What is double insulation and why does that make it so grounding is not necessary?
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It’s just any device where the live circuit has multiple layers of protection to the user.
It does not make grounding not necessary, it just reduces the risk such that no single point of failure will zap you. Regulators have decided that’s “good enough” to reasonably protect you from electrical shock.
Consider a hair dryer. It has an electrical heating circuit where an insulated cable goes into the handle, heats an electrically insulated heater element and gets switched on and off. Now that handle is made of insulating plastic, so let’s just say the wire wears down and touches that plastic, what happens to you? Nothing, because the plastic won’t connect the live wire to your body. It’s double insulated both from the wire touching the handle and the handle “touching” you electrically.
Now what if that handle was made of aluminum instead? Well the live wire wears down and touches the aluminum which touches you which touches the ground and you die. Single insulation.
What if you ground the metal handle then? Well the live wire touches the handle and it has a choice, go through you to ground or go through the metal handle to a ground wire. Well you’re a LOT harder to go through than a copper wire, so most of it goes to ground, and again you’re (probably, don’t try this at home) fine.
Basically, you want several things to have to go wrong for you to die, not just one, and that’s the point of double insulation.
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