calculate the fault current ...set voltage obtained by multiplying the secondary fault current by the CT and lead resistances sum.
Compare the result obtained to the Magnetising Characterictic of CT , if the knee point is higher than twice the value obtained than the setting should be done...
Sphincterboy is correct however SF6 is dangerous to health if decomposed especially the white dust which forms....this is caused by arcing thus it is important to avoid any kind of 'fire' when handling SF6.
Professional help can be obtained from manufacturers of the Switchgear ; also check this...
