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Trifluoroiodomethane CF 3 I has been recently considered as a potential alternative to SF 6 , thanks to its high electronegativity and extremely low environmental effect. However, due to its high boiling point, CF 3 I should be mixed with other gases such as N 2 or CO 2 in order to avoid the liquefaction at low temperature.In this paper, we present breakdown measurements carried out in CF 3 I and CF 3 I-N 2 gas mixtures, under dc voltage, and with a pressure-distance product (p.d) up to 5 mmbar. Pure CF 3 I has a higher breakdown voltage (V b ) than SF 6 . However, the favourable synergistic effect observed in SF 6 gas mixtures with N 2 , at low SF 6 content, is not observed in CF 3 I gas mixtures. In CF 3 I-N 2 mixtures, the dc breakdown voltage increases proportionally to the CF 3 I content. As a consequence, breakdown voltages for mixtures containing up to 50% CF 3 I are lower than with SF 6 at the same ratio. These results are in good agreement with calculated values using breakdown model and reduced critical field strength (E/N) crit reported by others authors.In CF 3 I, the appearance of solid by-products (particles) generated by breakdown is also observed. The nature of these particles is tentatively attributed to the formation of iodine. In our sphere-sphere geometry, the accumulation of such solid particles leads to a measurable conductivity.

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