phanom fail safe

[bladesufc]

hi all  just got my lad a dji phantom  and was wondering how the failsafe works does it have to be set up in the phantom naza sassistant ?when i go to the software the failsafe does not seem to trigger on the software when i turn the radio off ie the boxes that turn blue the gps goes blue  so does rthe atti but the failsafe
e is greyed out  any help appreciated cheers martyn..