For the balloon project it was decided that the balloon should be able to be separated from the parachute and pauload under certain conditions.
We developed and tested several methods for doing this and setteled with this very reliable method
A nylon filament is retained by a 1/4W resistor. When a drop is needed the resistor is powered for 3sec. thus melting the filament. This method has proven to be extremely reliable over many tests.
Parachute drop mechanism, a set on Flickr.