Got a call today from a machine shop owner. He tells me that his employees are occasionally getting a shock off one of their CNC milling machines. I originally installed the power drops in this shop 10 or so years ago.120/240v 3 phase delta supplies a xfmr that steps up to 415v (Australian equipment). A couple months ago a service tech came in to address Fanuc control issues. He explained to the owner that "noise" was being generated through the ground and disconnected it totally from the machine.Anybody here deal with machine tool wiring and heard of this being an acceptable practice. FWIW this machine does have a driven ground rod that was installed originally per mfgr's specs.
disconnecting egc
- Thread starter nizak
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- Location
- Placerville, CA, USA
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- Retired PV System Designer
The driven ground rod will not do much if anything to reduce "noise" in the system and will probably have a high enough resistance that it will not provide operator safety in the event of a wiring fault inside the machine.
You must have a effective EGC which connects to the building GES and the bond to the utility neutral or the secondary ground point of your isolation transformer if there is one.
You say that you are getting your input as three phase delta. Is that corner grounded or actually coming from a utility three phase wye?
If the output of the transformer is also delta, what have you done to provide a ground reference or ground detection?
PS: Disconnecting the EGC in response to "noise" issues seems to be a common practice among machine techs, but it is misguided and totally unacceptable.
If there actually is a noise problem, that is NOT the way to fix it. Unless the existing grounding and bonding setup is not NEC compliant, it is not likely to improve the noise situation at all. If the existing ground and bond setup is incorrect, it is not the right thing to do to fix it.
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mopowr steve
Senior Member
- Location
- NW Ohio
- Occupation
- Electrical contractor
I recently had a Mike Holt seminar and he touched on this situation that it's been found that driving a ground rod at a machine does create this phenominum. And that make sure you have an equipment ground attached to the equipment. You may actually be creating a ground loop or voltage gradients thru earth may be imposed up the rod to your equipment. He says that himself and others have been in communication with manufacturers about this. And are trying to get a change to there requirement of having rods drove at equipment. I guess I would call the manufacturer and see if they have made any recommendations or changes to there requirements about your equipment or at least report your findings and concerns.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Odds are that the building has common mode current (neutral) flowing through the grounding from neutrals bonded in subpanels, not too uncommon to find, if not then you might find a neutral faulted to ground someplace, or each CNC machine has its own transformer that has its secondary grounded which in most cases the NEC will require, this can also cause this problem, if the latter see the isolator fix below.
Many of these CNC machines use 485 or 232 networking and these networks reference the EGC of the machine and computer, with common mode current on the grounding the reference conductor in the network will also share this current and cause data problems, the best way to fix this would be to make sure every subpanel has the EGC's and neutral separated as long as they have a EGC path back to the service neutral, if it turns out you have a neutral to ground fault then using an amp meter on the EGCs should find it so it can be fixed, with the grounded SDS transformers the below option is the only method to fix it.
One way to eliminate this kind of problem with these types of networks is to install optical isolators in the network at each connection point to remove the networks reference to the system EGC, this will also allow for higher network speeds.
But no matter what the EGC to the machine has to be put back before someone gets hurt.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
140402-2345 EDT
I agree with the above comments for the most part.
It is absolutely essential that a proper EGC be provided. It will also help if it is greatly oversized. A supplemental ground rod at the CNC might marginally reduce noise problems.
If there is no connection to anything external to the CNC machine except input power and the necessary EGC, then the machine is essentially a shielded box, and that could be raised to 1,000,000 volts above earth ground (assuming the isolation resistance between the input energy and the machine components could tolerate this voltage) without a noise problem. This might require filtering of line noise at the input to the machine, and obviously the EGC connection to the machine is related to some point in the energy source which does not connect to earth. This is solely to illustrate the problem. Internally the electronics being in a shielded box shields it from noise other than conducted thru the power connection.
In all probability the noise problem that is of concern is communication to the machine for data or other reasons. The way to solve the communication problems is by dielectric isolation in the communication path. A simplified discussion for machine tool type people is at my web site http://beta-a2.com/noise_grnd.html .
I had one customer with two nearly identical HAAS machines spaced about 1 foot apart, and fed from the same 240 bus. One machine was a 2005 model and the other about a year newer. RS232 drip mode communication to the newer machine was impossible. With servos off communication was possible, but you can not machine a part with servos off. The HAAS service people said there was nothing wrong with the machine because their laptop communicated perfectly. Therefore, HAAS said the problem was with the RS232 source. You could interchange the cables between the two machines and the problem stayed with the one problem machine. Yes it is a machine problem.
Installation of our I232 isolators in the path with the problem machine solved the problem, and further allowed transmission at 115.2 kbaud over 100 ft of cable.
The problem was the newer machine had brushless servos that created a lot of internal noise between the RS232 common and the machine chassis. The older machine used DC servos. Note, both machines had separate proper EGCs and supplemental ground rods.
The RS232 common wire (pin 7 or 5) in now way was adequate to tie the two different systems (computer and CNC) together with a sufficiently low impedance to prevent noise between the two references from producing communication errors.
The CNC reps and manufacturers basically do not know how to solve RS232 communication problems, or they would never suggest a ground rod as a solution.
The reason the HAAS serviceman's laptop did not have a problem with communication to the problem machine was that the laptop was essentially floating from the building EGC and the RS232 computer pin 5 to CNC pin 7 common connection established the same reference potential at both the computer and CNC.
.
I agree with the above comments for the most part.
It is absolutely essential that a proper EGC be provided. It will also help if it is greatly oversized. A supplemental ground rod at the CNC might marginally reduce noise problems.
If there is no connection to anything external to the CNC machine except input power and the necessary EGC, then the machine is essentially a shielded box, and that could be raised to 1,000,000 volts above earth ground (assuming the isolation resistance between the input energy and the machine components could tolerate this voltage) without a noise problem. This might require filtering of line noise at the input to the machine, and obviously the EGC connection to the machine is related to some point in the energy source which does not connect to earth. This is solely to illustrate the problem. Internally the electronics being in a shielded box shields it from noise other than conducted thru the power connection.
In all probability the noise problem that is of concern is communication to the machine for data or other reasons. The way to solve the communication problems is by dielectric isolation in the communication path. A simplified discussion for machine tool type people is at my web site http://beta-a2.com/noise_grnd.html .
I had one customer with two nearly identical HAAS machines spaced about 1 foot apart, and fed from the same 240 bus. One machine was a 2005 model and the other about a year newer. RS232 drip mode communication to the newer machine was impossible. With servos off communication was possible, but you can not machine a part with servos off. The HAAS service people said there was nothing wrong with the machine because their laptop communicated perfectly. Therefore, HAAS said the problem was with the RS232 source. You could interchange the cables between the two machines and the problem stayed with the one problem machine. Yes it is a machine problem.
Installation of our I232 isolators in the path with the problem machine solved the problem, and further allowed transmission at 115.2 kbaud over 100 ft of cable.
The problem was the newer machine had brushless servos that created a lot of internal noise between the RS232 common and the machine chassis. The older machine used DC servos. Note, both machines had separate proper EGCs and supplemental ground rods.
The RS232 common wire (pin 7 or 5) in now way was adequate to tie the two different systems (computer and CNC) together with a sufficiently low impedance to prevent noise between the two references from producing communication errors.
The CNC reps and manufacturers basically do not know how to solve RS232 communication problems, or they would never suggest a ground rod as a solution.
The reason the HAAS serviceman's laptop did not have a problem with communication to the problem machine was that the laptop was essentially floating from the building EGC and the RS232 computer pin 5 to CNC pin 7 common connection established the same reference potential at both the computer and CNC.
.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Gar
The reason that a few of us state that a ground rod at the CNC machine can cause or increase the problem is similar to the stray voltage problem of pools, if you have a voltage drop on the neutral of the system supplying the CNC and even the MGN of a WYE MV system on the utilities primary side, this can cause a raised voltage between the buildings grounding system and Earth, without the ground rod at the CNC then theres no current flow on the EGC feeding the CNC, but install a ground rod, and you now have current flow on the EGC and the 232s pin 7 or 5 which is in parallel of the EGC, this is why the ground rod in reality doesn't serve any useful purpose and can cause or increase the problem of the 60hz noise on the 232 network or a 485 or any network that have a ground referenced conductor, your isolation system is the way to go.
Also the above applies to a corner grounded delta as well as a high impedance grounded system, any conductor that is used grounded and is also part of the current carrying of the source can cause this raised voltage on the grounding system to Earth.
The ground rod even if it has a low impedance to Earth connection will not remove this voltage drop but will increase the current flowing on the EGC, so the better the connection to earth actually works against you as it increases the amount of current in the EGC and the network reference conductor.
The reason that a few of us state that a ground rod at the CNC machine can cause or increase the problem is similar to the stray voltage problem of pools, if you have a voltage drop on the neutral of the system supplying the CNC and even the MGN of a WYE MV system on the utilities primary side, this can cause a raised voltage between the buildings grounding system and Earth, without the ground rod at the CNC then theres no current flow on the EGC feeding the CNC, but install a ground rod, and you now have current flow on the EGC and the 232s pin 7 or 5 which is in parallel of the EGC, this is why the ground rod in reality doesn't serve any useful purpose and can cause or increase the problem of the 60hz noise on the 232 network or a 485 or any network that have a ground referenced conductor, your isolation system is the way to go.
Also the above applies to a corner grounded delta as well as a high impedance grounded system, any conductor that is used grounded and is also part of the current carrying of the source can cause this raised voltage on the grounding system to Earth.
The ground rod even if it has a low impedance to Earth connection will not remove this voltage drop but will increase the current flowing on the EGC, so the better the connection to earth actually works against you as it increases the amount of current in the EGC and the network reference conductor.
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