So, if I understood correctly, through a power supply it is not possible that the AC current arrives inside the case?
There are two ways that could possibly happen.
First would be some kind of catastrophic failure of the power supply which resulted in AC short-circuiting to the case. This is highly unlikely but it is theoretically possible. The best protection against this kind of failure is good grounding. It is also worth pointing out that this kind of failure is a risk with any electronic device so it's probably not something that would keep you away from modular.
Second is what is generally meant by the term "leakage current". Basically this refers to very small amounts of electricity passing through insulation, and it is a very tiny % of the working voltage of the circuit. This is what I was referring to in my earlier post. Because Modular signals are very low voltage whatever tiny % of leakage may occur is probably not worth worrying about.
There is a good centralized and certified grounding system in the building where I live
That is good to hear. But when I wrote that "Good grounding" is important I don't mean just the building, I meant how the rack is put together. Ideally you'd have a metal rack, the rack itself should be wired to ground, and you'd make sure to tighten all the screws holding your modules in. Sometimes I see people online that have just rested modules inside a rack without any screws, or maybe they're just using one screw per module. That's not good practice, the metal front panel of the modules should have firm contact with the grounded rack. That is critical for safety, if there were any stray voltages present you want them going to ground through the case, not being present where you might touch a knob or grab a patch cable. If you are building a wooden case with metal rails to make your rack then run a ground wire to each rail.
Like JimHowell1970 wrote, most Modular racks use the same kind of power supplies as a computer, sometimes this is a literal laptop power supply "brick" wired externally, other times it is a similar power supply mounted inside the rack itself. But either way the power supply has the same basic function: it takes in AC mains and converts it to the lower voltages (+/-12V, 5V) used by the modules.