A small open-frame line powered power supply (Cincom CFM05S033) doesn't have a ground connection. It will power a tiny device which only has +3.3V/0V DC connections. This will be used in a 6x6x4 plastic enclosure so there is no chassis ground. An electrician will be wiring it in.
Is this inherently unsafe? Do I need to find another supply with a ground?
The AC-to-DC switching regulator power supply you're using has an isolated output, meaning it's constructed so there's no inherent connection between either the positive or negative end of the 3.3VDC supply and anything else.
Actually, the data sheet suggests there's more than 4KV isolation between the line voltage input side and the output. I'd take that with a grain of salt, but it does tell you there's no inherent danger involved in either leaving the supply output floating, or grounding it.
If you need a grounded supply (think twice before doing this unless you have to -- most times, in industrial applications grounding the supply causes more difficulty than it solves), you can just connect the earth ground wire to the negative terminal of the supply (or the positive terminal, for that matter).
Please clarify "external PSU". You imply...external to the product, like a wall wart. My design will have the PSU internal to the product. The entire product and PSU is in the IP65 housing, there are no other connections or outputs. So I think I am still looking at a Class I device, correct?
There are no "outputs". There will be an Earth Ground conductor coming in, should I just connect it to the AC Neutral terminal?
DO NOT connect the earth ground (safety ground, green or green/yellow wire) to the neutral in your equipment. The safety ground should be connected to any exposed metal (or otherwise conductive) parts of your equipment.
If your device has no exposed metal parts, do not connect the green (or green/yellow) safety ground to anything.
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
Why not buy a wall wart with the required output? It will be made to be ungrounded and all the energized parts will be enclosed by the maker so you won't need to worry. By the way, please don't pay attention to phil allison. He is probably in need of professional help but instead spends his time trying to rile up anybody he can online. Just like when CB radios were a big craze and all sorts of shy whackos would get their hands on a radio and broadcast all sorts of stuff pretending to be something or someone they were not. ERS