Interested in Language
Hello one more time!
As a rule however, no panels, connection boxes or similar are installed on deck, except where otherwise impossible.
Does "except where otherwise impossible" mean "except where (it is) impossible under/in other conditions?".
I will provide more context.
Cable connection boxes and other panels on deck are stainless steel, or bronze or composite materials for smaller sizes.
Except where it is impossible to install them anywhere other than on deck. In other words, "You can put them on deck if there is no alternative place to put them."