I want to measure input voltage with oscilloscope for my DUT which runs with 20v. Do i need to ground the oscilloscope? Wheb do i need to ground the instrument with ground mat and do i need to be also grounded? When is it necessary to geound the instrument??
The scope doesn't absolutely need a soil connection. Nor your DUT. In fact a good thing to install is an isolation transformer. It insulates the scope from soil.
There are situations where mains AC can find a short circuit through your setup, if you're not careful. More than once I have zapped my projects because I forgot to pay attention to hot and neutral mains wires.