On Tue, 15 Apr 2008 16:53:22 -0700, Jim Lux wrote:
>I don't think you need a neutral (ground*ed* conductor).. two hots and
>ground will do. (three phase motors have three hots and a ground, for
>instance)
That's right, and I noted that in an earlier post -- a 240V circuit in the
US does NOT require a neutral. But that's not the application -- the
original poster was talking about 120V, which in North America is phase,
neutral, and equipment ground. And if the application IS two hots and a
ground, that's still three wires.
>And then, is a grounding conductor needed even for an isolated circuit?
> There's all sorts of weird rules for isolated circuits, and in this
>case, the green wire ground is created brand new at the utilization
>equipment, so the safety issue is addressed (i.e. the case of the
>equipment is at "bare feet on floor" potential).
I don't pretend to know every exception in the code. BUT -- pro audio and
video systems make extensive use of what NEC calls isolated ground wiring.
For a 120V circuit, each outlet REQUIRES a phase (hot), a neutral, and a
green wire that is not raceway (conduit). AND there must be an additional
ground conductor that grounds any outlet boxes and raceway. If all raceway
is conductive, that raceway can serve as its own ground conductor, but any
lengths of non-conductive raceway (PVC, etc.) must have that fourth
conductor to bond between the non-conductive segments.
Building codes (NEC, City of LA, Chicago, etc.) ALL require an equipment
ground that is sufficiently robust to blow the breaker, AND THAT FOLLOWS
THE SAME ROUTE as the phase conductor. If the phase conductors are in
raceway, they must generally be in the same raceway. EARTH has NOTHING to
do with blowing that breaker. Continuity of the equipment ground back to
the panel is what blows the breaker, as well as minimizing the inductance
of the fault current.
There IS an exception within NEC that permits so-called "balanced power"
for 120V wiring. It places VERY stringent limitations on such a system and
how it can be installed, including a requirement that each and every
outlet have a GFCI.
It's also important to point out major regional differences in practice,
all of which are covered by NEC. In many large cities (NY, Chicago,
others) all electrical wiring is required to be in steel conduit, or use
an armored cable like BX. In other parts of the country, most homes are
wired with plastic-jacketed triplets of hot, neutral, and ground, with no
conduit or other form of protection. Steel conduit has the major benefit
of shielding wiring from magnetic fields and RFI.
Several years ago, I wrote a tutorial that addresses Power and Grounding
for pro audio and video systems. Nearly all of the concepts are
regulations are directly applicable to home systems and ham stations.
http://audiosystemsgroup.com/publish
I regularly teach this material in three hours classes at industry trade
shows. The next one on my schedule is at Infocomm (Lost Wages, June 20).
73,
Jim Brown K9YC
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