Minimum approach distances calculation and individuation of the limit conditions for safe live work. Claudio Valagussa*, Ugo Leva*, Giorgio De Donà**, Carlo Dante Milanello** Alfonso Posati**, Rocco Gallo** ICOLIM 2008-9th International Conference on Live Maintenance Torun, Polonia 4-6 Giugno 2008 *CESI RICERCA ** Terna SpA At present, composite insulators are widely used on High Voltage lines, all over the world. Nevertheless, some doubts about their diffused use subsist, since, in presence of damaged insulators, it is difficult to define the criteria allowing safe operation conditions with Live Line Working techniques. In particular, IEC 61472 Standard, which states the method for working distances calculation, in case of porcelain or glass insulators prescribes to assess the maximum amount of allowed damaged units, but does not deal with live work on composite insulators: in this case, the standard does not define neither the calculation method of the safety distances nor the limit damage length, above which live works have to be forbidden. The present paper describes the test activity carried on by CESI RICERCA in collaboration with TERNA, with the aim to characterize High Voltage composite insulators affected by defects leading to their electromechanic features loss, from the point of view of dielectric strength and damage detection. Test activities performed allowed the formulation of proposals in order to apply to composite insulators a distance calculation method and an assessment of the limit defect length, similar to those prescribe by IEC Standard 61472 for cap and pin insulators. Moreover, additional test activities allowed a sensitivity evaluation for the diagnostic method based on UV measurements, when applied to detection of defects having equal or lower length than the limit one.