The susceptibility of a CNC machine to different sources of noise and stray currents depends on its design, but grounding, bonding, and shielding play a vital role
As the use of CNC machines becomes more prevalent, minimizing production downtime, product loss, and expensive repair bills becomes increasingly important. Good practices for installation, powering and grounding, and maintenance procedures are all required to prevent malfunction, degradation, and damage to electronics.
Today's electronically controlled CNC machine requires a common signal reference ground for its logical circuits to operate reliably. Upsets related to grounding and ground reference are usually attributed to noise or stray currents that find their way into logic circuits. A common wiring-related source is the inductive coupling of noise currents into wires of control and power circuits run in the same raceway or placed in close proximity to each other inside a CNC machine. Additional sources include contactors, relays, solenoids, and motor drives that are internal to the CNC machine. (For more sources, see sidebar “Noise Sources and Coupling Modes” below.)
The susceptibility of a CNC machine to different sources of noise and stray currents will depend on its installation as well as how the manufacturer designed the machine. Grounding, bonding, and shielding also play a role, as do the use of certain types of communication cables that are more susceptible to high-frequency noise or stray ground currents.
Problem areas Most machine shops are supplied by either a 3-phase, 4-wire grounded service (at either 208Y/120V or 480Y/277V) or a 3-phase ungrounded service (at 480V delta or 240V delta). Both services require a grounding electrode conductor to reference the electrical power system to the building grounding electrode system, which consists of all available earth electrodes (building steel, metal water pipes, buried ground rings, and ground rods).
The intent of the grounding electrode conductor and grounding electrode system is three-fold:
To provide a low-impedance path to earth for lightning surge or electrical fault currents.
To provide a low-impedance fault return path back to the source in order to trip the breaker.
To reference the building electrical system to its surroundings. In other words, minimize voltage differences between exposed metal parts of the power system, including connected equipment and the surrounding parts of the building.
Typically, a CNC machine's signal reference is a common wire of its computer logic power supply connected to the machine's ground plate. The ground plate, in turn, is bonded to the machine's enclosure, equipment grounding conductor, or conduit — and, subsequently, to the building grounding electrode system. So, the equipment ground establishes the local signal reference both within the CNC machine and for any remotely connected devices (click here to see Fig. 1.).
CNC machines contained in a single cabinet with good bonding, grounding, and shielding should be relatively immune to noise. However, those machines with remote controllers or data links may be quite susceptible to noise or stray currents. That's because the other area of the factory will also have a signal reference ground. These different reference ground points may increase a CNC machine's sensitivity to power disturbances (click here to see Fig. 2.).
In these machines, you should consider two forms of ground-related noise:
The noise appearing on the input power conductors relative to the equipment ground (typically the enclosure or chassis ground), and
The noise or difference in electrical potential that appears between the grounds of interconnected equipment.
When trying to avoid electrical noise, distancing from the source of noise is usually a great help, but watch out for misperceptions about ground noise problems (see sidebar “Myths About Ground Loops,” below.)
Potential pitfalls Some CNC machine manufacturers recommend, or even require, the addition of a supplemental ground rod at the CNC machine. (See sidebar “Manufacturers Speak Out About Machine Grounding and Ground Rods,” below.) In most cases, this is an 8-ft copper rod driven next to the machine, often through the concrete floor, and bonded to the machine's ground plate. The NEC does permit such an installation, as long as the ground rod is bonded to the building electrode grounding system (click here to see Fig. 3.). However, this type of installation may still invite stray ground currents.
Different manufacturers state different benefits from the supplemental rod. However, field experience at sites with the supplemental ground rods has shown that the rod may actually increase the risk of CNC electronics damage. These sites were found to be prone to damage of internal electronics after thunderstorms or utility power system faults.
Basically, the supplemental ground invites stray currents into the machine by effectively connecting the electronic controller between the service grounding electrode system and the supplemental ground rod. A rise in earth ground potential near an electrode causes a large amount of current to flow on the grounding conductor between the electrodes and pass through the CNC machine. Such large currents are possible when lightning strikes nearby or when utility power line faults occur outside the facility. Current-induced electromagnetic stress is the suspected failure mode.
The NEC, being strictly an electrical fire safety code, does not consider this condition. Drawing a comparison with other electronic equipment (computers, electronic motor drives, and PLCs), none subscribe to such a local grounding practice. The analogy would be to drive a separate ground rod in the office, and connect it to the logic ground inside your personal computer.
IEEE 1100-2005 (The Emerald Book), which focuses on powering and grounding electronic equipment, does not recommend such a practice. Instead, it recommends that a single-point ground from individual electronic cabinets be individually bonded to a local ground grid.
Beware of isolated ground rod Code violation When a CNC machine manufacturer recommends or absolutely requires an additional earth ground rod located at the CNC machine, the end-user is ultimately responsible for the installation to meet NEC and local code requirements.
Although permitted by the NEC, the additional rod is intended as a supplemental grounding electrode, which means you must bond it to the rest of the building grounding system. Too often, this ground rod, which incorrectly might be thought of as “isolated and dedicated” to the CNC machine and from the otherwise “noisy” building ground, is not bonded to the building electrical system ground (Fig. 3). This installation creates an electric shock hazard and is a violation of the NEC.
In effect, this effort to further distance the CNC machine from the “noisy” building ground using an isolated ground rod creates new hazards to both people and machine during lightning storms or power system ground faults. When these occur, dangerous potential differences can exist between the incorrectly isolated ground rod and the rest of the building's grounding systems.
Temporary solutions Because existing codes or standards don't provide specific grounding practices for CNC machines, an initiative is currently under way to bring end-users, CNC machine manufacturers, consultants, and utility personnel together to work out and publish a best powering and grounding practice for these very important tools. Until then, what should you do to ground CNC equipment?
For safety, you must strictly follow the NEC's provisions. This starts with proper grounding of the power source (service entrance or separately derived system). Other requirements include bonding to the grounding electrode system, which connects all available electrodes (building steel, metal water pipes, ground rings, ground rods, etc.) and using properly sized and installed equipment grounding conductors routed with the associated power conductors. Also, you should avoid any connections of the grounded circuit conductor (typically the neutral) to ground, except at the power source.
Because the CNC machine manufacturer is most knowledgeable of the CNC's circuit design and its susceptibilities, it's in the best position to recommend additional grounding, shielding, or installation practices. You should follow these recommendations, as long as they do not violate local or national electrical codes. Also, make sure they aren't based on myths about ground loops or misconceptions about the need for an isolated ground.
Keep in mind that not all operational problems are the result of inadequate grounding, and that proper grounding can't solve all problems. Good system design, which includes facility lightning protection (where appropriate), surge protection, the use of dedicated circuits for the CNC machine, and the segregation of large cyclical or other “disturbing” loads on power feeders separate from the CNC machine, is important to avoid operational problems. For more severe power supply variations, such as voltage distortion, sags, swells, and interruptions, you may need some form of power conditioning.
Key is senior technical leader, EPRI, Knoxville, Tenn.
Sidebar: Noise Sources and Coupling Modes
Much of the focus of special grounding recommendations for electronic equipment is based on reducing electrical noise or any unwanted signal that may affect logic circuits.
Sources of electrical noise may be nearby transmitters, such as electric arc welders or chattering relay contacts. Also, lightning or switching surges may capacitively couple noise voltages into sensitive control circuits.
It should be noted that before evaluating solutions for noise-related nuisance tripping problems with CNC machines, the input power-related possibilities should be ruled out. This can usually be done by incorporating a power line monitor in conjunction with an equipment upset log to ensure that the nuisance trips do not correlate to power variations, such as voltage sags or capacitor switching transients.
Sidebar: Myths About Ground Loops
Sometimes there is a concerted effort to identify and eliminate “ground loops” in the name of reducing noise. However, this effort may be misguided because noise currents need a completed low-impedance path.
In any practical building electrical distribution system, there are numerous ground loops that can form complete paths for noise currents. Consider the number of loops formed by every connection of raceway and enclosure to the building structure or by the equipment ground conductor connections with parallel raceways. As you can see, we live with ground loops in nearly all building wiring systems.
But are they always bad? Certainly, when bonding creates large ground loops, this may be a problem for analog signals where induced noise currents degrade the signal. This happens because of exposure to unbalanced fields of other nearby conductors over longer distances.
In contrast, the digital-electronic system's voltage potential differences occur in the absence of bonding and can be a significant problem. For many high-speed digital systems, multipoint bonding and grounding connections are needed to control high-frequency resonant effects that occur in individual data cables between interconnected equipment. This is the reason why we use signal reference grids in computer rooms.
So bonding may create “small” ground loops, which are viewed as good, and are a necessary part of a well-bonded mesh of conductors required to achieve equal ground potentials among interconnected electronic system components.
Generally speaking, small ground loops, with loop areas less than a few square feet, are beneficial and desirable, while longer loops, with large loop areas (particularly those that involve signal cables to complete the loop), are undesirable and should be avoided.
Sidebar: Manufacturers Speak Out About Machine Grounding and Ground Rods
In 1995, Allen G. Morinec and Steven A. Watts of Centerior Energy Corp., Cleveland, conducted a survey of CNC machine tool manufacturers to determine the recommendations for proper grounding of their equipment. The results indicate that there are conflicting grounding recommendations and requirements.
There is a widespread belief by many CNC machine tool manufacturers that the building's equipment grounding system is “noisy” because of other equipment in the facility, such as welders, wire EDM machines, and motor drives.
To combat the problem of electrical noise on the equipment grounding system, 10 of the 15 CNC machine tool manufacturers surveyed require or recommend a ground rod be driven into the earth at each CNC machine and connected to either the CNC machine's frame or ground plate. Three of the 10 manufacturers who recommended a ground rod stated that the equipment warranty would be voided if a ground rod were not installed. The most commonly stated purpose of the ground rod was the carrying away or elimination of electrical noise from the CNC machine's signal reference.
Apparently, there is a general belief that the ground rod will provide a lower impedance path for noise than the equipment grounding system. The IEEE Emerald Book disputes this belief. The recommended impedance of this ground rod varies by manufacturer and ranges from less than 5 ohms to less than 100 ohms.
Of those manufacturers that recommend installing a ground rod to solve unexplained problems, about half report that unexplained operating problems go away more often than not after the ground rod is installed. In addition, at least one manufacturer feels that the ground rod provides some lightning protection. On the other hand, other CNC manufacturers specifically do not recommend a ground rod because of possible lightning damage.