With DC coupling you are obviously limited by the fact that the scope probably can only display 80Vp-p (10V/div x 8 divisions), plus you can use a certain amount of offset. So the 400Vpk is a safety or damage limitation and you typically need to stay well below that. In practice, most experienced users will use at least a 10X probe almost all the time and not get anywhere near the 400V limitation.
With AC coupling, the scope will not see a DC bias so you can look at the ripple of a 300VDC power supply using the 2mV/div range without any issues. That's fine, but if the AC coupling capacitor gets overloaded or breaks down, sparks will fly. Unless you really, really need the sensitivity, using a 10X probe (at least) puts 9M of resistance between the DUT and your scope. Note that the AC coupling capacitor still charges up to the full 300VDC, so the 400V limitation remains the same and the full 300VDC shows up at the input BNC. There are specific x100/10M probes with internal dividers that avoid this issue when using scopes with this design.
Many newer design scopes have AC coupling that works differently than your older scope and these don't react the same way--AC coupling is not limited in the same way. A 10X probe with the same 300VDC power supply on these scopes will only have 30VDC at the input BNC.
I wouldn't worry about whether the scope is 'Chinese' or not, but I would avoid cheap junk. I have Tektronix scopes that are made in China and I have Siglent scopes made in China and they are both protected just fine. However, the Siglents are of the latter design that doesn't have the AC coupling limitation. In any case, I wouldn't typically set the scope up with anything over 50V or so at the inputs, just because it is almost always unnecessary.
We've drifted a bit, with a large signal say like 300v the main reason not to try and view it (DC coupled) is dynamic range, if you can get it into view your'll maybe be able to tell 299v-301v say - by AC coupling you can go go a lot lower (that's the point)
My question was more about whether it'd hurt the amplifiers or the scope, exceeding the ratings means if it did it's not unexpected. I'm not trying to do that.
To re-iterate the example take a 300v digital signal switching between 0v and 300v for some reason, suppose we were only interested in viewing the bits near 0v, and we put our scope to 2mv/div
We can now very clearly see the lows of the signal and inspect any ringing and whatnot, but the 300v values are MASSIVELY out of range.
The question was can the scope damage itself here.
I'd like to know more about what protects it (is is just simply "the amps saturate and hit a maximum value"?) - I imagine it is, but I saw nothing in the manual about how it handles the amps basically being over-driven.
The main benefit for using a x10 probe in this situation is dynamic range, by dividing like that we can actually fit it on our screens say, if you step down that 300v signal by 100x say you're now going from 0v to 3v, and you have good odds of being able to use the 0.5v/div scale and offsetting it and getting a decent view of that 300v
This "happens" to be safer/easier and all that but it's not why we're doing it.
Does this make sense?
From other's thought it seems like 0-300v switching signal viewed at 2mv/div is totally fine (provided you only want to see the bottom of it!) for a scope rated say 400v (as it is in my case)
Thanks