This happened to a friend of mine a couple of days ago, but i'd like to share with you his experience.
Notwithstanding I clearly have advised him to switch the SWG machine off while backwashing and rinsing the filter, my friend, not only has forgotten it but he also had the great idea to close all the cell bypass valves.
As a result after a few minutes the sgw cell exploded and it was a quite big explosion, fortunately he was far enough to not be invested by clorine gases, but .......
The cell container was made with a transparent plastic (about 8 mm thick) and it was impressing how it got destroied.
In the next days I'll post some picture in orr to let you figure it out.
So don't keep the bypass valves closed, never!!!!
Unless you are really sure that the SWG machine is powered off !