SF6 and Greenhouse effect

Does anyone know the impact of SF6 in the greenhouse effect?

One word - Minimal. As a fellow supplier of American SF6 switchgear into New Zealand, here are two good links below which explain things nicely

http://en.wikipedia.org/wiki/Sulfur_hexafluoride

What does the guide IEC1634 says about it?

That SF6 is not considered an ozone depletion gas (if that's what you are concerned about). From the article linked below

"SF6 gas has a negligible impact on the environment according to Australian and International Standards (IEC 1634:1995 and AS2791-1996)".

http://www.poweronline.com/article.mvc/IEC-and-Australian-Standards-Information-on-S-0001?VNETCOOKIE=NO

Plenty of other links on the internet. If in doubt you could buy IEC1634.