Does it have to use a microprocessor ? Using a micro for this is trivial.

When the 3-phase power operates properly and is connected in the right order
then you can drive three 'analog memory' circuits mounted in series with
switching from the phases. This only has output if all phases are on and in the
right sequence. This needs little more than a 4066 analog switch and a few
resistors and capacitors.

Peter


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