Unusual use of M8/M9 to control a compressor

[Doddy]

Couple of caveats -

You don't 'need' to isolate the switch circuit. But doing so does no harm.

You'll be back - I reckon within a couple of weeks - to complain that, whilst under UCCNC control someone switched the compressor on accidentally, and after UCCNC has quit that the compressor has been left operating. That's a more complicated problem!