I appreciate the complete answer but I think my question was missing some information.
Lets forget that I live in Brazil, why is it safe to tie the neutral to the chassi but not the live if we are talking about Ac current? wouldnt the neutral have full 110Vac voltage potential for each half cycle of the wave? So why is it considered important to have a polarized plug or discern between live and neutral when tying one of those to the chassi?
In the bad old days, it was common to tie the neutral to the chassis but there was also a time before polarized receptacles. I suspect they became common in the '50s. There are a lot of homes in the US that predate polarized receptacles.
So, given that one wire is tied to the chassis, you need to deal with the situation that it is the hot wire. That works safely as long as the knobs are on the controls and the back cover is in place. However, when you work on the device, you probably open the cover. I got knocked on my butt touching the shaft of a radio control back in the mid '50s. The plug wasn't polarized (the receptacle may have been) and the chassis got 'hot'. That's why they talk about 'hot chassis' appliances.
These days, the receptacles are likely grounding (3 pin) and older houses probably have at least polarized receptacles. Here's what you need to know about 2 wire cord. One of the conductors will have a couple of 'ridges' that identify it. The 'identified conductor' is the neutral - always. For a table lamp, this identified conductor would connect to the lamp holder shell and the wide blade on the plug.
Now that we use mostly low voltage electronics, there is almost always a low voltage DC supply involved - maybe something like a wall wart. No issues with 'hot chassis' with this kind of equipment.
Remember, the neutral is tied to earth ground at the pole and very probably at the home service entrance. The neutral should always be very close to zero volts. Only the phase conductor is 'hot' to ground.