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Here's a much simplified version of a mains reversal correction circuit. By using a single DPDT 230VAC relay, or two SPDT 230VAC relays, you can create a circuit which will ensure that no matter how you plug the plug into the mains outlet, the live wire will always end up on the same place at the output. This is handy when working with non-isolated mains, to prevent shock as with this rectifier example.
A very very important remark here is that the circuit uses the neutral and live wires to switch between, and the protective ground wire as a reference point for the detection mechanism, so if your outlet is improperly wired and the ground output is referenced to live instead of neutral, the circuit will behave exactly the opposite and will seek to place the live wire at the wrong output, making this circuit dangerous. So you need to make sure your outlet's protective ground output is wired correctly first with a live wire detector.
A remark about the circuit. Switch at the bottom simulates neutral/live reversal and pertains to only the neutral and live wires. The ground symbol at the bottom is the protective ground wire, which should be constant. The rest is just a half bridge rectifier.
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