I read the sentence
Non posso disturbarle tranne / salvo / eccetto che non sia molto urgente.
illustrating the use of congiuntivo after tranno che.
In other languages, we would say
I can't disturb him except if this is very urgent.
It would be the same in French or German for instance. I don't really get why there is a negation. Is the english translation above correct and why is there a negation ? Couldn't we say
Non posso disturbarli tranne che sia molto urgente.
or what would be the sense of this last sentence ?