You must do whatever is required by code relative to grounding. That is
a must for safety.
Will this eliminate error or equipment damage problems in a directly
connected RS232 link?
It may not, and in many cases won't.
Relative to data errors we hear people say a good ground is required.
Do most of them know what a good ground is? No. So what is a good ground?
Do these people have a quantitative value to define a good ground? If so why
not specify the value? If they specify a value, then can they show how this
will prevent data errors? What is ground? Is it earth ground or something else?
How do you measure ground resistance? A
DC multimeter is not satisfactory.
Often it is suggested that an 8 foot ground rod adjacent to each CNC machine
and connected with a heavy wire to the machine will provide a good ground.
What is a heavy wire? What is material and diameter of ground rod? How deep is
ground rod driven? What is resistance to ground? These are not defined.
The NEC ( National Electrical Code )
does not allow equipment to be solely grounded by a ground rod at the machine,
and isolation of the machine from the Equipment Grounding Conductor. See comment
on page 178, Article 250 of NEC2002 Handbook.
The reason is that you can not get a low impedance thru a ground rod.
NEC requires that you ground the machine thru the "Equipment Grounding Conductor" ( this
is the safety ground wire run from the main breaker box ). Adding a ground rod
at the machine may and probably does nothing for you and may actually create
problems. Search the internet to see discussions on problems of adding ground
rods at machines.
I believe that in most cases that an added ground rod at a CNC machine is next
to useless, and probably a problem. The NEC
(National Electrical Code) specifies that a maximum of 25 ohms is allowed
for a ground rod system to meet code, this most likely requires multiple rods.
This implies that a lot of installations may
have fairly high ground resistance. If you look at the NEC2002 Handbook you will
find comments like the one on page 207 -- "Also assume that the power ground
has a resistance of 10 ohms, a very low value in most circumstances ( a single
ground rod in average soil has a resistance to ground in the neighborhood of
40 ohms)". A source on the Internet discusses 5 to
10 ohms. Measurements I have made on a clay soil are in the range of 5 to
If a ground rod alone is not satisfactory to protect a machine, then why are these
required at the main breakerbox (service entrance). One reason is under normal
conditions to keep earth
ground and the electrical system neutral at close to the same potential. Second
reason is that when lightning strikes you want the lightning energy to dissipate
in the ground where there is a large mass to absorb the energy rather than the
energy going into your building and setting it on fire. Suppose the ground resistance
is 10 ohms and the lightning current is 1000 amps, then the service entrance and
everything in your building tied together by the Equipment Grounding Conductor will
rise to 10,000 volts relative to some place down in the earth. If there are no
other paths to ground in the building except the one at the service entrance, then
no lightning current flows in the building. But if you have one or more machines
to their own rods, then a current will flow in the building.
This is an over simplification, but illustrates the general concept. You should
not use my comments here to make your decisions, but use this information to
help you search and study the subject. The web site reference below with a sexy
lightning rod is a good illustration of different points of view. Who is correct I
do not know.
What does a given amount of ground resistance mean? Consider a supplemental ground rod at
a machine with a ground resistance of
10 ohms back to the nuetral point of your main breaker box. This also means a ground
rod array at the breaker box. The ground rod array at the service is the primary
ground rod and the one absolutely
required by the NEC. In this experiment said supplemental ground rod is connected only
to earth ground. Apply 120 v to said supplemental
ground rod from a hot line (not neutral or ground) in that breaker box with #12 wire
which is fused by a 20
amp breaker. We will use a 50 ft piece of #12 copper wire which has a
resistance of 0.08 ohms to connect from the breaker to said supplemental ground rod. The
current that flows is I = E/R, or I = 120/10.08 = 11.9 amperes. This will
produce a voltage on said supplemental ground rod of E = I * R = 11.9 * 10 = 119 volts
relative to neutral and safety ground at the breaker box. This won't ever blow the
20 amp breaker.
The failure to blow the breaker and the high voltage produced are the reason
that the NEC does not allow this as a safety ground means. Do not run this experiment,
but rather use an ac voltage in the range of 20 to 25 volts.
Suppose you could get the ground
resistance down to 1 ohm ( I suggest for most applications a difficult task. Would
require many rods. May be virtually impossible to do. ).
Now use the same #12 wire and do the same experiment. Then I = 120/1.08 = 111 amperes.
The voltage on said supplemental ground rod will now be E = 111 * 1.0 = 111 volts. A little
less, but not much. In either case serious damage would occur to RS232 components.
This current will blow the breaker, but that won't protect the RS232 components
because no breaker is fast enough.
Now consider the safety ground wire, NEC's name for it is "Equipment Grounding Conductor",
from the breaker box to the machine by the NEC.
This safety ground wire will be of equal size to any power wires leading to the machine.
In any practical case these will be #14 or larger wire. Note: wire diameter
increases as the wire number decreases. You would have a very small machine if the
wire was #14. But 50 feet of #14 copper wire has a resistance of 0.126 ohms. This is
1/79 the resistance of a 10 ohm ground rod, or 1/8 the resistance of a 1 ohm
ground rod. If the wire diameter is larger then the ratio is even greater.
So typically any safety ground wire is going to have a resistance
very much lower than the ground rod resistance. Thus, this supports my statement
that the ground
rod is virtually useless for noise reduction.
There is another consideration and that is inductance of the ground path. Inductance
provides opposition to the flow of transient or ac current. A large area conductor
will have a lower inductance (lower impedance), than a small diameter. This favors
the earth ground,
but the resistance at the interface of the ground rod to the earth, and
the bulk resistance of the earth are so high that
this may not matter relative to a copper wire.
One Internet reference discusses use of 0000 copper wire ( 0.46" in diameter )
and connected to the I-beam structure of the building. This is the kind of grounding that is
needed to be effective in reducing noise. Of course the I-beam structure of the
building needs all the beams to be bonded to each other. Welding would be a good means.
Further aluminum should not be used anywhere in safety ground circuits.
Briefly some problems with supplemental ground rods:
(1) A conductor ( copper, brass, steel,
carbon, etc. ) in an electrolyte produces a voltage. If there is a closed circuit and
a voltage is present, then there is a current. If a current is unidirectional (one way),
and there is metal in an electrolyte, then metal will move (electro plating). This
in time causes disintegation of something, and therefore higher resistances.
(2) If you
have a good low impedance "Equipment Grounding Conductor", like maybe 0.01 ohms, then
by comparison the 10 ohm ground rod has little effect. However, if lightning strikes
with a 1000 amp current, then a large current ( magnitude will depend upon the
relative resistances of the various supplemental ground rods and the primary ground
array ) may flow to the supplemental ground rod(s). Although the voltage drop within
the "Equipment Grounding Conductor"s maybe small (1000 amps thru .01 ohm is 10 volts),
the current may thru it's magnetic field induce damaging currents in near by elements.
If there are no supplemental ground rods, then no current flows in the "Equipment
Grounding Conductor". Everything is relative to the magnitudes of resistance and
Alternatively it is a lot easier and better to use whatever is required by code
for grounding, this is for safety purposes, and then use an optically isolated, or
equivalent, communication link for RS232 which will do a better job of noise
problem elimination. For example the Betatronics I232 system.
The isolation approach can tolerate large noise voltages (1000 volts),
whereas the direct connection of RS232 ports requires ground impedances very much
lower than required by NEC to keep peak ground voltages (less than 1 volt) low enough to
avoid data errors
or component damage.
Some internet references:
Some are funny to read. You will see some generalities without specifics. You will find
the discussions to be confusing.
....... heavy ground wires, 0000 copper, and building steel structure. This produces
....... a low impedance safety ground network.
....... discusses sexy lightning rod with radioactive material
....... no quantitative values -- duplicate of first ref but different journal
....... usefull discussion on ground rod at machine. However, this article is not clearly
....... wrtitten. The article should have been longer with very precise definitions. There is
....... inadequate info on what CNC OEMs are specifying for ground requirements. In other words
....... are the OEMs requiring only a ground rod and prohibiting connection to power system
....... safety ground wire. If OEMs are spefifying this, then this violates NEC specs.
A search using .... ground rod resistance earth .... on www.google.com produced the following
....... interesting, to an extent surface area should make a difference, one person claims 1 ohm
....... gov report, tough to get as low as 5 ohms, good discussion on how to
....... measure rod to ground resistance
My conclusion from all this is that:
It is on average tough to get down to 5 ohms resistance to earth
ground thru a single ground rod. This in my opinion is nowhere low enough to
do much for reduction of ground noise. Thus, do not use or expect any great
benefit from supplementary ground rods at CNC machines.
Thus, use heavy copper wire or bus bars for a ground system. I think you
need to look at dc resistance more in the range of 0.01 to 0.001 ohms for
this ground system. Inductance of the ground system will probably be dominate
for high frequency noise ( 10 kH to 100 kH ). Let's make a wild guess on inductance
at 30 microhenrys ( inductive reactance = 2 * Pi * f * L ), at 10 kHz this is
3.77 ohms, and at 100 kHz it is 37.7 ohms. On one of our machines I have measured
0.2 amp rms safety ground current under very light load (engraving). Thus you can
see that inductance can be very important. The reason for wanting low dc resistance
is to keep voltages low when someone shorts a hot wire to the machine and causes
hundreds of amps of short circuit ground current.
Or use isolated means, like our I232 system, to communicate RS232 to
the CNC and use the minimum
safety ground required by code.
Why is a DIRECT RS232 to RS232 connection risky?
1. Direct connection means you directly run several copper wires from
one machine RS232 connector to another (for example a computer to
CNC). For safety reasons all CNCs and computers and other devices
have their chassis or enclosure tied to a safety electrical ground.
The exception is devices that are labeled "double insulated" for example
many electric drills. Laptop computer internal common is not usually
tied to ac power ground and is isolated by a "wall" transformer
or equivalent, or using it in battery mode.
2. Assumptions for the following discussion. Your 120, 240, 480 vac
main power source is from a transformer. At your main circuit breaker
box the "transformer neutral", "earth grounding system", all "branch
circuit neutral" wires, and all "safety ground" wires are connected
In any branch circuit from this breaker box the "safety ground", "netural",
and "hot" wires are of equal diameter. This is to provide equal resistances
in the following example, but also is required by code. If the safety
ground wire is smaller the problems get even worse (old installations).
I believe that in general for many different types of earth ground
that the electrical resistance between two 8' ground rods, even at
short spacing, will be much greater than the resistance of even a
#14 safety ground wire. #14 copper wire is 0.0025 ohms per foot or
0.126 ohms for 50 feet. Thus, for the following discussion a grounding
rod path between machines would not change the results. This comment
does not imply that you should or should not use ground rods at each
machine. This must be determined from local and national codes. How
you ground equipment is not what this discussion is about. The National
Electrical Code allows a maximum ground resistance of about 25 ohms for
one ground rod. Even getting ground resistance down to 1 ohm does not
have much effect in comparision to 0.126 ohms.
3. Consider one CNC machine and one computer. We will space them 5
feet apart with a 5 foot long RS232 cable. This is to illustrate that
short RS232 cable length will not protect against the described fault.
Each is fed from the main circuit breaker box with its own branch
circuit. For example to the computer a 120 vac circuit using #12 Romex
with ground. And to the CNC a separate 3 phase cable with ground wire
( possibly #8 or #6 wires ).
Our voltage reference point will be at the "safety ground" point of the
circuit breaker box. The "neutral" and "earth ground" of the circuit
breaker box are at this same voltage.
At the CNC the "electronics common" is connected to the "enclosure"
of the CNC and thus to the CNC "safety ground" wire from the circuit
breaker box. In turn this means the RS232 connector common is connected
to the CNC "safety ground" wire. Note any current in the safety ground
wire produces a voltage drop I x R. This voltage drop determines the
voltage difference between the breaker box ground and the CNC cabinet.
At the computer its electronics common is connected to its incoming
safety ground wire and enclosure.
With no fault conditions and no erroneous ground currents the voltage
at the CNC common, computer common, and breaker box common will
be the same.
Wires of equal diameter, length, and material have equal electrical
resistance. The midpoint of two equal resistors in series has a voltage
1/2 of that applied to the series string.
If I have 120 vac at the breaker box applied to the branch circuit
to the computer then under no load at the computer I have 120 vac.
Assuming a perfect voltage source at the circuit breaker box, then
independent of the load current the voltage will remain 120 vac at
the breaker box.
If at the computer end I place a "DEAD SHORT" (zero resistance, maybe
a screw driver) between "hot" and computer "safety ground" (hot and
safety ground wires are of equal resistance), then 1/2 of the 120
vac = 60 vac rms or 85 v peak is applied to the computer case and
computer electronics common relative to the circuit breaker box common
and also the CNC common. Thus, 85 v peak is applied between the computer
common and the CNC common. This will blowout RS232 components and
maybe much more at both CNC and computer.
If you do the shorting at the CNC instead with the 240 or 480 lines
then the fault voltage will be even higher.
Note: HAAS puts 100 ohm resistors in series with each of the pins 2, 3, 4, 5,
and 7 at the RS232 25 pin connector. Pin 1 goes to machine chassis. Pins 6, 8, and
20 are connected together. 18 v bidirectional transient limiters are connected
between the 100 ohm resistors and pin 1. The transient limiters put a substantial
capacitive load on each signal line.
Note that it takes time for the circuit breaker to trip and this time
constant is long in relation to the time to destroy semiconductor
ICs. Also note that the RS232 cable length really had nothing to
do with this analysis. However, the shorter the RS232 cable the higher
the current is thru the electronic components and the greater damage.
Even if there is no short circuit to safety ground there may be leakage
currents that cause current in the ground wire and thus a voltage.
These leakage currents may be large enough to produce data errors.
The problems may be worse if the communicating RS232 equipment is
operating from different main transformers and breaker boxes.
For your data and equipment protection you need Betatronics I232 Isolators
in your RS232 cable paths.