Basically, Double insulated is a little misleading, but in UL/CSA parlance, it means that the internal wiring is such that it would take TWO faults to occur, either of which would stop the product from working, before anything could become electrically dangerous to the user. Usage conditions are also taken into account. Among other things it also means that there cannot be any exposed metal on the product. ALL exposed metal must be grounded, which become a non-essential in double insulated products.
It also means it doesn't need a third "prong", and the blades do not have to be polarized on the two-blade plug, and it does not need a 3-hole outlet.
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