"Gary Coffman" wrote in message
...
On Sun, 28 Dec 2003 01:01:56 -0800, "\"PrecisionMachinisT\""
wrote:
If you like you could run a dedicated ground conductor alongside or
inside
the existing conduit--bare or insulated copper should be fine..........It
then becomes your ground path--at this juncture, the conduit is simply
grounded, where this is a requirement in the case of metallic conduit
anyways.

Always a good plan.

As to the fifteen foot ceiling drop, not sure it would be to code, but if
a
plug were installed in the ceiling it would technically be a disconnect
at
that point. This places the machinery into the "portable "
classification........

Wouldn't be Code. The disconnect has to be in sight and *in reach* of
the operator. Unless you're very tall, or have extremely long arms,
mounting
the disconnect 15 feet in the air wouldn't comply with the requirements.

The current trend is to avoid multiple ground paths.......a "star"
grounding
system is preferred, whereas all grounds lead to a central point--that
being
the service entrance grounding stake, via the entrance panel grounding
bussbar.

Yes. Star connection avoids ground loop problems, which can generate
EMI problems.

Gary

Two different issues here. SAFETY grounding (what the NEC code requires) is
all about ground loops, lots and lots of ground loops, the more the merrier.
For safety purposes you want to make a ground connection every chance you
get. So that ground wire in the pipe is going to be connected at both ends
and possibly at several different places. The reasons for all these
interconnections is simple. When a big motor running on 3 phase 240 0r 480
volt power suddenly shorts to ground some really interesting **** happens.
Look at the circuit breaker feeding that motor. Notice it has an
Interrupting Current rating of somewhere between 10,000 and 100,000 AMPS!
That's the amount of current that can pass through the breaker in short
circuit, and the breaker can open it up without being destroyed by the arc.
Those big numbers are NOT speculative, they are easily calculated and
demonstrated. You don't want to be near the results during a real
demonstration! So what happens to all those amps trying to get back to the
source of the power? They flow EVERYWHERE in your grounding system. If it
was done right and there are no loose connections, high resistance paths
etc, then all that happens is that a fuse blows or a breaker trips. Unless
of course the electrical engineer that designed your CNC picked up a couple
of different grounds inside the machine while looking for the mythical
'clean ground' If you're feeling real lucky maybe you just gouge a part.
If it really ain't your day you may lose your CNC in cloud of smoke.

Now inside you CNC is a whole different matter. The ground is used as a
zero reference for 24 vdc, 12vdc, 5 vdc, 3.3 vdc etc. For current to flow
though you need a complete circuit, a circle or loop. No current can flow,
at any voltage, without a complete circuit. By connecting every thing in
the CNC, including the external safety ground, to just one ground point (it
LOOKS like a star radiating outwards) you ensure that no current can flow
from the safety ground THROUGH your CNC. What causes all the problems is
all the help we get from the various manufacturers of the parts used to
build a machine. Some provide 'convenient' ground points on devices, that
just happen to connect to various ground wires in cables and on circuit
boards, through different paths. If you go connecting all these without
actually checking how they are wired you will inadvertently get ground
loops. The ground loops allow current to flow from various sources, say
from the enormous surge of that motor which shorted to ground, and then bad
things happen. Sometimes people drive ground rods in hopes of getting the
mythical 'clean ground'. Sometimes it seems to work, because providing a
new path changes the current flowing in other paths. Usually though the
relief is temporary. Something big shorts to ground elsewhere in the plant
and thousands of amps of current go looking for a new path home. That's
when your local part changer earns his big bucks.

Now if we could just get the damn electronics engineers to understand the
above we could stop fighting this bull****.

Gary H. Lucas