The common convention seems to be positive on the "hot" (brass screw),
and negative on "netural". This makes sense, as if either DC leg is
grounded it's usually the negative one... except in telco facilities.
DC CF bulbs with a standard Edison base are set up the same way,
expecting +12V at the tip.
When wiring some of these up recently, I considered tying the neutral
and ground contacts together on both ends, to reduce voltage drop a bit,
but wasn't sure if the NEC allowed it. Assuming battery-negative is
grounded, do the "groundED" vs. "groundING" conductor separation
requirements still apply to DC circuits? I've yet to encounter a DC
appliance that needed a dedicated ground wire.
You're probably not watching this thread any more. The idea of safety
ground is mainly to keep touchable parts at ground potential. The
ground wire should only ever carry current in a fault situation, e.g.
the hot insulation fails, connecting the hot to the chassis. Ground
wire carries the current back to the panel and the breaker trips.
With low voltage stuff the same principles apply but with somewhat
less concern I suppose.
NEC would never allow connecting ground and neutral (or negative) at
both ends of anything. They're supposed to come together at one point
only, and as I said ground is never supposed to carry current except
when something has gone wrong.
Typically nothing "needs" a dedicated ground wire to function, not
even a ground fault detector(!)
I know what you're saying though... you bring the safety ground to the
outlet box, connect it dutifully to the box and the outlet, but
nothing that you ever plug in there has a chassis ground connection
anyway. I figure oh well, at least the box is grounded. That and a
bloody chicken should keep the evil spirits away.
-=s