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wireday:
What is "MMI"?
From Wiki ---
https://en.wikipedia.org/wiki/MMI
So from that list Man Machine Interface seems the most logical.
From your comment you said the power supply was 24 VDC output, but that is not your input.
If your enclosure is metal I would ground it independent of any rules. You said metal box. So I would connect that box to adjacent metal objects. Note: an EGC can have up to or possibly more than the source voltage on it under short circuit conditions. Somewhere in the past I have indicated how this can happen.
If whatever this device is totally insulated, and it is designed such that anything an operator could touch is non-conductive, then I would see no need for grounding.
In general I would not assume any power supply from 120 to 24 V could not develop an internal short. I can design one that has an extremely low probability of a shorting failure, but I can not generally assume this for any commercial product.
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