Was the isolator that is shown in your later picture turned off?
You haven’t directly answered that, but I guess the answer is no, the isolator was not turned off.
That’s crazy. Most people don’t have the benefit of an isolator like that; you have one, but haven’t taken advantage of it. You must be insane.
I’m not going to try to help you fault-find why you get that spark. My suggestion to anyone else who does want to help: make sure your suggestions start with “turn off the isolator” and then proceed with safe testing procedures.
