下面就是在Enginova中的一篇文章.Enginova里面还有不少有趣的东西,应该都是电气工程师写的心得和意见,没有详细看.
在民用工程里的电气部分,还是不要问太多为什么了,很容易搞昏头的.知道是什么,知道怎样去做就已经不错了.
http://enginova.com/iso_ground.htm
Isolated Grounding: A Waste of Good Copper?
By Doug Criner
A Chicago-area public high school was recently rewired to add 120-V
receptacles for personal computers. The designer specified isolated
grounding�with those orange-colored receptacles that seem so
reassuring.
Each receptacle was served by four conductors: an AWG No. 12 black
(hot), a No. 10 white (neutral, or "grounded conductor"), a No. 12
green (redundant raceway ground), and a No. 12 striped green (isolated
equipment ground)�all installed within a grounded, steel conduit.
Let's consider what on Earth might be going on here.
Four Conductors
The black hot wire is about the only thing that makes absolute sense.
The oversized neutral is evidently based on the possibility that
harmonics, generated by nonlinear loads, such as computers, can
overload the neutral conductor of a 3-phase, 4-wire, wye-connected
circuit. However, for a single-phase circuit, even a 120/240-V circuit
with a shared neutral, it is virtually impossible for the neutral
current to exceed the phase current. Therefore, there is no reason for
over-sizing the neutral.
The raceway ground conductor is not a bad idea, since it provides
redundancy to the ground path provided by the steel conduit. Such a
redundant ground conductor, while not required by the National
Electrical Code, is often specified for peace of mind in case the
conduit develops loose or corroded connections. (In an isolated-ground
system, a redundant raceway ground conductor is bonded to the
receptacle box, but it is not connected to the ground terminal on the
receptacle.)
The isolated ground conductor is connected to the ground connector on
the receptacle and can then be run back to the service ground bus,
unconnected to the ground bus of intermediate panels. What is the
purpose of such an isolated ground system? Interestingly, you may get
several different answers, depending upon whom you ask.
Purpose of Iso-Ground?
One explanation is that some manufacturers of sensitive electronic
equipment "require" that their equipment be supplied by an isolated
ground system. This explanation is a little weak�first, because such a
system is certainly not required by manufacturers of personal
computers and, second, because equipment manufacturers are not
necessarily the sole authority on power distribution.
(An example comes to mind: some equipment manufacturers once
"required" that their equipment be grounded to a separate building
ground rod, isolated from the building's regular electrical ground�an
unsafe practice that is prohibited by the NEC. In my opinion,
equipment manufacturers should themselves be "required" to design
their equipment to be compatible with any power distribution system
design that meets the NEC.)
Another explanation is that an isolated ground system affords greater
personnel safety against shock�which is why the orange receptacles are
seen in hospital rooms, right? However, the NEC does not require an
isolated ground system in hospital rooms. (The NEC does require a
redundant ground conductor in the hospital's raceway but not an
isolated ground. Interestingly, the NEC allows an ungrounded system in
hospitals, which is the Gold Standard of shock protection, at least
for the first ground fault.)
Another theory is that in the event of a ground fault, the fault
current passing through the electrical raceway can raise the voltage
of the raceway, and any connected equipment grounds, enough above
ground potential to be a shock hazard. Studies at the Georgia
Institute of Technology do not support this concern.
Some people think that an isolated ground system somehow alleviates
power quality problems caused by nonlinear power supplies. Yes,
computers do introduce harmonics because their power supplies chop up
the sinusoidal current. Perhaps these harmonics might, in turn, cause
problems to other electronic equipment. But these harmonics are
propagated through the hot and neutral conductors, and no jiggering of
the branch grounding conductors will prevent this.
Another justification cited for isolated grounds has to do with the
effects of ground loops, those pesky things that can play havoc with
low-voltage data signals. But we're dealing here with 120-V power, not
low-voltage data, and if things are installed properly, there should
be no current in the grounding path, except during a ground fault�when
ground loops will be the least of our worries.
Noise Suppression
The final argument, and the only one sanctioned by the NEC, is that an
isolated ground system may reduce electromagnetic noise interference
from appearing on the branch circuit. (In fact, the NEC permits the
use of an isolated ground system only where required to reduce
electromagnetic noise interference; thus, anybody who insists there is
any other reason for installing isolated grounding is caught in a
Catch 22�and prohibited by the code from installing an isolated
ground.)
Large currents in nearby circuits can induce small, unwanted 60-Hz
voltages in the grounded raceway of another circuit and the grounded
cases of any plug-and-cord equipment. Possibly, some poorly isolated
equipment might be sensitive to this�certainly, old-fashioned AC-DC
tube-type radios, whose filaments were connected across the AC line,
were prone to an annoying 60-Hz hum.
But for most situations, this theoretical concern should be
negligible, unless you happen to be located next to an arc welder. To
the extent that a separate, isolated ground conductor will be shielded
by its steel raceway, it logically follows that current-carrying
conductors in another, adjacent circuit would also be shielded.
Further, induced voltages between separate circuits are unlikely to
occur when the conductors of each circuit are tightly bundled, which
will cause induced voltages to be self canceling. In any case, if a
redundant ground conductor is pulled inside the steel conduit, and
bonded to the receptacle and box, it seems likely that any 60-Hz hum
would be suppressed.
At What Cost?
So what is the cost of the over design for that high school rewiring
project? Well, we don't know the grand total, but here are the
elements: (1) the cost differential between the AWG 10 and a AWG 12
neutral, (2) the cost of the No. 12 isolated ground conductor, (3) the
extra cost for the orange receptacles, (4) a larger raceway to
accommodate the extra conductor, and (5) the extra labor for pulling
and terminating.
The question arises: why have isolated ground systems become so
popular and possibly overused? Is it a plot instigated by electricians
and wire manufacturers? No, I think that the name "isolated ground,"
even though a misnomer, sounds very appealing. A more accurate name
would be "insulated ground," since the branch ground conductor is no
more isolated than the phase and ground conductors are isolated. And
those orange receptacles sure are impressive.
(c) 2001 Doug Criner
