What do you call a scientist who had to take remedial classes?
An engineer.
On a serious note, think back to the EM semester of physics and consider the potential differences of the things you are connecting. You are a lot better off getting in the habit of approaching problems from first principle than relying on some rule of thumb that might not apply in a particular case.
Isolation transformers are for handling cases where a circuit ground is referenced to the mains neutral lead. AC/DC radios, TVs and guitar amplifiers are the notorious cases. They are NOT a substitute for thinking about what you're doing. There was a thread a while back started by someone who had blown up their Hantek scope by connecting it to 220 V mains without realizing it. Someone posted a surveillance camera video of 4 men getting killed by pushing a metal scaffold into contact with an overhead power line to the thread. They all died because none of them looked up.
One of the big advantages of "hobby" work is you can stop if you are tired or uncertain.
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