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SF6 gas has nowadays established itself as a reliable insulating medium for gas-insulated switchgear (GIS). In GIS however different kind of defects may occur, such as protrusions fixed to the conductors. Such defects locally enhance the electric field, which can result in partial discharge (PD) activity or even breakdown. Insulation condition assessment procedures are required for the application of condition based maintenance (CBM) on GIS. This condition assessment procedure requires aspects such as sensitive detection of partial discharge activity, identification and location of the discharging source and probability for breakdown. In this paper, the criticality or probability for breakdown of fixed defects has been investigated. For this purpose, AC voltages have been applied to study the breakdown probabilities under pure AC condition. Secondly, the defects have been tested with standard lightning impulses (LI) and switching impulses (SI) and finally a combined AC and transient overvoltage test (bias-test) was performed. Based on above-mentioned results, knowledge rules for risk assessment tools have been defined.