conclusions. A discussion of the SFs is presented in Appendix I. In Appendix II, the objective trees graphically represent how, for each relevant safety principle , the safety objectives of the different levels of defence can be achieved by establishing defence in depth provisions at different stages of the lifetime of the plant. A test application of the approach is summarized in Appendix III. Definitions are provided at the end of the book. 

PSFs influence the occurrence of a human error and their interaction determines the respective mode of failure. This can be represented by an influence tree, which requires the definition of the potential error. The selection of the appropriate PSFs from the respective set may follow the procedure of fault or event trees. The procedure has two stages, namely the pre-selection and the selection stages. 

The systems risk analysis is performed in two steps. As a first step there is a FMEA and a conventional fault tree analysis. This is done for two reasons. The FMEA is done at a very early point in the project. At this early stage, the system architecture definition is not yet complete. High level errors and possible unsafe states are... 

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