Hi runman_up,
protective earth (PE) has nothing to do with your low voltage part of the object.
Just use circuit ground for the entire low voltage part and connect PE to the metal enclosure of your device.
In any case make sure that all wires in and out do not allow to stick any foreign object (pins, needles etc.) through the openings using PG-cable glands with the proper clamping ability for your specific cable diameter.
The attached images are cable glands with IP68 safety rating and tightened properly they won't even allow spray water to enter the enclosure.
When connecting the mains cable to your device make sure the PE wire is the longest and will remain connected even when life and neutral wires are accidently pulled out of the terminals.
Do not connect any capacitor between PE and the circuit! Use an adequate mains filter instead.
Boncuk