If there is an internal failure that exposes the housing of the unit to live voltage, it immediately shunts it to ground causing a surge that will trip the upstream protection device.
Without it, it is possible for the housing to be energized after such a failure and wait for some poor soul to walk over and grab what they think is a perfectly safe electrical device.
Current likes to take the path of least resistance
You’ll usually only find ground wires in machines with a conductive casing (or has a lot of conductive material present). All of these metal pieces are connected to the ground wire
Now the purpose for this is, if there was a loose wiring in the machine and it accidentally touches the conductive casing and you proceed to touch said casing, you will create a short between that wire and the ground you’re standing on and thus getting shocked.
However, if the ground was connected to that metal casing you won’t feel the shock as it’s easier for the current to flow through the ground wire than your body as your body has a much higher resistance comparatively
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