Electrical potential and grounding in a circuity, why it works?

Question

I thought I understood the ground concept but now I'm a lot confused.

From: Pg.67 Practical Electronics for Inventors - Scherz Paul

As it's explained in the book, the first circuity is the one that's dangerous if someone would put a hand on the chassis while this person is grounded(or touching something that is). But the second one is safe. But grounding this circuity before the load wouldn't make the current go strait to the earth ground?

In the way I see, the second circuity is the same as the first when someone put a hand on the chassis, and as such, whatever the load is supposed to do, it won't work because there is no current passing through there.

Did I get something wrong???

Answer 1

The issue is certainly NOT that the chassis ground is at -100V, it's absurd. The chassis ground by definition is at equal potential for power supply and load, you can treat it like one big wire that's large enough to take all the current you could throw into it. If you put a voltmeter between a car's chassis & earth, I promise it will never read -12V.

The reason the chassis needs to be grounded is 1) leakage to ground (R-sub-leak) can form a capacitor and you don't want to be the thing that lets its current flow. The second is that a fault to the chassis can result in the chassis being live at 100V, which is a problem if you touch it just like a bare wire (electricians call this getting bit, depending on how it goes through the body you can call it the end of you). This is prevented in modern electrical systems via a dedicated and properly wired ground wire, which is directly connected to true earth. It would be unusual to have a high voltage setup like shown using just a chassis ground because a fault could kill you.