ewr, if your main flight battery system shorts out what good is a separate battery powering your naza/receiver going to do? if the main battery system fails (powering motors etc) the bird will come down regardless of wether you still have power to your naza/rx (its not a car or boat).
If worried about LED lights, switches/bomb drops etc, shorting out, install a fuse on those circuits that will protect the rest of the system.
regards bruce