i dont understand how electricity pass from the case and dont do damage to the components inside?
That's because the motherboard doesn't actually touch the case, it's isolated from the case by the standoffs that the motherboard is mounted on, so it follows that all the motherboard components are isolated from the case as well.
With the side-panel removed, scrape the paint off a small area on the edge of the chassis where it won't be seen when the side-panel is put back. There's always a way to connect to bare metal If you give it some thought.