Ok, let's separate things like this:
1. scenario when we wish to measure with a bench DSO a low voltage DUT like let's say an Arduino stuff, or a crankshaft sensor on the car. All this DUT are low voltage devices and powered from battery. (
I won't put here now a situation when the DUT is powered from a bench power supply which could be a tricky stuff if the negative terminal on the bench power supply is grounded.)
2. scenario
when we wish to measure a HV DUT which is powered from mains like lets say an SMPS.
Of course I used also handheld scopes which are battery powered but they are all so small compared to my bench DSO and all that tiny buttons on there are really a pain in the a_s.
I really wondering how to convert my bench DSO in a way so I can use it more safer in circuit testing like in the 1. scenario.
I was thinking to mod my power stage of my DSO in a way so I could hook up a battery an if I'm working on a car I could use a separated battery. That would be the same as I would use a handheld scope.
The second option would be to use a portable GFCI cable where my scope is connected, but that is not a handy device
if some HV is traveling through the ground wire (...for some reason the main fuse is not reacting or there is no GFCI installed in the house) which is connected to the DUT through the ground clip of the scope...
The GFCI will cut the live wire and so shut down my scope but won't protect the DUT also.
I always go back and watch Daves video here:
He talks also about two scenario, the 1. and the 2. scenario, where the DUT is isolated through a battery powering
or some sort of isolated oldstyle power adapter with transformer inside but for sure the ground of the secondary stage of the power adapter must be really isolated and not connected to ground.
If I use Daves introducing then a car should be also the same DUT.
Actually there is also everything powered through his battery and has no way referenced to the main earth connection.
So, why should that be dangerous to use on a car a bench scope?
I see only one reason:
If some AC HV could travel through the earth wiring.
This could make some damage on the battery powered DUT.