Watched the video what...5 times? I still don't get the concept so here is a scenario with pictures.
Couple of facts before I lay this out:
I have a pic16f690 microchip, and I am simply probing the clock out frequency on pin 3
I can use a 9v battery or a walwart power supply (no 3rd ground pin) on the breadboard to power it
I live in the USA where the neutral and ground pins of my power outlets are bonded together (Also verified with ohm meter)
Here's the rub...If I use a 9v battery on the breadboard, then this circuit becomes completely isolated UNTIL I connect the ground clip of the scope probe to any ground rail of the breadboard - WHY? Because the scope has a 3rd earth pin for the 120v 60hz wall power, WHICH IS BONDED TO THE NEUTRAL.
What I have discovered is that the second I use the ground probe from the scope on ANYTHING it will always take an isolated circuit and make it...well not isolated.
It seems to me no different than in the video watching Dave connect up a 5v usb port to power a board and thus de-isolating it.
I need some help in understand what's going on and why I haven't blown anything up yet - as per the video if I hook up the breadboard to earth ground I should be shorting things out. How is this not happening?
I have included 3 pictures to help...In all 3 pictures I am using a 2 pin wall power transformer - again the power supply itself IS isolated (checked) but the minute I plug it in, the neutral will automatically go to ground anyway.
Picture 1 - probe ground to nothing
Picture 2 - probe ground to scope ground
Picture 3 - probe ground to breadboard ground