On my bench I have an isolation variable Xfer for the DUT. For those less common times where I need to float my instrument I have a Tek safety device that does not let the instrument float more than about 30V above earth ground.

 

My bench too. Ever since I was 10 years old and that was nearly 50 years ago. Worked required me to work on 15 kV @ 1.5 A DC, 100 kV @ 0.1 A DC and 1000 W into 50 ohms 13.56 MHz RF. One slip-up and i'd be dead,

 

@AlbertHall
getting the scope in A-B mode sometimes isn't obvious. it's usually invert one channel and sum.

The isolated differential probe is the way to go. I finally got one for home

 

This is the problem, the scope should always stay at ground potential, and you should use an isolation transformer to isolate the measured circuit. It seems to me that often when people talk about isolation transformers it is not quite evident if it is the scope that should be isolated or the device.

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Most pre LCD scopes have all metal case - the dangers of that much metalwork at floating potential, should be obvious even to someone who was dropped on their head a lot when they were little..............................

 

The isolated differential probe is the way to go. I finally got one for home

It's on my list.........someday.