The green wire is household AC goes directly into a 10 foot copper rod that is driven into the soil to divert any direct shorting to a genuine ground. Often it is also connected to the white or neutral wire in 110 AC or the 'middle wire' in 220AC at the service box for the building. But in any event, it has no fuses on it and goes directly into the soil.
The problem is really about two questions: isolation and safety. When AC goes through a transformer, the far side is isolated. That is to say you don't complete a circuit by merely standing bare foot on the ground. For instance, a wall wart with a transformer will not provide you with a lethal shook, but one that is transformerless may actually kill you if you happen to touch a water pipe and the wire at the same time as the 120VAC has a possilbe·pathway.· And that is why you should look for the UL stamp·on wall warts or test them for isolation.
Since 12VDC is not really a safety hazard in the same way as 120v AC or DC, I would just provide an option for the green 3rd wire of the 120VAC to be connected to a cold water pipe [usually that goes eventually into the ground]. In that way, any 120VAC applicance that might need the proper safety of a ground would have good access. I might also tie the external chassic of the converter into the same ground system in case of a failure where shorting is occuring on the 120VAC side.
But I really wouldn't worry at all about the 12volt DC to ground as a safety feature.
Though in automotive, the negative is generally the equivalent of ground and all questions of polarity should follow the same. But I would skip tying it directly into the chassis ground in common with the AC if it would be on the 'other' side of the transformer that is creating isolation. In sum, keep the isolation and use it for safety. There is always the remote. possiblity of lightning or another AC source entering the 120VAC side and eventually reaching the DC side via a shared, but not properly grounded system.
Also, it is always easier to have a non-conducting, fully insulated chassis rather than a steel box for safety.
I hope that makes sense to you.
"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········
···················· Tropically,····· G. Herzog [·黃鶴 ]·in TaiwanPost Edited (Kramer) : 9/24/2007 6:48:13 PM GMT