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Old 25th December 2019
Lives for gear

Two blade plugs are not grounded. If they're polarized (one blade is larger than the other, so they only fit in the socket one way), the wider blade is the neutral and should be at ground potential, but the neutral and ground lines should not be directly tied together in general. If the converter uses an isolated transformer then tying the output neutral to the input neutral, or to ground, would be okay; neutral probably would be the right way to do that. If the converter uses an autotransformer, which I suspect it does, then its output neutral should already be tied to the input neutral directly and so you don't need to do anything more.

The three prong to two prong plus wire adapter is intended for use with wall outlets that lack a grounding prong but are grounded anyhow; the wire is attached under the screw holding the outlet cover plate in place, and connects to ground.

Grounding is primarily for safety. It provides a low-impedance path for any fault current to get back to the source, thus preventing you (a higher impedance) from forming the return path in a fault condition, and getting a shock or worse. It also in the case of a hard short would cause the breaker to trip or the fuse to blow, shutting off the current.