Human safety.What is the main purpose of the three wire 120v circuit with 3 prong grounded receptacles compared to the older 2 wire 2 prong receptacles? Human safety, lightning protection, etc.?
Thank you.
190626-9842 EDT
If there was a solid short circuit in the motor from hot to the motor frame, then for most branch circuit wiring the voltage motor frame voltage would be about 1/2 the supply voltage relative to remote earth. And local earth voltage at the motor might be about equal to remote earth.
Note: the branch circuit EGC gets its connection to earth back at its main panel or thereabouts.
.